Issue 1 - Volume 572 - The Astrophysical Journal (original) (raw)

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, Angela M. Linn, Robert J. Scherrer, and David H. Weinberg

The dark energy that appears to produce the accelerating expansion of the universe can be characterized by an equation of state p = _w_ρ with w < - . A number of observational tests have been proposed to study the value or redshift dependence of w, including Type Ia supernova distances, the Sunyaev-Zeldovich effect, cluster abundances, strong and weak gravitational lensing, galaxy and quasar clustering, galaxy ages, the Lyα forest, and cosmic microwave background anisotropies. The proposed observational tests based on these phenomena measure either the distance-redshift relation d(z), the Hubble parameter H(z), the age of the universe t(z), the linear growth factor _D_1(z), or some combination of these quantities. We compute the evolution of these four observables and of the combination H(z)d(z) that enters the Alcock-Paczyznski anisotropy test in models with constant w, in quintessence models with some simple forms of the potential V(ϕ), and in toy models that allow more radical time variations of w. Measurement of any of these quantities to a precision of a few percent is generally sufficient to discriminate between w = -1 and - . However, the time dependence predicted in quintessence models is extremely difficult to discern because the quintessence component is dynamically unimportant at the redshifts where w departs substantially from its low-z value. Even for the toy models that allow substantial changes in w at low redshift, there is always a constant-w model that produces very similar evolution of all of the observables simultaneously. We conclude that measurement of the effective equation of state of the dark energy may be achieved by several independent routes in the next few years but that detecting time variation in this equation of state will prove very difficult except in specialized cases.

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Aparna Venkatesan

Spectroscopic studies of high-redshift objects and increasingly precise data on the cosmic microwave background (CMB) are beginning to independently place strong complementary bounds on the epoch of hydrogen reionization. Parameter estimation from current CMB data continues, however, to be subject to several degeneracies. Here, we focus on those degeneracies in CMB parameter forecasts related to the optical depth to reionization. We extend earlier work on the mutual constraints that such analyses of CMB data and a reionization model may place on each other to a more general parameter set and to the case of data anticipated from the MAP satellite. We focus in particular on a semianalytic model of reionization by the first stars, although the methods here are easily extended to other reionization scenarios. A reionization model can provide useful complementary information for cosmological parameter extraction from the CMB, particularly for the degeneracies between the optical depth and either of the amplitude and index of the primordial scalar power spectrum, which are still present in the most recent data. Alternatively, by using a reionization model, known limits on astrophysical quantities can reduce the forecasted errors on cosmological parameters. Forthcoming CMB data also have the potential to constrain the sites of early star formation, as well as the fraction of baryons that participate in it, if reionization were caused by stellar activity at high redshifts. Finally, we examine the implications of an independent, e.g., spectroscopic, determination of the epoch of reionization for the determination of cosmological parameters from the CMB. This has the potential to significantly strengthen limits from the CMB on parameters such as the index of the power spectrum, while having the considerable advantage of being free of the choice of the reionization model.

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N. Dalal and C. S. Kochanek

We devise a method to measure the abundance of satellite halos in gravitational lens galaxies and apply our method to a sample of seven lens systems. After using Monte Carlo simulations to verify the method, we find that substructure comprises _f_sat = 0.02 (median, 0.006 < _f_sat < 0.07 at 90% confidence) of the mass of typical lens galaxies, in excellent agreement with predictions of cold dark matter (CDM) simulations. We estimate a characteristic critical radius for the satellites of 00001 < b < 0006 (90% confidence). For a dn/dM ∝ _M_-1.8 (_M_low < M < _M_high) satellite mass function, the critical radius provides an estimate for the upper mass limit of 106_M_☉ ≲ _M_high ≲ 109_M_☉. Our measurement confirms a generic prediction of CDM models and may obviate the need to invoke alternatives to CDM such as warm dark matter or self-interacting dark matter.

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S. M. Khairul Alam, James S. Bullock, and David H. Weinberg

Using an analytic model calibrated against numerical simulations, we calculate the central densities of dark matter halos in a "conventional" cold dark matter model with a cosmological constant (LCDM) and in a "tilted" model (TLCDM) with slightly modified parameters motivated by recent analyses of Lyα forest data. We also calculate how warm dark matter (WDM) would modify these predicted densities by delaying halo formation and imposing phase-space constraints. As a measure of central density, we adopt the quantity Δ_V_/2, the density within the radius R V/2 at which the halo rotation curve falls to half of its maximum value, in units of the critical density. We compare the theoretical predictions to values of Δ_V_/2 estimated from the rotation curves of dark matter-dominated disk galaxies. Assuming that dark halos are described by Navarro-Frenk-White profiles, our results suggest that the conventional LCDM model predicts excessively high dark matter densities, unless there is some selection bias in the data toward the low-concentration tail of the halo distribution. A WDM model with particle mass 0.5-1 keV provides a better match to the observational data. However, the modified cold dark matter model, TLCDM, fits the data equally well, suggesting that the solution to the "halo cores" problem might lie in moderate changes to cosmological parameters rather than radical changes to the properties of dark matter. If cold dark matter halos have the steeper density profiles found by Moore et al., then neither conventional LCDM nor TLCDM can reproduce the observed central densities.

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Joseph F. Hennawi and Jeremiah P. Ostriker

We consider the astrophysical consequences of the self-interacting dark matter (SIDM) scenario for a general velocity-dependent cross section per unit mass that varies as some power of velocity: σDM = σ0-a. Accretion of SIDM onto seed black holes can produce supermassive black holes that are too large for certain combinations of σ0_v_ and a, a fact that is used to obtain a new constraint on the dark matter interaction. Constraints due to other astrophysical considerations are presented and previous constraints for a constant cross section are generalized. The black hole constraint is extremely sensitive to the cusp slope, α, of the inner density profile ρ ~ _r_-α of dark halos. For the most probable value of α = 1.3, we find that there exists a tiny region in the parameter space for SIDM properties, with a ≈ 0.5 and a ≈ 0.5, such that all constraints are satisfied. However, the adiabatic compression of the dark halo by baryons as they cool and contract in normal galaxies yields a steeper cusp, ρ ~ r. We find that in both the highly collisional and collisionless limits invariance arguments require α' = (6 - α) (4 - α), where α and α' are the (4α) inner profile slope of the dark halo before and after compression, respectively. This gives the tighter constraint a ≲ 0.02, which would exclude SIDM as a possible solution to the purported problems with cold dark matter (CDM) on subgalactic scales in the absence of other dynamical processes. Nevertheless, SIDM with parameters consistent with this stronger constraint, can explain the ubiquity of supermassive black holes in the centers of galaxies. A "best-fit" model is presented with a = 0 and = 0.02, which reproduces the supermassive black hole masses and their observed correlations with the velocity dispersion of the host bulges. Specifically, the approximately fourth-power dependence of black hole mass on galactic velocity dispersion is a direct consequence of the power spectrum of primeval perturbations having an index of n ≈ -2 and the value of a. Although the dark matter collision rates for this model are too small to directly remedy problems with CDM, mergers between dark halos harboring supermassive black holes at high redshift could ameliorate the cuspy halo problem. This scenario also explains the lack of comparable supermassive black holes in bulgeless galaxies like M33.

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Henk Hoekstra, Howard K. C. Yee, Michael D. Gladders, L. Felipe Barrientos, Patrick B. Hall, and Leopoldo Infante

We have analyzed ~24 deg2 of R C_-band imaging data from the Red-Sequence Cluster Survey (RCS) and measured the excess correlations between galaxy ellipticities on scales ranging from 1' to 30'. We have used data from two different telescopes: ~16.4 deg2 of Canada-France-Hawaii Telescope data and ~7.6 deg2 of Cerro Tololo Inter-American Observatory 4 m data, distributed over 13 widely separated patches. For the first time, a direct comparison can be made of the lensing signal measured using different instruments, which provides an important test of the weak-lensing analysis itself. The measurements obtained from the two telescopes agree well. For the lensing analysis, we use galaxies down to a limiting magnitude of R C = 24, for which the redshift distribution is known relatively well. This allows us to constrain some cosmological parameters. For the currently favored ΛCDM model (Ω_m = 0.3, ΩΛ = 0.7, Γ = 0.21), we obtain σ8 = 0.81 (95% confidence), in agreement with the 2001 results from Van Waerbeke and coworkers, who used fainter galaxies (and consequently higher redshift galaxies). The good agreement between these two very different weak-lensing studies demonstrates that weak lensing is a useful tool in observational cosmology.

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J. S. Arabadjis, M. W. Bautz, and G. P. Garmire

We present Chandra observations of EMSS 1358+6245, a relaxed cooling flow cluster of galaxies at z = 0.328. We employ a new deprojection technique to construct temperature, gas, and dark matter profiles. We confirm the presence of cool gas in the cluster core, and our deprojected temperature profile for the hot component is isothermal over 30 kpc < r < 0.8 Mpc. Fitting the mass profile to a Navarro-Frenk-White model yields r s = 153 kpc and c = 8.4. We find good agreement between our dark matter profile and weak gravitational lensing measurements. We place an upper limit of 42 kpc (90% confidence limit) on the size of any constant-density core. We compare this result to recent simulations and place a conservative upper limit on the dark matter particle-scattering cross section of 0.1 cm2 g-1. This limit implies that the cross section must be velocity-dependent if the relatively shallow core mass profiles of dwarf galaxies are a direct result of dark matter self-interaction.

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James Chiang

Using a geometry consisting of a hot central Comptonizing plasma surrounded by a thin accretion disk, we model the optical through hard X-ray spectral energy distributions of the type 1 Seyfert galaxies NGC 3516 and NGC 7469. As in the model proposed by Poutanen, Krolik, & Ryde for the X-ray binary Cyg X-1 and later applied to Seyfert galaxies by Zdziarski, Lubiński, & Smith, feedback between the radiation reprocessed by the disk and the thermal Comptonization emission from the hot central plasma plays a pivotal role in determining the X-ray spectrum and, as we show, the optical and ultraviolet spectra as well. Seemingly uncorrelated optical/UV and X-ray light curves, similar to those that have been observed from these objects can, in principle, be explained by variations in the size, shape, and temperature of the Comptonizing plasma. Furthermore, by positing a disk mass accretion rate that satisfies a condition for global energy balance between the thermal Comptonization luminosity and the power available from accretion, one can predict the spectral properties of the heretofore poorly measured hard X-ray continuum above ~50 keV in type 1 Seyfert galaxies. Conversely, forthcoming measurements of the hard X-ray continuum by more sensitive hard X-ray and soft γ-ray telescopes, such as those aboard the International Gamma-Ray Astrophysics Laboratory, in conjunction with simultaneous optical, UV, and soft X-ray monitoring, will allow the mass accretion rates to be directly constrained for these sources in the context of this model.

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A. Rodríguez-Ardila, S. M. Viegas, M. G. Pastoriza, L. Prato, and Carlos J. Donzelli

UV, visible, and near-infrared spectroscopy is used to study the transitions of neutral oxygen leading to the emission of broad O I λ8446, λ11287, and λ1304 in active galactic nuclei. From the strength of the former two lines, contrary to the general belief, we found that in six of seven galaxies, Lyβ fluorescence is not the only mechanism responsible for the formation of these three lines. Because O I λ13165 is almost reduced to noise level, continuum fluorescence is ruled out as an additional excitation mechanism, but the presence of O I λ7774 in one of the objects suggests that collisional ionization may have an important role in the formation of O I λ8446. The usefulness of the O I lines as a reliable reddening indicator for the broad-line region is discussed. The values of E(B-V) derived from the λ1304/λ8446 ratio agree with those obtained using other reddening indicators. The observations point toward a break in the one-to-one photon relation between λ8446 and λ1304, attributable to several destruction mechanisms that may affect the latter line.

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Luigi Spinoglio, Paola Andreani, and Matthew A. Malkan

New far-infrared photometry with ISOPHOT aboard the Infrared Space Observatory (ISO) is presented for 58 galaxies with homogeneous published data for another 32 galaxies, all belonging to the 12 μm galaxy sample—in total, 29 Seyfert 1 galaxies, 35 Seyfert 2 galaxies, and 12 starburst galaxies, or about half of the 12 μm active galaxy sample, plus 14 normal galaxies for comparison. ISO and Infrared Astronomical Satellite (IRAS) data are used to define color-color diagrams and spectral energy distributions (SEDs). Thermal dust emission at two temperatures (one cold at 15-30 K and one warm at 50-70 K) can fit the 60-200 μm SED, with a dust emissivity law proportional to the inverse square of the wavelength. Seyfert 1 galaxies and Seyfert 2 galaxies are indistinguishable longward of 100 μm, while, as already seen by IRAS, the former have flatter SEDs shortward of 60 μm. A mild anticorrelation is found between the [200-100] color and the "60 μm excess." We infer that this is due to the fact that galaxies with a strong starburst component and thus a strong 60 μm flux have a steeper far-infrared turnover. In non-Seyfert galaxies, increasing the luminosity corresponds to increasing the star formation rate, which enhances the 25 and 60 μm emission. This shifts the peak emission from around 150 μm in the most quiescent spirals to shorter than 60 μm in the strongest starburst galaxies. To quantify these trends further, we identified with the IRAS colors three idealized infrared SEDs: pure quiescent disk emission, pure starburst emission, and pure Seyfert nucleus emission. Even between 100 and 200 μm, the quiescent disk emission remains much cooler than the starburst component. Seyfert galaxies have 100-200 μm SEDs ranging from pure disks to pure starbursts, with no apparent contribution from their active nuclei at those wavelengths.

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M. Contini, M. Radovich, P. Rafanelli, and G. M. Richter

Detailed modeling of the different regions of NGC 7130 is presented, accounting for its composite nature of an active galactic nucleus (AGN) and a starburst galaxy. Shock waves, created by stellar winds from hot massive stars and by supernova ejecta, are evident in the continuum and line spectra emitted from the clouds. Therefore, the SUMA code, which accounts consistently for photoionization and shocks, is adopted in model calculation. The results show that the nuclear region is dominated by gas ionized by a power-law radiation flux from the active center (AC). High-velocity (V s = 1000 km s-1) clouds, which account for the broad FWHM component of the line profile, are found close to the AC and are characterized by a high dust-to-gas ratio (>10-12, while the dust-to-gas ratio is about 10-14 throughout the galaxy). Massive stars with temperatures of (5-7) × 104 K photoionize and heat the gas in the outer regions, and old star population (T* = 3000 K) background radiation contributes to the fit of the continuum in the optical-near-IR range. The AGN-starburst connection is discussed on the basis of model results, considering, particularly, the distribution of densities and velocities throughout the galaxy.

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Scott Dodelson, Vijay K. Narayanan, Max Tegmark, Ryan Scranton, Tamas Budavári, Andrew Connolly, Istvan Csabai, Daniel Eisenstein, Joshua A. Frieman, James E. Gunn et al

Early photometric data from the Sloan Digital Sky Survey (SDSS) contain angular positions for 1.5 million galaxies. In companion papers, the angular correlation function w(θ) and two-dimensional power spectrum C l of these galaxies are presented. Here we invert Limber's equation to extract the three-dimensional power spectrum from the angular results. We accomplish this using an estimate of dn/dz, the redshift distribution of galaxies in four different magnitude slices in the SDSS photometric catalog. The resulting three-dimensional power spectrum estimates from w(θ) and C l agree with each other and with previous estimates over a range in wavenumbers 0.03 < k/(h Mpc-1) < 1. The galaxies in the faintest magnitude bin (21 < r* < 22, which have median redshift z m = 0.43) are less clustered than the galaxies in the brightest magnitude bin (18 < r* < 19 with z m = 0.17), especially on scales where nonlinearities are important. The derived power spectrum agrees with that of Szalay et al., who go directly from the raw data to a parametric estimate of the power spectrum. The strongest constraints on the shape parameter Γ come from the faintest galaxies (in the magnitude bin 21 < r* < 22), from which we infer Γ = 0.14 (95% CL).

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Caleb Scharf

In this short paper I present the results of a calculation that seeks the maximum, or optimal, signal-to-noise energy band for galaxy group or cluster X-ray emission detected by the Chandra and XMM-Newton observatories. Using a background spectrum derived from observations and a grid of models, I show that the "classical" 0.5-2 keV band is indeed close to optimal for clusters with gas temperatures greater than 2 keV and redshifts z < 1. For cooler systems, however, this band is generally far from optimal. Sub-keV plasmas can suffer 20%-60% signal-to-noise loss, compared to an optimal band, and worse for _z_ > 0. The implication is that current and forthcoming surveys should be carefully constructed in order to minimize bias against the low-mass, low-temperature end of the cluster/group population.

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Fabio Gastaldello and Silvano Molendi

We make a detailed measurement of the metal abundance profiles and metal abundance ratios of the inner core of M87/Virgo observed by XMM-Newton during the performance verification phase. We use multitemperature models for the inner regions, and we compare the plasma codes APEC and MEKAL. We confirm the strong heavy-element gradient previously found by ASCA and BeppoSAX, but also find a significant increase in light elements, in particular O. This fact, together with the constant O/Fe ratio in the inner 9', indicates an enhancement of contribution in the core of the cluster, not only by Type Ia supernovae but also by Type II supernovae.

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Y. Krongold, D. Dultzin-Hacyan, and P. Marziani

This paper systematically studies, for the first time, the circumgalactic environment of bright IRAS galaxies as defined in 1989 by Soifer and coworkers. While the role of gravitational interaction for luminous and ultraluminous IRAS galaxies has been well established by various studies, the situation is by far more obscure in the IR luminosity range of the bright IRAS sample, 1010_L_☉ ≲ _L_FIR ≲ 1011_L_☉. To easily identify nearby companion galaxies, the bright IRAS sample is restricted to 87 objects with redshift range 0.008 ≤ z ≤ 0.018 and Galactic latitude δ ≥ . A control sample, selected from the CfA redshift-survey catalog, includes 90 objects matching the IRAS bright galaxy survey sample for distribution of isophotal diameter, redshift, and morphological type. From a search of nearby companion galaxies within 250 kpc in the second-generation Digitized Sky Survey (DSS-II), we find that the circumgalactic environments of bright IRAS galaxies contain more large companions than the galaxies in the optically selected control sample and are similar to those of Seyfert 2 galaxies. We find a weak correlation over a wide range of far-IR luminosity (109_L_☉ ≲ _L_FIR ≲ 1012.5_L_☉) between projected separation and _L_FIR, which confirms a very close relationship between the star formation rate of a galaxy and the strength of gravitational perturbations. We also find that the far-IR colors depend on whether a source is isolated or interacting. Finally, we discuss the intrinsic difference between and evolution expectations for the bright IRAS galaxies and the control sample, as well as the relationship between starbursting and active galaxies.

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Kenneth R. Sembach, Brad K. Gibson, Yeshe Fenner, and Mary E. Putman

There is a fortuitous coincidence in the positions of the quasar Ton S210 and the compact H I high-velocity cloud CHVC 224.0-83.4-197 on the sky. Using Far Ultraviolet Spectroscopic Explorer observations of the metal line absorption in this cloud and sensitive H I 21 cm emission observations obtained with the multibeam system at Parkes Observatory, we determine a metallicity of (O/H) < 0.46 solar at a confidence of 3 σ. The metallicity of the high-velocity gas is consistent with either an extragalactic or Magellanic Cloud origin but is not consistent with a location inside the Milky Way unless the chemical history of the gas is considerably different from that of the interstellar medium in the Galactic disk and halo. Combined with measurements of highly ionized species (C III and O VI) at high velocities, this metallicity limit indicates that the cloud has a substantial halo of ionized gas; there is as much ionized gas as neutral gas directly along the Ton S210 sight line. We suggest several observational tests that would improve the metallicity determination substantially and help to distinguish between possible origins for the high-velocity gas. Additional observations of this sight line would be valuable, since the number of compact HVCs positioned in front of background sources bright enough for high-resolution absorption-line studies is extremely limited.

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Jaime Alvarez-Muñiz, Ralph Engel, and Todor Stanev

Recently, the Akeno Giant Air Shower Array Collaboration presented data suggesting a significant clustering of ultrahigh-energy cosmic rays coming from the outer Galaxy region. In this paper we calculate expected cosmic-ray arrival distributions for several simple source location scenarios that bracket the more realistic ones and investigate the possibility of clustering and correlation effects. The role of the Galactic magnetic field is discussed in detail.

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Cindy Tam, Mallory S. E. Roberts, and Victoria M. Kaspi

We present a high-resolution radio study of the supernova remnant (SNR) G11.2-0.3 using archival Very Large Array data. Spectral tomography is performed to determine the properties of this composite-type SNR's individual components, which have only recently been distinguished through X-ray observations. Our results indicate that the spectral index of the pulsar wind nebula (PWN), or plerion, is α_P_ = 0.25. We observe a spectral index of α_S_ = 0.56 ± 0.02 throughout most of the SNR shell region but also detect a gradient in α in the southeastern component. We compare the spectral index and flux density with recent single-dish radio data of the source. Also, the radio efficiency and morphological properties of this PWN are found to be consistent with results for other known PWN systems.

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Ben E. K. Sugerman, Stephen S. Lawrence, Arlin P. S. Crotts, Patrice Bouchet, and Steve R. Heathcote

We present ground-based near-infrared imaging and Hubble Space Telescope optical imaging and spectroscopy of the interaction between the ejecta of SN 1987A and its equatorial circumstellar ring. This interaction has made a transition, from emission originating in just a few "hot spots" at restricted locations in position angle around the ring, to a collision producing optical emission over a nearly continuous distribution, with few breaks larger than 45°. The centroids of the first three spots are measured to move at 2000-3000 km s-1, which we interpret as a lower limit of the velocity of the forward blast front. Multiwavelength light curves of the spots show that they do not evolve uniformly and change significantly on timescales as short as 1 month; in particular, the first spot shows a significant break in its light curve. Implications of observed delays between spot appearances are discussed, which leads to a generalized model of hot spot evolution and suggests that the early appearance of the first hot spot is explained by its inward radial position and a fairly uniform forward blast wave, rather than extraordinary physical circumstances. Data further suggest that the forward blast is reaching the bulk of the inner ring material to the east, the density of which appears higher than elsewhere in the ring. We study the ring geometry, finding evidence suggestive of an intrinsic ellipticity of 0.95, and find lower and upper distance limits of 47.9 ± 0.92 and 54.4 ± 2.1 kpc, respectively.

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Luis Salas and Irene Cruz-González

We present an analysis of velocity-resolved near-IR molecular hydrogen observations of a variety of protostellar outflows with very different energetics, degrees of collimation, and morphologies. Observations in the 2.12 μm line of H2 were obtained using an IR Fabry-Pérot interferometer with a spectral resolution of 23 km s-1. The integrated flux-velocity diagrams for each outflow show a flat spectrum for low velocities followed by a decreasing power law dF/dv ∝ _v_γ, with γ between -1.8 and -2.6, for velocities higher than a clearly defined break velocity at 2-17 km s-1. Contrary to shock model predictions, it is shown that the H2 intensity is constant with velocity. We argue that the flux-velocity relation can then be interpreted as a mass-velocity relation, in striking similarity to the power-law mass spectra observed in CO outflows. By comparing H2 and CO mass-velocity spectra, it is shown that there is a velocity regime in which both molecules coexist and produce similar γ-values. Evolution effects in outflows appear as a correlation between outflow length and γ; as outflows age, the spectra becomes steeper. Our results support a common physical origin for both CO and H2 emission and a strong association between the molecular outflows traced in each molecule.

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Aigen Li and B. T. Draine

One of the major challenges to identification of the 3.3, 6.2, 7.7, 8.6, and 11.3 μm interstellar infrared (IR) emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the recent detection of these bands in regions with little ultraviolet (UV) illumination, since small, neutral PAH molecules have little or no absorption at visible wavelengths and therefore require UV photons for excitation. We show here that our "astronomical" PAH model, incorporating the experimental result that the visual absorption edge shifts to longer wavelength upon ionization and/or as the PAH size increases, can closely reproduce the observed IR emission bands of vdB 133, a UV-poor reflection nebula. We also show that single-photon heating of "astronomical" PAHs in reflection nebulae near stars as cool as _T_eff = 3000 K can result in observable emission at 6.2, 7.7, 8.6, and 11.3 μm. Illustrative mid-IR emission spectra are calculated for reflection nebulae illuminated by cool stars with _T_eff = 3500, 4500, and 5000 K. These will allow comparison with future Space Infrared Telescope Facility observations of vdB 135 (_T_eff = 3600 K), vdB 47 (_T_eff = 4500 K), and vdB 101 (_T_eff = 5000 K). The dependence on the effective temperature of the exciting star of the observed 12 μm IRAS emission (relative to the total far-IR emission) is consistent with the PAH model for 3000 K ≤ _T_eff ≤ 30,000 K.

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Paola Caselli, Priscilla J. Benson, Philip C. Myers, and Mario Tafalla

We present results of an extensive mapping survey of N2H+ (1-0) in about 60 low-mass cloud cores already mapped in the NH3 (1, 1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution of 54'', about 1.5 times finer than the previous ammonia observations made at the Haystack telescope. The comparison between N2H+ and NH3 maps shows strong similarities in the size and morphology of the two molecular species, indicating that they are tracing the same material, especially in starless cores. Cores with stars typically have map sizes about a factor of 2 smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. The mean aspect ratio is ~2. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Starless cores are less massive ( ≃ 3 _M_☉) than cores with stars ( ≃ 9 _M_☉). Velocity gradients range between 0.5 and 6 km s-1 pc-1, similar to what has been found with NH3 data, and the ratio β of rotational kinetic energy to gravitational energy has magnitudes between ~10-4 and 0.07, indicating that rotation is not energetically dominant in the support of the cores. "Local" velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complex motions not interpretable as simple solid-body rotation. Integrated intensity profiles of starless cores present a "central flattening" and are consistent with a spherically symmetric density law n ∝ _r_-α, where α = 1.2 for r < _r_break and α = 2 for _r_ > _r_break, with r_break ~ 0.03 pc. Cores with stars are better modeled with single density power laws with α ≥ 2, in agreement with observations of submillimeter continuum emission. Line widths change across the core, but we did not find a general trend: there are cores with significant positive as well as negative linear correlations between Δ_v and the impact parameter b. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~10 coherence lengths. The corresponding value of the coherence length, ~0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased "coherence" of line widths on small scales. Despite finer angular resolution, the majority of N2H+ and NH3 maps show a similar "simple" structure, with single peaks and no elongation.

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J. Christopher Howk, Blair D. Savage, Kenneth R. Sembach, and Charles G. Hoopes

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Miwa Goto, Naoto Kobayashi, Hiroshi Terada, and A. T. Tokunaga

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Michael P. Egan, J. Simon Clark, Donald R. Mizuno, Sean J. Carey, Iain A. Steele, and Stephan D. Price

The Midcourse Space Experiment (MSX) Galactic plane survey discovered a nearly perfectly circular ring nebula around the suspected planetary nebula Wray 17-96. Using near-IR spectral typing and modeling of the mid-IR nebula, we find that Wray 17-96 is more likely a candidate to be a luminous blue variable (LBV) surrounded by a large spherical ejecta shell. It is very similar to the G79.29+0.46 LBV candidate in Cygnus and the Pistol Star. The _K_-band spectrum and the mid-IR data indicate a stellar temperature of 13,000 K. The most likely distance to the source is 4.5 kpc, leading to a luminosity of 1.8 × 106_L_☉. We suggest that the nebula consists of multiple shells and that an evolution from oxygen-rich to carbon-rich chemistry may be indicated.

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Kensuke Imanishi, Masahiro Tsujimoto, and Katsuji Koyama

We present the Chandra Advanced CCD Imaging Spectrometer and ASCA Gas Imaging Spectrometer results for a series of four long-term observations on DoAr 21, ROXs 21, and ROXs 31, the X-ray-brightest T Tauri stars in the ρ Ophiuchi cloud. In the four observations with a net exposure of ~600 ks, we found six, three, and two flares from DoAr 21, ROXs 21, and ROXs 31, respectively; hence, the flare rate is fairly high. The spectra of DoAr 21 are well fitted with a single-temperature plasma model, while those of ROXs 21 and ROXs 31 need an additional soft plasma component. Since DoAr 21 is younger (~105 yr) than ROXs 21 and ROXs 31 (~106 yr), these results may indicate that the soft component gradually increases as T Tauri stars age. The abundances are generally subsolar and vary from element to element. Both high first ionization potential (FIP) and low-FIP elements show enhancement over the mean abundances. An unusual giant flare is detected from ROXs 31. The peak luminosity and temperature are ~1033 ergs s-1 and ~10 keV, respectively. The temperature reaches its peak value before the flux maximum and is nearly constant (4-5 keV) during the decay phase, indicating successive energy release during the flare. The abundances and absorption show dramatic variability from the quiescent to flare phase.

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Gregory J. Herczeg, Jeffrey L. Linsky, Jeff A. Valenti, Christopher M. Johns-Krull, and Brian E. Wood

We observed the classical T Tauri star TW Hya with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST) using the E140M grating, from 1150 to 1700 Å, with the E230M grating, from 2200 to 2900 Å, and with the Far Ultraviolet Spectroscopic Explorer from 900 to 1180 Å. Emission in 146 Lyman-band H2 lines, representing 19 progressions, dominates the spectral region from 1250 to 1650 Å. The total H2 emission line flux is 1.94 × 10-12 ergs cm-2 s-1, which corresponds to 1.90 × 10-4_L_☉ at TW Hya's distance of 56 pc. A broad stellar Lyα line photoexcites the H2 from excited rovibrational levels of the ground electronic state to excited electronic states. The C II λ1335 doublet, C III λ1175 multiplet, and C IV λ1550 doublet also electronically excite H2. The velocity shift of the H2 lines is consistent with the photospheric radial velocity of TW Hya, and the emission is not spatially extended beyond the 005 resolution of HST. The H2 lines have an intrinsic FWHM of 11.91 ± 0.16 km s-1. One H2 line is significantly weaker than predicted by this model because of C II wind absorption. We also do not observe any H2 absorption against the stellar Lyα profile. From these results we conclude that the H2 emission is more consistent with an origin in a disk rather than in an outflow or circumstellar shell. We also analyze the hot accretion region lines (e.g., C IV, Si IV, O VI) of TW Hya, which are formed at the accretion shock, and discuss some reasons why Si lines appear significantly weaker than other TR region lines.

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Dana S. Balser, Joseph P. McMullin, and T. L. Wilson

Standard stellar evolution theory is inconsistent with the observed isotopic carbon ratio, 12C/13C, in evolved stars. This theory is also inconsistent with the 3He/H abundance ratios observed in Galactic H II regions, when combined with chemical evolution theory. These discrepancies have been attributed to an extra, nonstandard mixing, which further processes material during the red giant branch and should lower both the 12C/13C and 3He/H abundance ratios for stars with masses ≤2 _M_☉. Measurements of isotopic ratios in planetary nebulae probe material that escapes the star to be further processed by future generations of stars. We have measured the carbon isotopic abundance ratio, 12C/13C, in 11 planetary nebulae (PNe) by observing the J = 2 → 1 and J = 3 → 2 millimeter transitions of 12CO and 13CO in molecular clouds associated with the PNe. A large velocity gradient (LVG) model has been used to determine the physical conditions for each PN for which both transitions have been detected. We detect both 12CO and 13CO in nine PNe. If 12CO/13CO = 12C/13C, the range of 12C/13C is 2.2-31. Our results support theories that include some form of extra mixing.

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Eric D. Feigelson, Gordon P. Garmire, and Steven H. Pravdo

To address the role of energetic processes in the solar nebula, we provide a detailed characterization of magnetic flaring in stellar analogs of the pre-main-sequence Sun based on two 0.5 day observations of the Orion Nebula cluster obtained with the Chandra X-Ray Observatory. The sample consists of 43 stars with masses between 0.7 and 1.4 _M_☉ and ages from less than 0.3 to ≃10 Myr. We find that the X-ray luminosities measured in the 0.5-8 keV band are strongly elevated over main-sequence levels with an average = 30.3 ergs s-1 and = -3.9. The X-ray emission is strongly variable within our exposures in nearly all solar analogs; about 30 flares with 29.0 ergs s-1 < log _L_X(peak) < 31.5 ergs s-1 on timescales from 0.5 to more than 12 hr are seen during the Chandra observations. Analogs of the ≤1 Myr old pre-main-sequence Sun exhibited X-ray flares that are 101.5 times more powerful and 102.5 times more frequent than the most powerful flares seen on the contemporary Sun. Radio observations indicate that acceleration of particles to relativistic energies is efficient in young stellar flares. Extrapolating the solar relationship between X-ray luminosity and proton fluence, we infer that the young Sun exhibited a 105-fold enhancement in energetic protons compared to contemporary levels. Unless the flare geometries are unfavorable, this inferred proton flux on the disk is sufficient to produce the observed meteoritic abundances of several important short-lived radioactive isotopes. Our study thus strengthens the astronomical foundation for local proton spallation models of isotopic anomalies in carbonaceous chondritic meteorites. The radiation, particles, and shocks produced by the magnetic reconnection flares seen with Chandra may also have flash-melted meteoritic chondrules and produce excess 21Ne seen in meteoritic grains.

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Claes Fransson, Roger A. Chevalier, Alexei V. Filippenko, Bruno Leibundgut, Aaron J. Barth, Robert A. Fesen, Robert P. Kirshner, Douglas C. Leonard, Weidong Li, Peter Lundqvist et al

Optical and ultraviolet observations of the Type IIn supernova SN 1995N at epochs between 321 and 1799 days after the explosion show three distinct velocity components. The narrow lines come from circumstellar gas and show both low and high ionization. This component has a low filling factor and is photoionized by X-rays from the shock. The intermediate component, which is dominated by newly processed oxygen, originates in a shell with velocity of 2500-5000 km s-1 and most likely comes from the ejecta. The hydrogen- and helium-dominated gas has a low ionization, a high density, and velocities that extend out to ≳10,000 km s-1. Strong signatures of Lyα-pumped fluorescence lines of Fe II are seen in the near-infrared and ultraviolet. The He/H ratio, ~0.3 by number, and the nitrogen overabundance provide strong evidence for CNO-burning products. The fluxes of the broad hydrogen and helium lines decrease considerably faster than the oxygen lines. The Hα line profile shows strong evolution, with the red wing decreasing faster than the blue. Possible scenarios, involving either a clumpy circumstellar medium or an aspherical distribution of the surrounding gas, are discussed based on the line profiles and physical conditions. Finally, we propose that Type IIn supernovae have their origin in red supergiants in a superwind phase.

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Pinaki Chatterjee, Lars Hernquist, and Abraham Loeb

We develop a simple physical model to describe the dynamics of a massive pointlike object, such as a black hole, near the center of a dense stellar system. It is shown that the total force on this body can be separated into two independent parts, one of which is the slowly varying influence of the aggregate stellar system, and the other being the rapidly fluctuating stochastic force due to discrete encounters with individual stars. For the particular example of a stellar system distributed according to a Plummer model, it is shown that the motion of the black hole is then similar to that of a Brownian particle in a harmonic potential, and we analyze its dynamics using an approach akin to Langevin's solution of the Brownian motion problem. The equations are solved to obtain the average values, time autocorrelation functions, and probability distributions of the black hole's position and velocity. By comparing these results with _N_-body simulations, we demonstrate that this model provides a very good statistical description of the actual black hole dynamics. As an application of our model, we use our results to derive a lower limit on the mass of the black hole Sgr A* in the Galactic center.

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Daniel Proga, Timothy R. Kallman, Janet E. Drew, and Louise E. Hartley

We describe a method of calculating synthetic line profiles using a generalized version of the Sobolev approximation. We apply this method to calculate line profiles predicted by the models of two-dimensional line-driven winds from luminous disks due to Proga, Stone, & Drew. We describe the main properties of the model line profiles and compare them with recent Hubble Space Telescope observations of the cataclysmic variable IX Vel. The model wind consists of a dense, slow outflow that is bounded on the polar side by a high-velocity stream. We find that these two wind components produce distinct spectral features. The fast stream produces profiles that show features consistent with observations. These include the appearance of the classical P Cygni shape for a range of inclinations, the location of the maximum depth of the absorption component at velocities less than the terminal velocity, and the transition from net absorption to net emission with increasing inclination. However, the model profiles have too little absorption or emission equivalent width compared to observed profiles. This quantitative difference between our models and observations is not a surprise because the line-driven wind models predict a mass-loss rate, mostly due to the fast stream, that is lower than the rate required by the observations. We note that the model profiles exhibit a double-humped structure near the line center that is not echoed in observations. We identify this structure with a nonnegligible redshifted absorption that is formed in the slow component of the wind where the rotational velocity dominates over expansion velocity. We conclude that the next generation of disk wind models, developed for application to cataclysmic variables, needs to yield stronger wind driving out to larger disk radii than do the present models.

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Tomaso Belloni, Dimitrios Psaltis, and Michiel van der Klis

We study an empirical model for a unified description of the power spectra of accreting neutron stars and black holes. This description is based on a superposition of multiple Lorentzians and offers the advantage that all quasi-periodic oscillation and noise components are dealt with in the same way, without the need of deciding in advance the nature of each component. This approach also allows us to compare frequencies of features with high and low coherences in a consistent manner and greatly facilitates comparison of power spectra across a wide range of source types and states. We apply the model to six sources: the low-luminosity X-ray bursters 1E 1724-3045, SLX 1735-269, and GS 1826-24; the high-latitude transient XTE J1118+480; the bright system Cir X-1; and the Z source GX 17+2. We find that it provides a good description of the observed spectra without the need for a scale-free (1/f) component. We update previously reported correlations between characteristic frequencies of timing features in the light of this new approach and discuss similarities between different types of systems that may point toward similar underlying physics.

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Krzysztof Belczynski, Vassiliki Kalogera, and Tomasz Bulik

A new generation of ground-based interferometric detectors for gravitational waves is currently under construction or has entered the commissioning phase (Laser Interferometer Gravitational-wave Observatory [LIGO], VIRGO, GEO600, TAMA300). The purpose of these detectors is to observe gravitational waves from astrophysical sources and help improve our understanding of the source origin and physical properties. In this paper we study the most promising candidate sources for these detectors: inspiraling double compact objects. We use population synthesis methods to calculate the properties and coalescence rates of compact object binaries: double neutron stars, black hole-neutron star systems, and double black holes. We also examine the formation channels available to double compact object binaries. We explicitly account for the evolution of low-mass helium stars and investigate the possibility of common-envelope evolution involving helium stars as well as two evolved stars. As a result we identify a significant number of new formation channels for double neutron stars, in particular, leading to populations with very distinct properties. We discuss the theoretical and observational implications of such populations, but we also note the need for hydrodynamical calculations to settle the question of whether such common-envelope evolution is possible. We also present and discuss the physical properties of compact object binaries and identify a number of robust, qualitative features as well as their origin. Using the calculated coalescence rates we compare our results to earlier studies and derive expected detection rates for LIGO. We find that our most optimistic estimate for the first LIGO detectors reach a couple of events per year and our most pessimistic estimate for advanced LIGO detectors exceed ≃10 events per year.

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Dmitri A. Uzdensky

In this paper I analyze the process of formation of thin current structures in the magnetosphere of a conducting accretion disk in response to the field-line twisting brought about by the rotation of the disk relative to the central star. I consider an axisymmetric force-free magnetically linked star-disk configuration and investigate the expansion of the poloidal field lines and partial field-line opening caused by the differential rotation between the star and a nonuniformly rotating disk. I present a simple analytical model that describes the asymptotic behavior of the field in the strong-expansion limit. I demonstrate the existence of a finite (of order 1 rad) critical twist angle, beyond which the poloidal field starts inflating very rapidly. If the relative star-disk twist is enhanced locally, in some finite part of the disk (which may be the case for a Keplerian disk that extends inward significantly closer to the central star than the corotation radius), then, as the twist is increased by a finite amount, the field approaches a partially open configuration, with some field lines going out to infinity. Simultaneous with this partial field opening, a very thin, radially extended current layer forms, thus laying out a way toward reconnection in the disk magnetosphere. Reconnection, in turn, leads to a very interesting scenario for a quasi-periodic behavior of magnetically linked star-disk systems with successive cycles of field inflation, opening, and reconnection.

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R. V. E. Lovelace, H. Li, A. V. Koldoba, G. V. Ustyugova, and M. M. Romanova

We give further consideration to the problem of the evolution of a coronal, force-free magnetic field that threads a differentially rotating, conducting Keplerian disk, extending the recent work of Li and coworkers. This situation is described by the force-free Grad-Shafranov (GS) equation for the flux function Ψ(r, z) that labels the poloidal field lines (in cylindrical coordinates). The GS equation involves a function H(Ψ) describing the distribution of the poloidal current, which is determined by the differential rotation or "twist" of the disk that increases linearly with time. We numerically solve the GS equation in a sequence of volumes of increasing size corresponding to the expansion of the outer perfectly conducting boundaries at (R m , Z m). The outer boundaries model the influence of an external nonmagnetized plasma. The sequence of GS solutions provides a model for the dynamical evolution of the magnetic field in response to (1) the increasing twist of the disk and (2) the pressure of external plasma. We find solutions with magnetically collimated Poynting jets in which there is a continuous outflow of energy, angular momentum, and toroidal magnetic flux from the disk into the external space. This behavior contradicts the commonly accepted "theorem" of solar plasma physics that the motion of the footpoints of a magnetic loop structure leads to a stationary magnetic field configuration with zero power, angular momentum, and flux outflows. In addition, we discuss magnetohydrodynamic simulations that show quasi-stationary collimated Poynting jets similar to our GS solutions. In contrast with the GS solutions, the simulations show a steady uncollimated hydromagnetic (nonforce-free) outflow from the outer part of the disk. The Poynting jets are of interest for the understanding of the jets from active galactic nuclei, microquasars, and possibly gamma-ray burst sources.

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Edward M. Sion and Joel Urban

We present the results of the first multicomponent synthetic spectral analysis of International Ultraviolet Explorer (IUE) archival spectra of the long-period dwarf nova RU Peg during quiescence. The best-fit, high-gravity, solar composition photosphere models yield _T_eff = 50,000-53,000 K with scale factor distances of 250 pc. Optically thick accretion disk models imply accretion rates between 1 × 10-9 and 1 × 10-10_M_☉ yr-1 in order to match the steeply sloping far-UV continuum. However, the best-fit accretion disk models yield distances from 600 to 1300 pc, well beyond the estimated distance range of 130-300 pc. Using rough theoretical flux arguments and the distance estimates, we find better agreement between the observed far-UV luminosity and the predicted far-UV luminosity of a hot, massive, white dwarf model than with an accretion disk model. RU Peg appears to contain the hottest white dwarf yet found in a dwarf nova. We cannot rule out that the far-UV energy distribution is due to a multitemperature white dwarf with cooler, more slowly rotating higher latitudes and a rapidly spinning, hotter equatorial belt. We discuss implications of our analysis for theoretical predictions of the disk instability theory of dwarf nova outbursts. We discuss a comparison between RU Peg's white dwarf and the observed properties of other analyzed white dwarfs in dwarf novae.

461

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Keke Zhang and Gerald Schubert

In the deep interiors of rapidly rotating stars and planets the Coriolis force plays an essential role in determining the depth of penetrative convection from a convectively unstable region into a neighboring stably stratified region. The linear Coriolis force, not the nonlinear inertial force, dominates the dynamics. Accordingly, this paper investigates linear penetrative convection in a rapidly rotating, convectively unstable plane or spherical fluid layer bounded above by a less unstable or a stable corotating plane or spherical fluid layer. For the plane-layer geometry, exact solutions are obtained for two stress-free bounding walls. Asymptotic relations for the onset of penetrative convection in a rapidly rotating plane-layer system are obtained for an asymptotically small Ekman number. It is shown that the interface between the stable and unstable layer forms an effective wall that prevents convective flows from penetrating deeply into the stable fluid layer. This is the ordinary situation of penetrative convection. In spherical-layer geometry, the problem is fundamentally different. Several new and novel forms of convection are discovered. In one phenomenon that we term teleconvection, convection is thermally driven in the deep unstable interior, but the resulting convective motions concentrate primarily in the stable region near the outer spherical surface far away from the location of the thermal forcing. In another case we find a multilayer roll structure in rotating spherical two-layer convection as a result of a stable spherical outer layer. Our findings suggest that observed motions in the atmospheres of planets or stars could be driven by remote energy sources in their deep interiors.

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Stephen L. Skinner, Svetozar A. Zhekov, Manuel Güdel, and Werner Schmutz

We have used the excellent sensitivity of XMM-Newton to obtain the first high-quality X-ray spectrum of a Wolf-Rayet (W-R) star that is not known to be a member of a binary system. Our target, the nitrogen-type star WR 110 (HD 165688), was also observed and detected with the Very Large Array at four different frequencies. The radio flux density increases with frequency according to a power law _S_ν ∝ ν+0.64±0.10, in very good agreement with the behavior expected for free-free wind emission. The radio data give an ionized mass-loss rate = 4.9 × 10-5_M_☉ yr-1 for an assumed spherical constant-velocity wind. The undispersed CCD X-ray spectra reveal strong emission lines from He-like ions of Mg, Si, and S. The emission measure distribution shows a dominant contribution from cool plasma with a characteristic temperature _kT_cool ≈ 0.5 keV (≈6 MK). Little or no excess absorption of this cool component above the value expected from the visual extinction is present. We conclude that the bulk of the cool plasma detected by XMM-Newton lies at hundreds of stellar radii or more if the wind is approximately spherical and homogeneous, but it could lie closer to the star if the wind is clumped. If the cool plasma is due to instability-driven wind shocks, then typical shock velocities are v s ≈ 340-550 km s-1 and the average filling factor of X-ray-emitting gas in the wind is no larger than f ~ 10-6.

A surprising result is the unambiguous detection of a hard X-ray component that is clearly seen in the hard-band images and the spectra. This hard component accounts for about half of the observed flux and can be acceptably fitted by a hot, optically thin thermal plasma or a power-law model. If the emission is thermal, then a temperature _kT_hot ≥ 3 keV is derived. Such high temperatures are not predicted by current instability-driven wind shock models, and a different mechanism is thus required to explain the hard X-rays. We examine several possible mechanisms and show that the hard emission could be accounted for by the W-R wind shocking onto a close stellar companion that has so far escaped detection. However, until persuasive evidence for binarity is found, we are left with the intriguing possibility that the hard X-ray emission is produced entirely by the Wolf-Rayet star.

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R. R. Ransom, N. Bartel, M. F. Bietenholz, D. E. Lebach, M. I. Ratner, I. I. Shapiro, and J.-F. Lestrade

We present the first VLBI images of the RS CVn binary star HR 1099 (=V711 Tauri, HD 22468) obtained from observations at 8.4 GHz in 1996 May and September made in support of the NASA/Stanford Gravity Probe B project. The first set of observations was made during a decay stage of a flare event. The second set of observations was made during a quiescent period. The detected emission region for the active epoch had an estimated major-axis length (FWHM) of 2.7 ± 0.2 mas. This region consisted of a halo and two superimposed compact condensations that were oriented approximately east-west. The centers of the compact condensations were separated by 1.7 ± 0.1 mas. Compared to the ~1.3 mas separation on the sky at this epoch of the centers of the stellar components of the binary, the observed separation of the condensations differs by only 0.4 mas, which is less than the ~0.6 mas angular radius of the larger component. During the observations, the compact western condensation rotated north-northwestward by 24° ± 4°, or by 0.7 ± 0.1 mas, relative to the eastern condensation. We speculate that (1) either both condensations originate from the corona of the larger stellar component or one condensation is close to the surface of each of the two stellar components of the binary and (2) the relative rotation of the two condensations is a consequence of the rotation of the binary system. We speculate further that the halo is a consequence of flare-energized electrons confined by the magnetosphere of the larger stellar component or by the combined magnetospheres of the two stellar components. Our quiescent epoch, in contrast, was characterized by a single emission region with a major axis estimated to be 1.7 ± 0.1 mas (FWHM).

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E. Berger

We present Very Large Array observations of 12 late M and L dwarfs in the solar neighborhood. The observed sources were chosen to cover a wide range of physical characteristics—spectral type, rotation, age, binarity, and X-ray and Hα activity—to determine the role of these properties in the production of radio emission and hence magnetic fields. Three of the 12 sources, TVLM 513-46546, 2MASS J0036159+182110, and BRI 0021-0214, were observed to flare and also exhibit persistent emission, indicating that magnetic activity is not quenched at the bottom of the main sequence. The radio emission extends to spectral type L3.5, and there is no apparent decrease in the ratio of flaring luminosities to bolometric luminosities between M8 and L3.5. Moreover, contrary to the significant drop in persistent Hα activity beyond spectral type M7, the persistent radio activity appears to steadily increase between M3 and L3.5. Similarly, the radio emission from BRI 0021-0214 violates the phenomenological relations between the radio and X-ray luminosities of coronally active stars, hinting that radio and X-ray activity are also uncorrelated at the bottom of the main sequence; an even stronger violation was found for the brown dwarf LP 944-20. The radio-active sources that have measured rotational velocities are rapid rotators, v sin i > 30 km s-1, while the upper limits on radio activity in slowly rotating late M dwarfs (v sin i < 10 km s-1) from this survey and from the literature are lower than these detections. These observations provide tantalizing evidence that rapidly rotating late M and L dwarfs are more likely to be radio active. This possible correlation is puzzling given that the observed radio emission requires sustained magnetic fields of ~10-103 G and densities of ~1012 cm-3, indicating that the active sources should have slowed down considerably as a result of magnetic braking.

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Piotr Popowski and Charles Alcock

We entertain the idea that robust theoretical expectations can become a tool in removing hidden observational or data-reduction biases. We illustrate this approach for a specific problem associated with gravitational microlensing. Using the fact that a group is more than just a collection of individuals, we derive formulae for correcting the distribution of the dimensionless impact parameters of events, _u_min. We refer to the case when undetected biases in the _u_min distribution can be alleviated by multiplication of impact parameters of all events by a common constant factor. We show that in this case the general maximum likelihood problem of solving an infinite number of equations reduces to two constraints, and we find an analytic solution. Under the above assumptions, this solution represents a state in which the "entropy" of a microlensing ensemble is at its maximum; that is, the distribution of _u_min resembles a specific, theoretically expected, boxlike distribution to the highest possible extent. We also show that this technique does not allow one to correct the parameters of individual events on an event-by-event basis, independently from each other.

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Jin H. An, M. D. Albrow, J.-P. Beaulieu, J. A. R. Caldwell, D. L. DePoy, M. Dominik, B. S. Gaudi, A. Gould, J. Greenhill, K. Hill et al

We analyze PLANET photometric observations of the caustic-crossing binary lens microlensing event, EROS BLG-2000-5, and find that modeling the observed light curve requires incorporation of the microlens parallax and the binary orbital motion. The projected Einstein radius (E = 3.61 ± 0.11 AU) is derived from the measurement of the microlens parallax, and we are also able to infer the angular Einstein radius (θE = 1.38 ± 0.12 mas) from the finite source effect on the light curve, combined with an estimate of the angular size of the source given by the source position in a color-magnitude diagram. The lens mass, M = 0.612 ± 0.057 _M_☉, is found by combining these two quantities. This is the first time that parallax effects are detected for a caustic-crossing event and also the first time that the lens mass degeneracy has been completely broken through photometric monitoring alone. The combination ofE and θE also allows us to conclude that the lens lies in the near side of the disk, within 2.6 kpc of the Sun, while the radial velocity measurement indicates that the source is a Galactic bulge giant.

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Lam Hui and Sara Seager

Future high-precision photometric measurements of transiting extrasolar planets promise to tell us much about the characteristics of these systems. We examine how atmospheric lensing and (projected) planet oblateness/ellipticity modify transit light curves. The large density gradients expected in planet atmospheres can offset the unfavorably large observer lens-to-source lens distance ratio and allow the existence of caustics. Under such conditions of strong lensing, which we quantify with an analytic expression, starlight from all points in the planet's shadow is refracted into view, producing a characteristic slowing down of the dimming at ingress (vice versa for egress). A search over several parameters, such as the limb-darkening profile, the planet radius, the transit speed, and the transit geometry, cannot produce a nonlensed transit light curve that can mimic a lensed light curve. The fractional change in the diminution of starlight is approximately the ratio of atmospheric scale height to planet radius, expected to be 1% or less. The lensing signal varies strongly with wavelength—caustics are hidden at wave bands where absorption and scattering are strong. Planet oblateness induces an asymmetry to the transit light curve about the point of minimum flux, which varies with the planet orientation with respect to the direction of motion. The fractional asymmetry is at the level of 0.5% for a projected oblateness of 10%, independent of whether or not lensing is important. For favorable ratios of planet radius to stellar radius (i.e., gas giant planets), the above effects are potentially observable with future space-based missions. Such measurements could constrain the planet shape and its atmospheric scale height, density, and refractive coefficient, providing information on its rotation, temperature, and composition. We have examined a large range of planetary system parameter space including the planetary scale height and orbital distance. For HD 209458b, the only currently known transiting extrasolar planet, caustics are absent because of the very small lens-source separation (and a large scale height caused by a high temperature from the small separation). Its oblateness is also expected to be small because of the tidal locking of its rotation to orbital motion. Finally, we provide estimates of other variations to transit light curves that could be of comparable importance—including rings, satellites, stellar oscillations, star spots, and weather.

556

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John H. Debes and Steinn Sigurdsson

The presence of planets around solar-type stars suggests that many white dwarfs should have relic planetary systems. While planets closer than ~5 AU will most likely not survive the post-main-sequence lifetime of their parent star, any planet with semimajor axis greater than 5 AU will survive, and its semimajor axis will increase as the central star loses mass. Since the stability of adjacent orbits to mutual planet-planet perturbations depends on the ratio of the planet mass to the central star's mass, some planets in previously stable orbits around a star undergoing mass loss will become unstable. We show that when mass loss is slow, systems of two planets that are marginally stable can become unstable to close encounters, while for three planets the timescale for close approaches decreases significantly with increasing mass ratio. These processes could explain the presence of anomalous IR excesses around white dwarfs that cannot be explained by close companions, such as G29-38, and may also be an important factor in explaining the existence of DAZ white dwarfs. The onset of instability through changing mass ratios will also be a significant effect for planetary embryos gaining mass in protoplanetary disks.

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R. R. Rafikov

Gap formation in a gas disk triggered by disk-planet tidal interaction is considered. Density waves launched by the planet are assumed to be damped as a result of their nonlinear evolution leading to shock formation and its subsequent dissipation. As a consequence, wave angular momentum is transferred to the disk, leading to evolution of its surface density. Planetary migration is an important ingredient of the theory; effects of the planet-induced surface density perturbations on the migration speed are considered. A gap is assumed to form when a stationary solution for the surface density profile is no longer possible in the frame of reference migrating with the planet. An analytical limit on the planetary mass necessary to open a gap in an inviscid disk is derived. The critical mass turns out to be smaller than the mass _M_1 for which the planetary Hill radius equals the disk scale height by a factor of at least _Q_5/7 (Q is the Toomre stability parameter), depending on the strength of the migration feedback. In viscous disks the critical planetary mass could vary from ~0.2_M_1 to _M_1, depending on the disk viscosity. This implies that a gap could be formed by a planet with mass of 2-15 _M_⊕, depending on the disk aspect ratio, viscosity, and the planet's location in the nebula.

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Tongjiang Wang, Yihua Yan, Jialong Wang, H. Kurokawa, and K. Shibata

On 1998 May 2 a class X1/3B flare occurred at 13:42 UT in NOAA Active Region 8210 near disk center, which was followed by a halo coronal mass ejection (CME) at 15:03 UT observed by SOHO/LASCO. Using the boundary element method (BEM) on a global potential model, we reconstruct the large-scale coronal field structure from a composite boundary by SOHO/MDI and Kitt Peak magnetograms. The extrapolated large field lines well model a transequatorial interconnecting loop (TIL) seen in the soft X-ray (SXR) between AR 8210 and AR 8214, which disappeared after the CME. The EUV Imaging Telescope (EIT) observed the widely extending dimmings, which noticeably deviate from the SXR TIL in position. We find that the major dimmings are magnetically linked to the flaring active region but some dimmings are not. The spatial relationships of these features suggest that the CME may be led by a global restructuring of multipolar magnetic systems due to flare disturbances. Mass, magnetic energy, and flux of the ejected material estimated from the dimming regions are comparable to the output of large CMEs, derived from the limb events. At the CME source region, Huairou vector magnetograms show that a strong shear was rapidly developed in a newly emerging flux region (EFR) near the main spot before the flare. Magnetic field extrapolations reveal the presence of a "bald patch" (defined as the locations where the magnetic field is tangent to the photosphere) at the edge of the EFR. The preflare features such as EUV loop brightenings and SXR jets appearing at the bald patch suggest a slow reconnection between the TIL field system and a preexisting overlying field above the sheared EFR flux system. High-cadence Yohkoh/SXT images reveal a fast expanding motion of loops above a bright core just several minutes before the hard X-ray onset. This may be a precursor for the eruption of the sheared EFR flux to produce the flare. We propose a scenario, similar to the "breakout" model in principle, that can interpret many observed features.

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Hiroki Kurokawa, Tongjiang Wang, and Takako T. Ishii

An emerging twisted flux rope model to explain the drastic evolution of a flare-productive NOAA Active Region 9026 is presented. The drastic changes in the δ-type sunspot configuration were found to start shortly before the big flares of 2000 June 6: (1) rapid proper motions of sunspots started at the both sides of the central δ sunspot about 7 hr prior to the strong flare activity, (2) the collapse of the central δ sunspot with its disintegration and partial disappearance started about 3 hr before the strong flare activity, (3) a _switchback_-shaped and strongly sheared magnetic neutral line was formed with intruding motions of sunspots into the opposite magnetic polarities, and (4) the direction of the neutral line rapidly rotated clockwise at the same time when the switchback neutral line was formed. To explain these outstanding features of the sunspot evolution, we constructed a schematic model of an emerging twisted flux rope in which the central writhe helicity of the flux rope could be formed by continuous transformation of the twist helicity by means of the kink instability in the course of its emergence through the convection zone.

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Jeongwoo Lee, Dale E. Gary, Jiong Qiu, and Peter T. Gallagher

We discuss injection and transport of high-energy electrons during a GOES X-ray class M9.8 flare observed in microwaves with the Owens Valley Solar Array (OVSA) and in hard X-rays (HXRs) with the hard X-ray telescope (HXT) on board Yohkoh. Observed at 1 s timescales or better in both wavelength regimes, the event shows (1) a large difference in scale between the microwave source and the HXR source; (2) an unusually hard HXR spectrum (maximum spectral index ~-1.6), followed by rapid spectral softening; and (3) a microwave light curve containing both impulsive peaks (3 s rise time) simultaneous with those of the HXRs and a long, extended tail with a uniform decay rate (2.3 minutes). We analyze the observations within the framework of the electron trap-and-precipitation model, allowing a time-dependent injection energy spectrum. Assuming thick-target bremsstrahlung for the HXRs, we infer the electron injection function in the form Q(E, t) ~ (E/_E_0)-δ(t), where the timescale for δ(t) to change by unity is ~7 s. This injection function can account for the characteristics of the impulsive part of the microwave burst by considering the bulk of the electrons to be directly precipitating without trapping. The same injection function also accounts for the gradual part of the microwave emission by convolving the injection function with a kernel representing the trapping process, which at late times gives N(E, t) ~ e_-ν_t(E/_E_0)-b. We require b ~ 1.4 and ν ~ 6 × 10-3β s-1, where β is the electron speed divided by the speed of light. Therefore, the derived form of the precipitation rate ν itself indicates strong pitch-angle diffusion, but the slow decay of the microwave radiation requires a small loss cone (~4°) and a low ambient density in the coronal trap. Also, the numbers of electrons needed to account for the two components of the microwave emission differ by an order of magnitude. We estimate that the ≥100 keV number of the directly precipitating electrons is ~1033, while the trapped population requires ~1032 electrons. This leads us to a model of two interacting loops, the larger of which serves as an efficient trap while the smaller provides the impulsive source. These characteristics are consistent with the spatially resolved observations.

626

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Mats Carlsson and R. F. Stein

We investigate the ionization of hydrogen in a dynamic solar atmosphere. The simulations include a detailed non-LTE treatment of hydrogen, calcium, and helium but lack other important elements. Furthermore, the omission of magnetic fields and the one-dimensional approach make the modeling unrealistic in the upper chromosphere and higher. We discuss these limitations and show that the main results remain valid for any reasonable chromospheric conditions. As in the static case, we find that the ionization of hydrogen in the chromosphere is dominated by collisional excitation in the Lyα transition followed by photoionization by Balmer continuum photons—the Lyman continuum does not play any significant role. In the transition region, collisional ionization from the ground state becomes the primary process. We show that the timescale for ionization/recombination can be estimated from the eigenvalues of a modified rate matrix where the optically thick Lyman transitions that are in detailed balance have been excluded. We find that the timescale for ionization/recombination is dominated by the slow collisional leakage from the ground state to the first excited state. Throughout the chromosphere the timescale is long (103-105 s), except in shocks where the increased temperature and density shorten the timescale for ionization/recombination, especially in the upper chromosphere. Because the relaxation timescale is much longer than dynamic timescales, hydrogen ionization does not have time to reach its equilibrium value and its fluctuations are much smaller than the variation of its statistical equilibrium value appropriate for the instantaneous conditions. Because the ionization and recombination rates increase with increasing temperature and density, ionization in shocks is more rapid than recombination behind them. Therefore, the ionization state tends to represent the higher temperature of the shocks, and the mean electron density is up to a factor of 6 higher than the electron density calculated in statistical equilibrium from the mean atmosphere. The simulations show that a static picture and a dynamic picture of the chromosphere are fundamentally different and that time variations are crucial for our understanding of the chromosphere itself and the spectral features formed there.

636

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J. M. Fontenla, E. H. Avrett, and R. Loeser

In this paper we extend our previous modeling of energy balance in the chromosphere-corona transition region to cases with particle and mass flows. The cases considered here are quasi-steady and satisfy the momentum and energy balance equations in the transition region. We assume one-dimensional geometry and include the flow velocity terms in all equations, but we neglect the partial derivatives with respect to time. We present a complete and physically consistent formulation and method for solving the non-LTE and energy balance equations in these situations, including both particle diffusion and flows of H and He. Our calculations include partial frequency redistribution in the Lyα and Lyβ lines. Our results show quantitatively how mass flows affect the ionization and radiative losses of H and He, thereby affecting the structure and extent of the transition region. Furthermore, our computations show that the H and He line profiles are greatly affected by flows. We find that line shifts are much less important than the changes in line intensity and central reversal as a result of the influence of flows on the excitation and ionization. In this paper we use fixed conditions at the base of the transition region and in the underlying chromosphere. Our intent is to show the physical effects of flows on the transition region, not to match any particular observations. However, our computed Lyα profiles can account for the range of observed high spectral and spatial resolution from the quiet Sun. We suggest that dedicated modeling of specific sequences of observations based on physically consistent methods like those presented here will substantially improve our understanding of the energy balance in the chromosphere and corona.

663

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R. Komm, R. Howe, and F. Hill

We present the first attempt at localizing in latitude the temporal variation of mode energy, energy supply rate, and lifetime of global acoustic modes. We use Global Oscillation Network Group (GONG) and Michelson Doppler Imager data analyzed with the GONG peak-fitting algorithm to measure mode width and amplitude of individual (l, n, m) modes. While measured amplitude and width values are inherently noisier than frequency measurements, it is possible to use the (m/l) dependence of these mode parameters to extract their variation in latitude. With the currently analyzed data sets, we construct maps in time and latitude of acoustic mode energy, lifetime (inverse of mode width), and energy supply rate covering the rising phase of the current solar cycle from the previous minimum to the current maximum. We find that the energy and width of global modes vary in latitude as well as in time and that the variation is clearly related to the distribution of magnetic flux. After removing the average quantity, the residual mode width shows a linear correlation with magnetic activity with a correlation coefficient of 0.88, while the corresponding residual mode energy is anticorrelated with magnetic activity with a correlation coefficient of -0.90. These mode parameters derived from global _p_-modes respond to the local distribution of surface magnetic activity. The energy supply rate shows no correlation with the latitudinal distribution of magnetic activity within the limits of the current measurements. We estimate the variation of global mode energy in response to an individual magnetic feature, such as a plage, and find that the global mode energy and the mode lifetime are reduced by about 40% by an active region compared to the quiet Sun.

674

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Robert Rubinstein and Ye Zhou

This paper considers the question of whether Eulerian or Lagrangian time correlations should be used to evaluate sound radiation by turbulent convection. Whereas previous analyses have either explicitly or implicitly used Lagrangian correlations, this paper argues in favor of Eulerian correlations. The physics of turbulent time correlations is briefly reviewed, and the implications of using Eulerian instead of Lagrangian correlations in sound-radiation calculations are discussed. The assumption made about time correlations alters the frequency distribution of the radiated sound; assuming Eulerian time correlations results in a more shallow acoustic power spectrum and consequently greater relative weighting of higher frequencies. The consequences of assuming Bolgiano scaling of buoyancy-generated turbulence are also discussed.

679

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Zdenek Sekanina, Emmanuel Jehin, Hermann Boehnhardt, Xavier Bonfils, Oliver Schuetz, and Daniel Thomas

Analysis of the visual light curve and fragmentation sequence of comet C/2001 A2 (LINEAR) shows a strong temporal correlation between the onset of outbursts and separation of companion nuclei. This scenario conforms to Sekanina's conceptual model for the release of sizable fragments of an inert dust mantle from the nucleus surface: an outburst is triggered as some of the mass rapidly disintegrates into fine dust.

685

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George B. Field and Eric G. Blackman

We present a two-scale approximation for the dynamics of a nonlinear α2 dynamo. Solutions of the resulting nonlinear equations agree with the numerical simulations of Brandenburg and show that α is quenched by the buildup of magnetic helicity at the forcing scale 1/_k_2 as the α effect transfers it from the large scale 1/k_1(> 1/k_2). For times t > (k_1/k_2)Re_M,2 in eddy turnover units (where Re_M,2 is the magnetic Reynolds number of the forcing scale), α is limited resistively in the form predicted for the steady state case. However, for t ≪ Re_M,2, α takes on its kinematic value independent of Re_M,2, allowing the production of large-scale magnetic energy equal to _k_1/_k_2 times equipartition. Thus, the dynamic theory of α predicts substantial "fast" growth of a large-scale field despite being "slow" at large times.

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J. P. Halpern, M. Eracleous, R. Mukherjee, and E. V. Gotthelf

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M. J. Reid and J. E. G. Peek

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S. C. Chapman, A. Shapley, C. Steidel, and R. Windhorst

We present new Hubble Space Telescope, high-resolution optical imaging of the submillimeter luminous Lyman break galaxy (LBG), Westphal-MMD 11, an interacting starburst at z = 2.979. The new imaging data, in conjunction with reanalysis of Keck optical and near-IR spectra, demonstrate MMD 11 to be an interacting system of at least three components: a luminous blue source, a fainter blue source, and an extremely red object (ERO) with R_-K ≳ 6. The separations between components are ~8 kpc (Λ = 0.7, Ω_M = 0.3, h = 0.65), similar to some of the local ultraluminous infrared galaxies (ULIGs). The lack of obvious active galactic nucleus in MMD 11, along with the fragmented, early-stage merger morphology, suggest a young forming environment. While we cannot unambiguously identify the location of the far-IR emission within the system, analogy to similar ULIGs suggests the ERO as the likely far-IR source. The greater than 1012_L_☉ bolometric luminosity of MMD 11 can be predicted reasonably from its rest-frame UV properties once all components are taken into account; however, this is not typically the case for local galaxies of similar luminosities. While LBGs as red in _g_-R and _R_-K as MMD 11 are rare, they can only be found over the restricted 2.7 < z < 3.0 range. Therefore, a substantial number of MMD 11-like galaxies (≃0.62 arcmin-2) may exist when integrated over the likely redshift range of Submillimeter Common-User Bolometric Array (SCUBA) sources (z = 1-5), suggesting that SCUBA sources should not necessarily be seen as completely orthogonal to optically selected galaxies.

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David A. Turnshek and Sandhya M. Rao

Hubble Space Telescope (HST) UV spectroscopy of 12 candidate low-redshift damped Lyα (DLA) systems in 11 quasi-stellar objects (z = 0.103 in Q0054+144, z = 0.969 and z = 0.987 in Q0302-223, z = 0.478 in Q0454-220, z = 1.476 in Q1047+550, z = 1.070 in Q1206+459, z = 1.228 in Q1247+267, z = 0.399 in Q1318+290B, z = 0.519 in Q1329+412, z = 0.276 in Q1451-375, z = 0.204 in Q2112+059, z = 0.263 in Q2251+113) are presented; the observations demonstrate that they are not DLAs with N ≥ 2 × 1020 atoms cm-2. In all cases except two, the systems either do not exist or are well below the DLA threshold column density; the exceptions are a z = 0.474 system in Q0454-220 that has N = 3 × 1019 atoms cm-2 and a z = 1.223 system in Q1247+267 that has N = 8 × 1019 atoms cm-2. Thus, observations of these objects in the Chandra, Gemini, and HST archives are not suitable for doing follow-up work on low-redshift DLAs. Furthermore, these results indicate that the low-redshift DLA statistics derived from International Ultraviolet Explorer spectra and presented by Lanzetta, Wolfe, & Turnshek and Wolfe et al. in 1995 are invalid.

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Jens Hjorth, Ingunn Burud, Andreas O. Jaunsen, Paul L. Schechter, Jean-Paul Kneib, Michael I. Andersen, Heidi Korhonen, Jacob W. Clasen, A. Amanda Kaas, Roy Østensen et al

We present optical light curves of the gravitationally lensed components A (≡A1+A2+A3) and B of the quadruple quasar RX J0911.4+0551 (z = 2.80). The observations were primarily obtained at the Nordic Optical Telescope between 1997 March and 2001 April and consist of 74 _I_-band data points for each component. The data allow the measurement of a time delay of 146 ± 8 days (2 σ) between A and B, with B as the leading component. This value is significantly shorter than that predicted from simple models and indicates a very large external shear. Mass models including the main lens galaxy and the surrounding massive cluster of galaxies at z = 0.77, responsible for the external shear, yield _H_0 = 71 ± 4 (random, 2 σ) ± 8 (systematic) km s-1 Mpc-1. The systematic model uncertainty is governed by the surface-mass density (convergence) at the location of the multiple images.

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Yun Wang, Daniel E. Holz, and Dipak Munshi

We present an approximate form for the weak-lensing magnification distribution of standard candles, valid for all cosmological models, with arbitrary matter distributions, over all redshifts. Our results are based on a universal probability distribution function (UPDF), P(η), for the reduced convergence, η. For a given cosmological model, the magnification probability distribution, P(μ), at redshift z is related to the UPDF by P(μ) = P(η)/ , where η = 1 + (μ - 1)/(2|κmin|), and κmin (the minimum convergence) can be directly computed from the cosmological parameters (Ω_m_ and ΩΛ). We show that the UPDF can be well approximated by a three-parameter stretched Gaussian distribution, where the values of the three parameters depend only on ξη, the variance of η. In short, all possible weak-lensing probability distributions can be well approximated by a one-parameter family. We establish this family, normalizing to the numerical ray-shooting results for a Λ cold dark matter (CDM) model by Wambsganss et al. (1997). Each alternative cosmological model is then described by a single function ξη(z). We find that this method gives P(μ) in excellent agreement with numerical ray-tracing and three-dimensional shear matrix calculations, and we provide numerical fits for three representative models (SCDM, ΛCDM, and OCDM). Our results provide an easy, accurate, and efficient method to calculate the weak-lensing magnification distribution of standard candles and should be useful in the analysis of future high-redshift supernova data.

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Xiang-Ping Wu and Yan-Jie Xue

It is widely believed that the global baryon content and the mass-to-light ratio of groups and clusters of galaxies are fair representatives of the matter mix of the universe and therefore can be used to reliably determine the cosmic mass density parameter Ω_M_. However, this fundamental assumption is challenged by growing evidence from optical and X-ray observations that the average gas mass fraction and mass-to-light ratio increase mildly with scale from poor groups to rich clusters. Although a number of time-consuming hydrodynamical simulations combined with semianalytic approaches have been carried out that permit a sophisticated treatment of some complicated processes in the formation and evolution of cosmic structures, the essential physics behind the phenomenon still remains a subject of intense debate. In this Letter, using a simple analytic model, we show that radiative cooling of the hot intragroup/intracluster gas may allow one to reproduce the observed scale dependence of the global stellar and gas mass fractions and mass-to-light ratio of groups and clusters, provided that about half of the cooled gas is converted into stars. Together with the recent success in the recovery of the entropy excess and the steepening of the X-ray luminosity-temperature relations detected in groups and clusters, radiative cooling provides a simple, unified scheme for the evolution of hot gas and the formation of stars in the largest virialized systems of the universe.

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Rachel S. Somerville

Cold dark matter theory predicts that the Local Group should contain many more dwarf-sized objects than the observed number of dwarf galaxies—the so-called substructure problem. We investigate whether the suppression of star formation in these small objects due to the presence of a photoionizing background can resolve the problem. We make use of results from recent hydrodynamic simulations to build a recipe for the suppression of gas infall into semianalytic galaxy formation models and use these to predict the luminosity function of dwarf galaxies in the Local Group. In the models without photoionization "squelching," we predict a large excess of faint dwarf galaxies compared with the observed number in the Local Group—thus, the usual recipe for supernova feedback used in semianalytic models does not solve the substructure problem on its own. When we include photoionization squelching, we find good agreement with the observations. We have neglected tidal destruction, which probably further reduces the number of dwarf galaxies. We conclude that photoionizing squelching easily solves the substructure problem. In fact, it is likely that once this effect is taken into account, models with reduced small-scale power (e.g., warm dark matter) would underproduce dwarf galaxies.

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G. Polenta, P. A. R. Ade, J. J. Bock, J. R. Bond, J. Borrill, A. Boscaleri, C. R. Contaldi, B. P. Crill, P. de Bernardis, G. De Gasperis et al

We search the BOOMERANG (Balloon Observations Of Millimetric Extragalactic Radiation ANd Geophysics) maps of the anisotropy of the cosmic microwave background (CMB) for deviations from Gaussianity. In this Letter, we focus on analysis techniques in pixel space and compute skewness, kurtosis, and Minkowski functionals for the BOOMERANG maps and for Gaussian simulations of the CMB sky. We do not find any significant deviation from Gaussianity in the high galactic latitude section of the 150 GHz map. We do find deviations from Gaussianity at lower latitudes and at 410 GHz, and we ascribe them to Galactic dust contamination. Using non-Gaussian simulations of instrumental systematic effects, of foregrounds, and of sample non-Gaussian cosmological models, we set upper limits to the non-Gaussian component of the temperature field in the BOOMERANG maps. For fluctuations distributed as a 1 degree of freedom χ2 mixed to the main Gaussian component, our upper limits are in the few percentile range.

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Roberto Soria and Albert K. H. Kong

The face-on spiral galaxy M74 (NGC 628) was observed by XMM-Newton on 2002 February 2. In total, 21 sources are found in the inner 5' from the nucleus (after rejection of a few sources associated to foreground stars). Hardness ratios suggest that about half of them belong to the galaxy. The higher luminosity end of the luminosity function is fitted by a power law of slope -0.8. This can be interpreted as evidence of ongoing star formation, in analogy with the distributions found in disks of other late-type galaxies. A comparison with previous Chandra observations reveals a new ultraluminous X-ray transient (_L_X ≈ 1.5 × 1039 ergs s-1 in the 0.3-8 keV band) about 4' north of the nucleus. We find another bright transient source (_L_X ≈ 5 × 1038 ergs s-1) about 5' northwest of the nucleus. The UV and X-ray counterparts of SN 2002ap are also found in this XMM-Newton observation; the hardness ratio of the X-ray counterpart suggests that the emission comes from the shocked circumstellar matter.

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Masaru Shibata and Stuart L. Shapiro

We follow the collapse in axisymmetry of a uniformly rotating, supermassive star (SMS) to a supermassive black hole in full general relativity. The initial SMS of arbitrary mass M is marginally unstable to radial collapse and rotates at the mass-shedding limit. The collapse proceeds homologously early on and results in the appearance of an apparent horizon at the center. Although our integration terminates before final equilibrium is achieved, we determine that the final black hole will contain about 90% of the total mass of the system and will have a spin parameter J/_M_2 ~ 0.75. The remaining gas forms a rotating disk about the nascent hole.

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J. S. Bloom, S. R. Kulkarni, P. A. Price, D. Reichart, T. J. Galama, B. P. Schmidt, D. A. Frail, E. Berger, P. J. McCarthy, R. A. Chevalier et al

Using observations from an extensive monitoring campaign with the Hubble Space Telescope, we present the detection of an intermediate-time flux excess that is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red "bump," which exhibits a spectral rollover at ~7200 Å, is well described by a redshifted Type Ic supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova SN 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts.

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P. A. Price, E. Berger, D. E. Reichart, S. R. Kulkarni, S. A. Yost, R. Subrahmanyan, R. M. Wark, M. H. Wieringa, D. A. Frail, J. Bailey et al

Of the cosmological gamma-ray bursts, GRB 011121 has the lowest redshift, z = 0.36. More importantly, the multicolor excess in the afterglow detected in the Hubble Space Telescope (HST) light curves is compelling observational evidence of an underlying supernova. Here we present near-infrared and radio observations of the afterglow, and from our comprehensive afterglow modeling, we find evidence favoring a wind-fed circumburst medium. Lacking X-ray data, we are unable to conclusively measure the mass-loss rate, , but obtain an estimate, ~ 2 × 10-7/v _w_3_M_☉ yr-1, where v _w_3 is the speed of the wind from the progenitor in units of 103 km s-1. This is similar to that inferred for the progenitor of the Type Ibc supernova SN 1998bw that has been associated with the peculiar burst GRB 980425. Our data, taken in conjunction with the HST results of Bloom et al., provide a consistent picture: the long-duration GRB 011121 had a massive star progenitor that exploded as a supernova at about the same time as the gamma-ray burst event. Finally, we note that the gamma-ray profile of GRB 011121 is similar to that of GRB 980425.

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Davide Lazzati, Enrico Ramirez-Ruiz, and Martin J. Rees

We compute the luminosity of Kα emission lines produced by astrophysically abundant elements in the soft X-ray spectra of the early afterglow of gamma-ray bursts. We find that the detection of these lines can be a diagnostic for the geometrical setup of the reprocessing material. In particular, we can distinguish between a "geometry-dominated" model, in which the line emission is coming from an extended region and its duration arises from light-travel time effects, and an "engine-dominated" model, where the line-emitting gas is in a smaller region, irradiated for a longer period. These lines therefore offer clues to the dynamics and timescale of the explosion leading to a gamma-ray burst.

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P. A. Mazzali, J. Deng, K. Maeda, K. Nomoto, H. Umeda, K. Hatano, K. Iwamoto, Y. Yoshii, Y. Kobayashi, T. Minezaki et al

Photometric and spectroscopic data of the energetic Type Ic supernova (SN) 2002ap are presented, and the properties of the SN are investigated through models of its spectral evolution and its light curve. The SN is spectroscopically similar to the "hypernova" SN 1997ef. However, its kinetic energy [~(4-10) × 1051 ergs] and the mass ejected (2.5-5 _M_☉) are smaller, resulting in a faster evolving light curve. The SN synthesized ~0.07 _M_☉ of 56Ni, and its peak luminosity was similar to that of normal SNe. Brightness alone should not be used to define a hypernova, whose defining character, namely very broad spectral features, is the result of high kinetic energy. The likely main-sequence mass of the progenitor star was 20-25 _M_☉, which is also lower than that of both hypernovae SN 1997ef and SN 1998bw. SN 2002ap appears to lie at the low-energy and low-mass end of the hypernova sequence as it is known so far. Observations of the nebular spectrum, which is expected to dominate by the summer of 2002, are necessary to confirm these values.

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M. Plionis

Evidence is presented for the recent evolution of the relaxation processes in clusters of galaxies, using large optical and X-ray cluster samples. The criteria of the cluster relaxation used are the cluster ellipticity, the intracluster medium (ICM) temperature, and X-ray cluster luminosity. We find evidence of varying strength and significance of all three indicators evolving with redshift for z ≲ 0.15. This result supports the view that clusters have mostly stopped undergoing mergers and accreting matter, as expected in a low-Ω_m_ universe, and are now in the process of gravitational relaxation, which reduces their flattening, their ICM temperature (shock heated during the merging phase), and their X-ray luminosity. These results support similar recent claims of Melott, Chambers, & Miller.

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Alejandra Recio-Blanco, Giampaolo Piotto, Antonio Aparicio, and Alvio Renzini

We present high-resolution Very Large Telescope/Ultraviolet Visual Echelle Spectrograph observations of 56 stars in the extended horizontal-branch (EHB) of the Galactic globular clusters NGC 1904, NGC 2808, NGC 6093, and NGC 7078. Our data reveal for the first time the presence in NGC 1904 of a sizable population of fast (v sin i ≥ 20 km s-1) horizontal-branch (HB) rotators, confined to the cool end of the EHB, similar to that found in M13. We also confirm the fast rotators already observed in NGC 7078. The cooler stars (_T_eff < 11,500 K) in these three clusters show a range of rotation rates, with a group of stars rotating at ~15 km s-1 or less and a fast rotating group at ~30 km s-1. Apparently, the fast rotators are relatively more abundant in NGC 1904 and M13 than in NGC 7078. No fast rotators have been identified in NGC 2808 and NGC 6093. All the stars hotter than _T_eff ~ 11,500 K have projected rotational velocities of v sin i < 12 km s-1, but less than 20% have v sin i < 2 km s-1. The connection between photometric gaps in the HB and the change in the projected rotational velocities is not confirmed by the new data. However, our data are consistent with a relation between this discontinuity and the HB jump. We discuss a number of possibilities for the origin of the stellar rotation distribution along the HB. We conclude that none of them can yet provide a satisfactory explanation of the observations.

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Mary Barsony, Thomas P. Greene, and Geoffrey A. Blake

We present high-resolution λ = 2.7 mm imaging of the close triple pre-main-sequence system WL 20. Compact dust emission with integrated flux density of 12.9 ± 1.3 mJy is associated with two components of the triple system, WL 20W and WL 20S. No emission above a 3 σ level of 3.9 mJy is detected toward the third component, WL 20E, which lies 317 (400 AU) due east in projection from its neighbors. A possibly warped structure of ~0.1 _M_☉ and ≤32 extent encompasses WL 20W and WL 20S, which have a projected separation of 225 (~280 AU) along a north-south axis. This structure is most likely a tidally disrupted disk surrounding WL 20S. New near-infrared spectra of the individual components show a remarkable similarity between the two T Tauri stars of the system: WL 20E has a K7 spectral type (_T_eff = 4040 K) with r K = 0.2, and WL 20W has an M0 spectral type (_T_eff = 3800 K) with r K = 0.2. The spectrum of WL 20S is consistent with that of a source intrinsically similar to WL 20W, with r K < 0.9, but seen through an A V = 25 in addition to the A V = 16.3 to the system as a whole. Taken together, these millimeter and infrared data help explain the peculiar nature of the infrared companion, WL 20S, as resulting from a large enhancement in its dusty, circumstellar environment in relation to its companions.

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Patrick J. Lowrance, J. Davy Kirkpatrick, and Charles A. Beichman

Upsilon Andromedae is an F8 V star known to have an extrasolar system of at least three planets in orbit around it. Here we report the discovery of a low-mass stellar companion to this system. The companion shares common proper motion, lies at a projected separation of ~750 AU, and has a spectral type of M4.5 V. The effect of this star on the radial velocity of the brighter primary is negligible, but this system provides an interesting test bed for stellar planetary formation theory and understanding dynamical stability since it is the first multiple planetary system known in a multiple stellar system.

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Curtis Struck, Babak E. Cohanim, and Lee Anne Willson

We present numerical hydrodynamical models of the effects of planets or brown dwarfs orbiting within the extended atmosphere and wind formation zone of Mira variables. We find time-dependent wake dynamics and episodic accretion phenomena that may give rise to observable optical events and affect SiO maser emission.

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Vladimir V. Usov

We discuss the polarization selection rules for the splitting of the two principal electromagnetic modes that propagate in a vacuum polarized by a superstrong magnetic field (B > 0.1_B_cr ≃ 4 × 1012 G). We show that below the threshold of free pair creation, the selection rules found by Adler in the limit of weak dispersion remain unaffected by taking the resonant effects into consideration; i.e., splitting of one mode is strictly forbidden, while splitting of the other is allowed.

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Yu-Qing Lou

Radio pulses emitted from a pulsar magnetosphere travel across the light cylinder and through a magnetized relativistic pulsar wind composed mainly of an electron-positron (_e_∓) pair plasma embedded in a spiral magnetic field. Besides the Doppler-shifted cyclotron resonance and wind advections of hybrid oscillations or low-frequency waves, we identify two cutoff frequencies at ωpe and ~(ω + ω/γ2)1/2 for radio waves of two orthogonal polarizations perpendicular and parallel to the projected spin axis in the sky plane, where ωpe is the proper e_∓ pair plasma frequency, ω_c is the relativistic Larmor frequency, and γ is the Lorentz factor of _e_∓ bulk flow. Radio pulse spectra of most pulsars show low-frequency cutoffs at νcut ≲ 100 MHz. For (ω + ω/γ2)1/2 < 2πνcut, one may bracket the _e_∓ particle loss rate of a pulsar wind. If for some cases, alternative to the wisdom that all such spectral cutoffs are intrinsic to the emission mechanism(s) near a pulsar, the magnetized e_∓ pair plasma of a pulsar wind actually causes the low-frequency cutoffs of radio pulse spectra, then spectral polarization studies of the two different frequency cutoffs can yield ωpe and ω_c/γ. Together with the observed rates of pulsar spin and spin-down, one may derive key properties of a pulsar wind, including the _e_∓ particle loss rate.

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Yaron Sheffer, S. R. Federman, and David L. Lambert

In an echelle spectrum of X Per acquired with the Space Telescope Imaging Spectrograph, we have identified individual rotational lines of 11 triplet-singlet (intersystem) absorption bands of 12CO. Four bands provide first detections for interstellar clouds. From a comparison with the ζ Oph sight line, we find that X Per is obscured by a higher 12CO column density of 1.4 × 1016 cm-2. Together with the high spectral resolution of 1.3 km s-1, this allows (1) an improved measurement of previously published interstellar _f_-values for seven bands and (2) an extraction of the first astrophysical oscillator strengths for _d_-X (8-0), (9-0), and (10-0), as well as for _e_-X (12-0). The 13CO _d_-X (12-0) band, previously suspected to exist toward ζ Oph, is now readily resolved and modeled. Our derived intersystem _f_-values for 12CO include a few mild (≤34%) disagreements with recent predictions from a perturbation analysis calculated for the interstellar excitation temperature. Overall, the comparison confirms the superiority of employing multiple singlet levels in the calculations of mixing coefficients over previous single-level predictions.

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P. F. Chen, S. T. Wu, K. Shibata, and C. Fang

Solar coronal mass ejections (CMEs) are associated with many dynamical phenomena, among which EIT waves have always been a puzzle. In this Letter MHD processes of CME-induced wave phenomena are numerically simulated. It is shown that as the flux rope rises, a piston-driven shock is formed along the envelope of the expanding CME, which sweeps the solar surface as it propagates. We propose that the legs of the shock produce Moreton waves. Simultaneously, a slower moving wavelike structure, with an enhanced plasma region ahead, is discerned, which we propose corresponds to the observed EIT waves. The mechanism for EIT waves is therefore suggested, and their relation with Moreton waves and radio bursts is discussed.

L103

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N. Gopalswamy, S. Yashiro, G. Michałek, M. L. Kaiser, R. A. Howard, D. V. Reames, R. Leske, and T. von Rosenvinge

We studied the association between solar energetic particle (SEP) events and coronal mass ejections (CMEs) and found that CME interaction is an important aspect of SEP production. Each SEP event was associated with a primary CME that is faster and wider than average CMEs and originated from west of E45°. For most of the SEP events, the primary CME overtakes one or more slower CMEs within a heliocentric distance of ~20 _R_☉. In an inverse study, we found that for all the fast (speed greater than 900 km s-1) and wide (width greater than 60°) western hemispheric frontside CMEs during the study period, the SEP-associated CMEs were ~4 times more likely to be preceded by CME interaction than the SEP-poor CMEs; i.e., CME interaction is a good discriminator between SEP-poor and SEP-associated CMEs. We infer that the efficiency of the CME-driven shocks is enhanced as they propagate through the preceding CMEs and that they accelerate SEPs from the material of the preceding CMEs rather than from the quiet solar wind. We also found a high degree of association between major SEP events and interplanetary type II radio bursts, suggesting that proton accelerators are also good electron accelerators.

L109

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N. Narukage, H. S. Hudson, T. Morimoto, S. Akiyama, R. Kitai, H. Kurokawa, and K. Shibata

We report the observation of a Moreton wave in Hα (line center and ±0.8 Å) with the Flare Monitoring Telescope at the Hida Observatory of Kyoto University at 4:36-4:41 UT on 1997 November 3. The same region (NOAA Active Region 8100) was simultaneously observed in soft X-rays with the soft X-ray telescope on board Yohkoh, and a wavelike disturbance ("X-ray wave") was also found. The position of the wave front as well as the direction of propagation of the X-ray wave roughly agree with those of the Moreton wave. The propagation speeds of the Moreton wave and the X-ray wave are about 490 ± 40 and 630 ± 100 km s-1, respectively. Assuming that the X-ray wave is an MHD fast-mode shock, we can estimate the propagation speed of the shock, on the basis of MHD shock theory and the observed soft X-ray intensities ahead of and behind the X-ray wave front. The estimated fast shock speed is 400-760 km s-1, which is in rough agreement with the observed propagation speed of the X-ray wave. The fast-mode Mach number of the X-ray wave is also estimated to be about 1.15-1.25. These results suggest that the X-ray wave is a weak MHD fast-mode shock propagating through the corona and hence is the coronal counterpart of the Moreton wave.

L113

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Boris Vilhelm Gudiksen and Åke Nordlund

The heating of the solar corona and the puzzle of the slender high reaching magnetic loops seen in observations from the Transition Region and Coronal Explorer (TRACE) has been investigated through three-dimensional numerical simulations and found to be caused by the well-observed plasma flows in the photosphere displacing the footpoints of magnetic loops in a nearly potential configuration. It is found that even the small convective displacements cause magnetic dissipation sufficient to heat the corona to temperatures of the order of a million K. The heating is intermittent in both space and time—at any one height and time it spans several orders of magnitude, and localized heating causes transonic flows along field lines, which explains the observed nonhydrostatic stratification of loops that are bright in emission measure.

L117

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M. Ishiguro, J. Watanabe, F. Usui, T. Tanigawa, D. Kinoshita, J. Suzuki, R. Nakamura, M. Ueno, and T. Mukai

We present the first evidence of a cometary dust trail in optical wavelengths along the orbit of 22P/Kopff, observed when the parent comet was at a heliocentric distance of 3.01 AU. We find that the surface brightness and the width of the trail become, respectively, fainter and wider as the distance from the comet nucleus increases, except for a region with delta mean anomaly ΔMA ≤ 002. This suggests that the majority of the centimeter-sized dust particles were ejected before the comet's previous perihelion passage and that they spread due to their initial velocity with respect to the comet. By comparing this trail with the IRAS data at wavelengths of 12 and 25 μm, we infer that the trail is composed of very low albedo particles (~0.01).

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Alister W. Graham