Issue 1 - Volume 684 - The Astrophysical Journal (original) (raw)
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and Tom Abel
The initial conditions and relevant physics for the formation of the earliest galaxies are well specified in the concordance cosmology. Using ab initio cosmological Eulerian adaptive mesh refinement radiation hydrodynamical calculations, we discuss how very massive stars start the process of cosmological reionization. The models include nonequilibrium primordial gas chemistry and cooling processes and accurate radiation transport in the case B approximation using adaptively ray-traced photon packages, retaining the time derivative in the transport equation. Supernova feedback is modeled by thermal explosions triggered at parsec scales. All calculations resolve the local Jeans length by at least 16 grid cells at all times and as such cover a spatial dynamic range of ~106. These first sources of reionization are highly intermittent and anisotropic and first photoionize the small-scale voids surrounding the halos they form in, rather than the dense filaments they are embedded in. As the merging objects form larger, dwarf-sized galaxies, the escape fraction of UV radiation decreases and the H II regions only break out on some sides of the galaxies, making them even more anisotropic. In three cases, SN blast waves induce star formation in overdense regions that were formed earlier from ionization front instabilities. These stars form tens of parsecs away from the center of their parent DM halo. Approximately five ionizing photons are needed per sustained ionization when star formation in 106_M_☉ halos is dominant in the calculation. As the halos become larger than ~107_M_☉, the ionizing photon escape fraction decreases, which in turn increases the number of photons per ionization to 15-50, in calculations with stellar feedback only. Radiative feedback decreases clumping factors by 25% when compared to simulations without star formation and increases the average temperature of ionized gas to values between 3000 and 10,000 K.
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Xuelei Chen and Jordi Miralda-Escudé
We predict the 21 cm signature of the first metal-free stars. The soft X-rays emitted by these stars penetrate the atomic medium around their host halos, generating Lyα photons that couple the spin and kinetic temperatures. These create a region we call the "Lyα sphere," visible in 21 cm against the CMB, which is much larger than the H II region produced by the same star. The spin and kinetic temperatures are strongly coupled before the X-rays can substantially heat the medium, implying that a 21 cm absorption signal from the adiabatically cooled gas in Hubble expansion around the star is expected when the medium has not been heated previously. A central region of emission from the gas heated by the soft X-rays is also present, although with a weaker signal than the absorption. The Lyα sphere is a universal signature that should be observed around any first star illuminating its vicinity for the first time. The 21 cm radial profile of the Lyα sphere can be calculated as a function of the luminosity, spectrum, and age of the star. For a star of a few hundred _M_☉ and zero metallicity (as expected for the first stars), the physical radius of the Lyα sphere can reach tens of kiloparsecs. The first metal-free stars should be strongly clustered because of high cosmic biasing; this implies that the regions producing a 21 cm absorption signal may contain more than one star and will generally be irregular and not spherical, because of the complex distribution of the gas. We discuss the feasibility of detecting these Lyα spheres, which would be present at redshifts z ∼ 30 in the cold dark matter model. Their observation would represent a direct proof of the detection of a first star.
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Mansi M. Kasliwal, Richard Massey, Richard S. Ellis, Satoshi Miyazaki, and Jason Rhodes
We assess the relative merits of weak-lensing surveys, using overlapping imaging data from the ground-based Subaru telescope and the Hubble Space Telescope (HST). Our tests complement similar studies undertaken with simulated data. From observations of 230,000 matched objects in the 2 deg2 COSMOS field, we identify the limit at which faint galaxy shapes can be reliably measured from the ground using well-established shape-measurement techniques. Our ground-based shear catalog achieves subpercent calibration bias compared to high-resolution space-based data for galaxies brighter than i' ≃ 24.5 and with half-light radii larger than 1.8''. This selection corresponds to a surface density of 15 galaxies arcmin−2 compared to ~71 arcmin−2 from space. On the other hand, the survey speed of current ground-based facilities is much faster than that of HST, although this gain is mitigated by the increased depth of space-based imaging desirable for tomographic (3D) analyses. As an independent experiment, we also reconstruct the projected mass distribution in the COSMOS field using both data sets, and compare the derived cluster catalogs with those from X-ray observations. The ground-based catalog achieves a reasonable degree of completeness, with minimal contamination and no detected bias, for massive clusters at redshifts 0.2 < z < 0.5. The space-based data provide improved precision and a greater sensitivity to clusters of lower mass or at higher redshift.
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Rosanne Di Stefano
In a variety of astronomical situations, there is a relatively high probability that a single isolated lens will produce a detectable event. The high probability is caused by some combination of a large Einstein angle, fast angular motion, and a dense background field. We refer to high-probability lenses as "mesolenses." Planetary and stellar masses located within 1-2 kpc are examples of mesolenses. We show that their presence can be detected against a wide variety of background fields, using signatures of both time variability and spatial effects. Time signatures can be identical to those of microlensing, but can also include baseline jitter, extreme apparent chromaticity, photometric events well fit by lens models in which several independent sources are simultaneously lensed, and sequences of events. Spatial signatures include sequential astrometric lensing of surface brightness fluctuations, as well as patterns of time variability that sweep across the background field as the lens moves. Wide-field monitoring programs, such as Pan-STARRS and LSST, are well suited to the study of nearby masses. In addition, targeted high-resolution observations of the region behind a known mass traveling across a dense background field can use lensing effects to measure the lens mass and study its multiplicity.
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Rosanne Di Stefano
Nearby masses can have a high probability of lensing stars in a distant background field. This high-probability lensing, or mesolensing, can be used to dramatically increase our knowledge of dark and dim objects in the solar neighborhood, where it can discover and facilitate the study of members of the local dark matter population (free-floating planets, low-mass dwarfs, white dwarfs, neutron stars, and stellar-mass black holes). We can measure the mass and transverse velocity of those masses discovered (or already known) and determine whether or not they are in binaries with dim companions. Here we explore these and other applications of mesolensing, including the study of forms of matter that have been hypothesized but not discovered, such as intermediate-mass black holes, dark matter objects free-streaming through the Galactic disk, and planets in the outermost regions of the solar system. In each case we discuss the feasibility of deriving results based on present-day monitoring systems, and we also consider the vistas that will open with the advent of all-sky monitoring in the era of Pan-STARRS and LSST.
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Ryan J. Foley, Alexei V. Filippenko, C. Aguilera, A. C. Becker, S. Blondin, P. Challis, A. Clocchiatti, R. Covarrubias, T. M. Davis, P. M. Garnavich et al
We present a large-scale effort of creating composite spectra of high-redshift SNe Ia and comparing them to low-redshift counterparts in an attempt to understand possible cosmic evolution of SNe Ia, which has major implications for studies of dark energy. Through the ESSENCE project, we have obtained 107 spectra of 88 high-redshift SNe Ia with excellent light-curve information. In addition, we have obtained 397 spectra of low-redshift SNe Ia through a multiple-decade effort at the Lick and Keck Observatories, and we have used 45 UV spectra obtained by HST and IUE. The low-redshift spectra act as a control sample when comparing to the ESSENCE spectra. In all instances, the ESSENCE and Lick composite spectra appear very similar. The addition of galaxy light to the Lick composite spectra allows an excellent match of the overall SED with the ESSENCE composite spectra, indicating that the high-redshift SNe are more contaminated with host galaxy light than their low-redshift counterparts. This is caused by observing objects at all redshifts with similar angular slit widths, which corresponds to different projected physical distances. After correcting for the galaxy light contamination, a few marginally significant differences in the spectra remain. We have estimated the systematic errors when using current spectral templates for _K_-corrections to be ~0.02 mag. The variance in the composite spectra gives an estimate of the intrinsic variance in low-redshift maximum light SN spectra of ~3% relative flux in the optical and growing toward the UV. The difference between the maximum light low- and high-redshift spectra constrains the evolution of SN spectral features between our samples to be <10% relative flux in the rest-frame optical. Currently, galaxy contamination and the small samples of rest-frame UV spectra at low and high redshifts are the limiting factors for future studies.
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Jeffrey A. Newman
We describe a new method that can measure the true redshift distribution of any set of objects that are studied only photometrically. Measuring the angular cross-correlation between objects in the photometric sample with objects in some spectroscopic sample as a function of the spectroscopic z, along with other, standard correlation measurements, provides sufficient information to reconstruct the redshift distribution of the photometric sample. The spectroscopic sample need not resemble the photometric sample in galaxy properties, but must fall within its sky coverage. We test this hybrid, photometric-spectroscopic cross-correlation technique with Monte Carlo simulations based on realistic error estimates (including sample variance). The rms errors in recovering both the mean redshift and σ of the redshift distribution for a single photometric redshift bin with true distribution given by a Gaussian are 1.4 × 10−3(σ_z_/0.1)(Σ_p_/10)−0.3(d_N_ s/dz/25,000)−1/2, where σ_z_ is the true Gaussian σ , Σ_p_ is the surface density of the photometric sample in galaxies arcmin −2, and dN s/dz is the number of galaxies with a spectroscopic redshift per unit z. We test the impact of non-Gaussian redshift outliers and of systematic errors due to unaccounted-for bias evolution, errors in measuring autocorrelations, photometric zero-point variations, or mistaken cosmological assumptions, and find that none will dominate measurement uncertainties in reasonable scenarios. The true redshift distributions of even arbitrarily faint photometric samples may be determined to the precision required by proposed dark energy experiments (Δ⟨_z_⟩ ≲ 3 × 10−3 at z ∼ 1) with this method.
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G. Bono, F. Caputo, G. Fiorentino, M. Marconi, and I. Musella
We perform a detailed analysis of Cepheids in NGC 4258, the Magellanic Clouds, and Milky Way in order to verify the reliability of the theoretical scenario based on a large set of nonlinear convective pulsation models. We derive Wesenheit functions from the synthetic BVI magnitudes of the pulsators, and we show that the sign and the extent of the metallicity effect on the predicted period-Wesenheit (P − W) relations change according to the adopted passbands. These P − W relations are applied to measured BVI magnitudes of NGC 4258, Magellanic, and Galactic Cepheids available in the literature. We find that Magellanic and Galactic Cepheids agree with the metallicity dependence of the predicted P − W relations. Concerning the NGC 4258 Cepheids, the results strongly depend on the adopted metallicity gradient across the galactic disk. The most recent nebular oxygen abundances support a shallower gradient and provide a metallicity dependence that agrees well with current pulsation predictions. Moreover, the comparison of Cepheid distances based on VI magnitudes with distance estimates based on the revised TRGB method for external galaxies, on the HST trigonometric parallaxes for Galactic Cepheids, and on eclipsing binaries in the Magellanic Clouds seems to favor the metallicity correction predicted by pulsation models. The sign and the extent of the metallicity dependence of the P − W and of the period-luminosity (P − L) relations change according to the adopted passbands. Therefore, distances based on different methods and/or bands should not be averaged. The use of extragalactic Cepheids to constrain the metallicity effect requires new accurate and extensive nebular oxygen measurements.
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Miguel A. Urbaneja, Rolf-Peter Kudritzki, Fabio Bresolin, Norbert Przybilla, Wolfgang Gieren, and Grzegorz Pietrzyński
The quantitative analysis of low-resolution spectra of A and B supergiants is used to determine a distance modulus of 24.99 ± 0.10 mag (995 ± 46 kpc) to the Local Group galaxy WLM. The analysis yields stellar effective temperatures and gravities, which provide a distance through the flux-weighted gravity-luminosity relationship (FGLR). Our distance is 0.07 mag larger than the most recent results based on Cepheids and the tip of the red giant branch. This difference is within the 1 σ overlap of the typical uncertainties quoted in these photometric investigations. In addition, non-LTE spectral synthesis of the rich metal-line spectra (mostly iron, chromium, and titanium) of the A supergiants is carried out, which allows the determination of stellar metallicities. An average metallicity of –0.87 ± 0.06 dex with respect to solar metallicity is found.
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C. Gruppioni, F. Pozzi, M. Polletta, G. Zamorani, F. La Franca, N. Sacchi, A. Comastri, L. Pozzetti, C. Vignali, C. Lonsdale et al
We present the broadband SEDs of the largest available highly complete (72%) spectroscopic sample of MIR-selected galaxies and AGNs at intermediate redshift. The sample contains 203 extragalactic sources from the 15 μm ELAIS-SWIRE survey, all with measured spectroscopic redshift. Most of these sources have full multiwavelength coverage from the FUV (GALEX) to the FIR (Spitzer) and lie in the redshift range 0.1 < z < 1.3. This large sample allows us for the first time to characterize the spectral properties of sources responsible for the strong evolution observed in the MIR. Based on SED-fitting, we have classified the MIR sources, identifying AGN signatures in about 50% of them. This fraction is significantly higher than that derived from optical spectroscopy (~29%) and is due in particular to the identification of AGN activity in objects spectroscopically classified as galaxies (the spectroscopic classification may be somewhat unreliable because of host galaxy dilution in the optical). It is likely that in most of our objects, the AGN is either obscured or low luminosity, and thus dominates the energetic output only in the MIR, showing up just in the range where the host galaxy SED has a minimum. The fraction of AGNs strongly depends on flux density, with that derived through the SED-fitting about 20% at _S_15μ m ∼ 0.5–1 mJy and gradually increasing to 100% at _S_15μ m > 10 mJy, while that obtained from optical spectroscopy is never >30%, even at higher flux densities. Our results will be very useful for updating all models aimed at interpreting the deep IR survey data and in particular for constraining the nature and role of dust-obscured systems in the intermediate/high-redshift universe.
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S. E. Tremblay, G. B. Taylor, J. F. Helmboldt, C. D. Fassnacht, and T. J. Pearson
We present multifrequency, multiepoch, Very Long Baseline Array (VLBA) observations of J11584+2450. These observations clearly show this source, previously classified as a core jet, to be a compact symmetric object (CSO). Comparisons between these new data and data taken over the last 11 years shows the edge-brightened hot spots retreating toward the core (and slightly to the west) at approximately 0.3_c_. Whether this motion is strictly apparent or actually physical in nature is discussed, as well as possible explanations, and what implications a physical contraction of J11584+2450 would have for current CSO models.
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Cheng-Jiun Ma, Harald Ebeling, David Donovan, and Elizabeth Barrett
We present the results of a wide-field spectroscopic analysis of the galaxy population of the massive cluster MACS J0717.5+3745 and the surrounding filamentary structure (z = 0.55), as part of our systematic study of the 12 most distant clusters in the MACS sample. Of 1368 galaxies spectroscopically observed in this field, 563 are identified as cluster members; of those, 203 are classified as emission-line galaxies, 260 as absorption-line galaxies, and 17 as E+A galaxies (defined by (H δ + H γ )/2 > 6 Å and no detection of [O II] and Hβ in emission). The variation of the fraction of emission- and absorption-line galaxies as a function of local projected galaxy density confirms the well-known morphology-density relation, and becomes flat at projected galaxy densities less than ~20 Mpc−2. Interestingly, 16 out of 17 E+A galaxies lie (in projection) within the ram-pressure stripping radius around the cluster core, which we take to be direct evidence that ram-pressure stripping is the primary mechanism that terminates star formation in the E+A population of galaxy clusters. This conclusion is supported by the rarity of E+A galaxies in the filament, which rules out galaxy mergers as the dominant driver of evolution for E+A galaxies in clusters. In addition, we find that the 42 e(a) and 27 e(b) member galaxies, i.e., the dusty-starburst and starburst galaxies respectively, are spread out across almost the entire study area. Their spatial distribution, which shows a strong preference for the filament region, suggests that starbursts are triggered in relatively low-density environments as galaxies are accreted from the field population.
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Keiichi Umetsu and Tom Broadhurst
We derive a projected 2D mass map of the well-studied galaxy cluster A1689 based on an entropy-regularized maximum likelihood combination of the lens magnification and distortion of red background galaxies registered in deep Subaru images. The method is not restricted to the weak regime but applies to the whole area outside the tangential critical curve, where nonlinearity between the surface mass density and the observables extends to a radius of a few arcminutes. The known strong-lensing information is also readily incorporated in this approach, represented as a central pixel with a mean surface density close to the critical value. We also utilize the distortion measurements to locally downweight the intrinsic clustering noise, which otherwise perturbs the depletion signal. The projected mass profile continuously steepens with radius and is well fitted by the Navarro-Frenk-White model, but with a surprisingly large concentration _c_vir = 13.4+ 5.3−3.3, lying far from the predicted value of _c_vir ∼ 5, corresponding to the measured virial mass, _M_vir = (2.1 ± 0.2) × 1015_M_☉, posing a challenge to the standard assumptions defining the ΛCDM model. We examine the consistency of our results with estimates derived with the standard weak-lensing estimators and by comparison with the inner mass profile obtained from strong lensing. All the reconstructions tested here imply a virial mass in the range _M_vir = (1.5–2.1) × 1015_M_☉, and the combined ACS and Subaru 2D mass reconstruction yields a tight constraint on the concentration parameter, _c_vir = 12.7 ± 1 ± 2.8 (_c_200 ∼ 10), improving upon the statistical accuracy of our earlier 1D analysis. Importantly, our best-fitting profile properly reproduces the observed Einstein radius of 45'' (z s = 1), in contrast to other weak-lensing work, reporting lower concentration profiles, which underestimate the observed Einstein radius.
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Alexey Voevodkin, Christopher J. Miller, Konstantin Borozdin, Katrin Heitmann, Salman Habib, Paul Ricker, and Robert C. Nichol
We present a combined optical and X-ray analysis of three optically selected X-ray-bright groups with giant elliptical galaxies in the center. These massive ellipticals were targeted for XMM-Newton X-ray observations on the basis of their large-velocity dispersions and their proximity to a nearby ROSAT X-ray source. In addition, these targets are significantly brighter in the optical than their nearest neighbors. We show that one of these systems meets the standard criteria for a fossil group. While the other two systems have a prominent magnitude gap in the E/S0 ridgeline, they do not appear to have reached the fossil-like final stage of group evolution.
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Todd A. Thompson
I consider the physics of gravitational instabilities in the presence of dynamically important radiation pressure and gray radiative diffusion. For any nonzero radiation diffusion rate on an optically thick scale, the medium is unstable unless the classical gas-only isothermal Jeans criterion is satisfied. When diffusion is "slow," even though the Jeans instability is stabilized by radiation pressure on scales smaller than the adiabatic Jeans length, on these same spatial scales the medium is unstable to a diffusive mode. In this regime, neglecting gas pressure, the characteristic growth timescale is independent of spatial scale and given by (3κ_c_ s_2)/(4π_Gc) , where c s is the adiabatic sound speed. This timescale is that required for a fluid parcel to radiate away its thermal energy content at the Eddington limit, the Kelvin-Helmholz timescale for a radiation pressure-supported self-gravitating object. In the limit of "rapid" diffusion, radiation does nothing to suppress the Jeans instability and the medium is dynamically unstable unless the gas-only Jeans criterion is satisfied. I connect with treatments of Silk damping in the early universe. I discuss several applications, including photons diffusing in regions of extreme star formation (starburst galaxies and parsec-scale AGN disks), and the diffusion of cosmic rays in normal galaxies and galaxy clusters. The former (particularly, starbursts) are "rapidly" diffusing and thus cannot be supported against dynamical instability in the linear regime by radiation pressure alone. The latter are more nearly "slowly" diffusing. I speculate that the turbulence in starbursts may be driven by the dynamical coupling between the radiation field and the self-gravitating gas, perhaps mediated by magnetic fields, and that this diffusive instability operates in individual massive stars.
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Hajime Susa
We perform radiation hydrodynamics simulations on the evolution of galactic gas disks irradiated by the ultraviolet radiation background. We find that gas disks with _N_H≳ 1021 cm−2 that are exposed to ultraviolet radiation at a level of _I_21 = 1 can be self-shielded from photoheating, whereas disks with _N_H≲ 1021 cm−2 cannot. We also find that the unshielded disks keep a smooth density distribution without any sign of fragmentation, while the self-shielded disks easily fragment into small pieces through self-gravity, possibly followed by star formation. The suppression of star formation in unshielded disks is different from the photoevaporation effect, since the assumed dark halo potential is deep enough to retain the photoheated gas. The presence of such a critical threshold column density would be one of the reasons for the so-called downsizing feature of present-day galaxies.
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Philip Chang
I calculate the action of a satellite, infalling through dynamical friction, on a coplanar gaseous disk of finite radial extent. The disk tides, raised by the infalling satellite, couple the satellite and disk. Dynamical friction acting on the satellite then shrinks the radius of the coupled satellite-disk system. Thus, the gas is "shepherded" to smaller radii. In addition, gas shepherding produces a large surface density enhancement at the disk edge. If the disk edge then becomes gravitationally unstable and fragments, it may give rise to enhanced star formation. On the other hand, if the satellite is sufficiently massive and dense, the gas may be transported from ~100 pc to inside of 10 to tens of parsecs before completely fragmenting into stars. I argue that gas shepherding may drive the fueling of active galaxies and central starbursts, and I compare this scenario to competing scenarios. I argue that sufficiently large and dense super star clusters (acting as the shepherding satellites) can shepherd a gas disk down to 10 to tens of parsecs. Inside of 10 to tens of parsecs, another mechanism may operate, i.e., cloud-cloud collisions or a marginally (gravitationally) stable disk, that drives the gas ≲1 pc, where it can be viscously accreted, feeding a central engine.
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Adam S. Bolton, Tommaso Treu, Léon V. E. Koopmans, Raphaël Gavazzi, Leonidas A. Moustakas, Scott Burles, David J. Schlegel, and Randall Wayth
We use a sample of 53 massive early-type strong gravitational lens galaxies with well-measured redshifts (ranging from z = 0.06 to 0.36) and stellar velocity dispersions (between 175 and 400 km s−1) from the Sloan Lens ACS (SLACS) Survey to derive numerous empirical scaling relations. The ratio between central stellar velocity dispersion and isothermal lens-model velocity dispersion is nearly unity within errors. The SLACS lenses define a fundamental plane (FP) that is consistent with the FP of the general population of early-type galaxies. We measure the relationship between strong-lensing mass _M_lens within one-half effective radius (R e/2) and the dimensional mass variable _M_dim ≡ _G_−1σe22(R e/2) to be log (_M_lens/1011_M_☉) = (1.03 ± 0.04) log (_M_dim/1011_M_☉) + (0.54 ± 0.02) (where σe2 is the projected stellar velocity dispersion within R e/2). The near-unity slope indicates that the mass-dynamical structure of massive elliptical galaxies is independent of mass and that the "tilt" of the SLACS FP is due entirely to variation in total (luminous plus dark) mass-to-light ratio with mass. Our results imply that dynamical masses serve as a good proxies for true masses in massive elliptical galaxies. Regarding the SLACS lenses as a homologous population, we find that the average enclosed two-dimensional (2D) mass profile goes as log [M(< R)/_M_dim] = (1.10 ± 0.09) log (R/R e) + (0.85 ± 0.03) , consistent with an isothermal (flat rotation curve) model when deprojected into three dimensions (3D). This measurement is inconsistent with the slope of the average projected aperture luminosity profile at a confidence level greater than 99.9%, implying a minimum dark matter fraction of _f_DM = 0.38 ± 0.07 within 1 effective radius. We also present an analysis of the angular mass structure of the lens galaxies, which further supports the need for dark matter inside one effective radius.
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Arjen van der Wel and Roeland P. van der Marel
We use the spatial information of our previously published VLT/FORS2 absorption-line spectroscopy to measure mean stellar velocity and velocity dispersion profiles of 25 field early-type galaxies at a median redshift z = 0.97 (full range 0.6 < z < 1.2). This provides the first detailed study of early-type galaxy rotation at these redshifts. From surface brightness profiles from HST imaging we calculate two-integral oblate axisymmetric Jeans equation models for the observed kinematics. Fits to the data yield for each galaxy the degree of rotational support and the mass-to-light ratio M/_L_Jeans. S0 and Sa galaxies are generally rotationally supported, whereas elliptical galaxies rotate less rapidly or not at all. Down to M B = − 19.5 (corrected for luminosity evolution), we find no evidence for evolution in the fraction of rotating early-type (E+S0) galaxies between z ∼ 1 (63% ± 11% ) and the present (61% ± 5% ). We interpret this as evidence for little or no change in the field S0 fraction with redshift. We compare M/_L_Jeans with M/_L_vir inferred from the virial theorem and globally averaged quantities and assuming homologous evolution. There is good agreement for nonrotating (mostly E) galaxies. However, for rotationally supported galaxies (mostly S0) M/_L_Jeans is on average ~40% higher than M/_L_vir. We discuss possible explanations and the implications for the evolution of M/L between z = 1 and the present and its dependence on mass.
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H. Kaneda, T. Onaka, I. Sakon, T. Kitayama, Y. Okada, and T. Suzuki
We performed mid-infrared spectroscopic observations of 18 local dusty elliptical galaxies by using the Infrared Spectrograph (IRS) on board Spitzer. We have significantly detected polycyclic aromatic hydrocarbon (PAH) features from 14 out of the 18 galaxies and, thus, found that the presence of PAHs is not rare but rather common in dusty elliptical galaxies. Most of these galaxies show an unusually weak 7.7 μm emission feature relative to 11.3 and 17 μm emission features. A large fraction of the galaxies also exhibit H2 rotational line and ionic fine-structure line emissions, which have no significant correlation with the PAH emissions. The PAH features are well correlated with the continuum at 35 μm, whereas they are not correlated with the continuum at 6 μm. We conclude that the PAH emission of the elliptical galaxies is mostly of interstellar origin rather than of stellar origin, and that the unusual PAH interband strength ratios are likely to be due to a large fraction of neutral to ionized PAHs.
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Douglas A. Swartz, Roberto Soria, and Allyn F. Tennant
A thorough search for ultraluminous X-ray source candidates within the Local Volume is made. The search spatially matches potential ULXs detected in X-ray images or cataloged in the literature with galaxies tabulated in the Catalog of Neighboring Galaxies compiled by Karachentsev et al. The specific ULX frequency (occurrence rate per unit galaxy mass) is found to be a decreasing function of host galaxy mass for host masses above ~108.5_M_☉. There is too little mass in galaxies below this point to determine whether this trend continues to lower galaxy mass. No ULXs have yet been detected in lower mass galaxies. Systematic differences between dwarf and giant galaxies that may explain an abundance of ULXs in dwarf galaxies and what they may imply about the nature of ULXs are discussed.
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Željko Ivezić, Branimir Sesar, Mario Jurić, Nicholas Bond, Julianne Dalcanton, Constance M. Rockosi, Brian Yanny, Heidi J. Newberg, Timothy C. Beers, Carlos Allende Prieto et al
Using effective temperature and metallicity derived from SDSS spectra for ~60,000 F- and G-type main-sequence stars (0.2 < g − r < 0.6), we develop polynomial models for estimating these parameters from the SDSS u − g and g − r colors. These photometric estimates have similar error properties as those determined from SDSS spectra. We apply this method to SDSS photometric data for over 2 million F/G stars and measure the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, which correspond to disk and halo. The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component is spatially invariant, while the median disk metallicity smoothly decreases with distance from the Galactic plane from –0.6 at 500 pc to –0.8 beyond several kiloparsecs. The absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks. We detect coherent substructures in the kinematics-metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of [Fe/H] = −0.95, with an rms scatter of only ~0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper-motion measurements accurate to ~0.5 mas yr−1, for about 200 million F/G dwarf stars within a distance limit of ~100 kpc (g < 23.5).
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Andrew McWilliam and Rebecca A. Bernstein
We describe the detailed chemical abundance analysis of a high-resolution (R ∼ 35,000), integrated-light (IL), spectrum of the core of the Galactic globular cluster 47 Tuc, obtained using the du Pont echelle at Las Campanas. We develop an abundance analysis strategy that can be applied to spatial unresolved extragalactic clusters. We have computed abundances for Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Ba, La, Nd, and Eu. For an analysis with the known color-magnitude diagram (CMD) for 47 Tuc we obtain a mean [Fe/H] value of -0.75 ± 0.026 ± 0.045 dex (random and systematic error), in good agreement with the mean of five recent high-resolution abundance studies, at –0.70 dex. Typical random errors on our mean [X/Fe] ratios are 0.07-0.10 dex, similar to studies of individual stars in 47 Tuc. Na and Al appear enhanced, perhaps due to proton burning in the most luminous cluster stars. Our IL abundance analysis with an unknown CMD employed theoretical Teramo isochrones; however, we apply zero-point abundance corrections to account for the factor of 3 underprediction of stars at the AGB bump luminosity. While line diagnostics alone provide only mild constraints on the cluster age (ruling out ages younger than ~2 Gyr), when theoretical IL B − V colors are combined with metallicity derived from the Fe I lines, the age is constrained to 10-15 Gyr and we obtain [ Fe/H ] = − 0.70 ± 0.021 ± 0.052 dex. We find that Fe I line diagnostics may also be used to constrain the horizontal-branch morphology of an unresolved cluster. Lastly, our spectrum synthesis of 5.4 million TiO lines indicates that the 7300-7600 Å TiO window should be useful for estimating the effect of M giants on the IL abundances, and important for clusters more metal-rich than 47 Tuc.
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Cara E. Rakowski, J. Martin Laming, and Parviz Ghavamian
Understanding the heating of electrons to quasi-thermal energies at collisionless shocks has broad implications for plasma astrophysics. It directly impacts the interpretation of X-ray spectra from shocks, is important for understanding how energy is partitioned between the thermal and cosmic-ray populations, and provides insight into the structure of the shock itself. In previous work by Ghavamian et al. we presented observational evidence for an inverse-square relation between the electron-to-proton temperature ratio and the shock speed at the outer blast waves of supernova remnants in partially neutral interstellar gas. There we outlined how lower hybrid waves generated in the cosmic-ray precursor could produce such a relationship by heating the electrons to a common temperature independent of both shock speed and the strength of the ambient magnetic field. Here we explore the mechanism of lower hybrid wave heating of electrons in more detail. Specifically, we examine the growth rate of the lower hybrid waves for both the kinetic (resonant) and reactive cases. We find that only the kinetic case exhibits a growing mode. At low Alfvén Mach numbers the growth of lower hybrid waves can be faster than the magnetic field amplification by modified Alfvén waves.
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Teresa Ross, Emily J. Baker, Theodore P. Snow, Joshua D. Destree, Brian L. Rachford, Meredith M. Drosback, and Adam G. Jensen
The negative ion H− is widely understood to be important in many astrophysical environments, including the atmospheres of late-type stars like the Sun. However, the ion has never been detected spectroscopically outside the laboratory. A search for the far-ultraviolet autodetaching transitions of H− in interstellar and circumstellar matter seems to be the best hope for directly detecting this ion. We undertook a highly sensitive search using data from the FUSE instrument. We concentrated on two types of sight lines: planetary nebulae, where model calculations suggest a sufficient abundance of H− to be determined, and translucent clouds, where H− might form on dust grains as an intermediate step in molecular hydrogen formation. Upper limits on H− abundances were set.
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C. Thom, J. E. G. Peek, M. E. Putman, Carl Heiles, K. M. G. Peek, and R. Wilhelm
We report an accurate distance of d = 10 ± 2.5 kpc to the high-velocity cloud Complex C. Using high signal-to-noise ratio Keck HIRES spectra of two horizontal-branch stars, we have detected Ca II K absorption lines from the cloud. Significant nondetections toward a further three stars yield robust lower distance limits. The resulting H I mass of Complex C is _M_H I = 4.9+2.9−2.2 × 106_M_☉; a total mass of _M_tot = 8.2+ 4.6−2.6 × 106_M_☉ is implied, after corrections for helium and ionization. At 10 kpc, Complex C has physical dimensions 3 × 15 kpc, and if it is as thick as it is wide, then the average density is log ⟨_n_⟩ ≃ −2.5. We estimate the contribution of Complex C to the mass influx may be as high as ~0.14 _M_☉ yr−1.
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Ellen G. Zweibel and Fabian Heitsch
The turnover of interstellar gas on ~109 yr timescales argues for the continuous operation of a galactic dynamo. The conductivity of interstellar gas is so high that the dynamo must be "fast"; i.e., the magnetic field must be amplified at a rate nearly independent of the magnetic diffusivity. Yet all the fast dynamos so far known, and all direct numerical simulations of interstellar dynamos, yield magnetic power spectra that peak at the resistive scale, while galactic magnetic fields have substantial power on large scales. In this paper we show that in weakly ionized gas the limiting scale may be the ion-neutral decoupling scale, which, although still small, is many orders of magnitude larger than the resistive scale.
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Pak Shing Li, Christopher F. McKee, Richard I. Klein, and Robert T. Fisher
Most numerical investigations on the role of magnetic fields in turbulent molecular clouds (MCs) are based on ideal magnetohydrodynamics (MHD). However, MCs are weakly ionized, so that the timescale required for the magnetic field to diffuse through the neutral component of the plasma by ambipolar diffusion (AD) can be comparable to the dynamical timescale. We have performed a series of 2563 and 5123 simulations on supersonic but sub-Alfvénic turbulent systems with AD using the heavy-ion approximation developed by Li and coworkers. Our calculations are based on the assumption that the number of ions is conserved, but we show that these results approximately apply to the case of time-dependent ionization in molecular clouds as well. Convergence studies allow us to determine the optimal value of the ionization mass fraction when using the heavy-ion approximation for low Mach number, sub-Alfvénic turbulent systems. We find that ambipolar diffusion steepens the velocity and magnetic power spectra compared to the ideal MHD case. Changes in the density PDF, total magnetic energy, and ionization fraction are determined as a function of the AD Reynolds number. The power spectra for the neutral gas properties of a strongly magnetized medium with a low AD Reynolds number are similar to those for a weakly magnetized medium; in particular, the power spectrum of the neutral velocity is close to that for Burgers turbulence.
395
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Patrick Hennebelle and Gilles Chabrier
We derive an analytical theory of the prestellar core initial mass function (IMF) based on an extension of the Press-Schechter statistical formalism. Our approach relies on the general concept of the gravothermal and gravoturbulent collapse of a molecular cloud, with a selection criterion based on the thermal or turbulent Jeans mass, which yields the derivation of the mass spectrum of self-gravitating objects in a quiescent or a turbulent environment. The same formalism also yields the mass spectrum of non-self-gravitating clumps produced in supersonic flows. The mass spectrum of the self-gravitating cores reproduces well the observed IMF. The theory predicts that the shape of the IMF results from two competing contributions, namely, a power law at large scales and an exponential cutoff (lognormal form) centered around the characteristic mass for gravitational collapse. The cutoff exists both in the case of thermal or turbulent collapse, provided that the underlying density field has a lognormal distribution. Whereas pure thermal collapse produces a power-law tail steeper than the Salpeter value, dN/_d_log M ∝ M_−x with x ≃ 1.35, the latter is recovered exactly for the (three-dimensional) value of the spectral index of the velocity power spectrum, n ≃ 3.8, found in observations and in numerical simulations of isothermal supersonic turbulence. Indeed, the theory predicts that x = (n + 1)/(2_n − 4) for self-gravitating structures and x = 2 − n'/3 for non-self-gravitating structures, where n' is the power spectrum index of log ρ . We show that, whereas supersonic turbulence promotes the formation of both massive stars and brown dwarfs, it has an overall negative impact on star formation, decreasing the star formation efficiency. This theory provides a novel theoretical foundation to understand the origin of the IMF and provides useful guidance to numerical simulations exploring star formation, while making testable predictions.
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Luke D. Keller, G. C. Sloan, W. J. Forrest, S. Ayala, P. D'Alessio, S. Shah, N. Calvet, J. Najita, A. Li, L. Hartmann et al
We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained with the Infrared Spectrograph on Spitzer. All but one of the Herbig stars show emission from PAHs, and seven of the spectra show PAH emission, but no silicate emission at 10 μm. The central wavelengths of the 6.2, 7.7-8.2, and 11.3 μm emission features decrease with stellar temperature, indicating that the PAHs are less photoprocessed in cooler radiation fields. The apparent low level of photoprocessing in HAeBe stars, relative to other PAH emission sources, implies that the PAHs are newly exposed to the UV-optical radiation fields from their host stars. HAeBe stars show a variety of PAH emission intensities and ionization fractions but a narrow range of PAH spectral classifications based on positions of major PAH feature centers. This may indicate that, regardless of their locations relative to the stars, the PAH molecules are altered by the same physical processes in the protoplanetary disks of intermediate-mass stars. Analysis of the mid-IR SEDs indicates that our sample likely includes both radially flared and more flattened/settled disk systems, but we do not see the expected correlation of overall PAH emission with disk geometry. We suggest that the strength of PAH emission from HAeBe stars may depend not only on the degree of radial flaring but also on the abundance of PAHs in illuminated regions of the disks and possibly on the vertical structure of the inner disk as well.
430
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Charles D. Dermer
An analysis of the interaction between a spherical relativistic blast-wave shell and a stationary cloud with a spherical cap geometry is performed assuming that the cloud width Δcl≪ x, where x is the distance of the cloud from the gamma-ray burst (GRB) explosion center. The interaction is divided into three phases: (1) a collision phase with both forward and reverse shocks; (2) a penetration phase when either the reverse shock has crossed the shell while the forward shock continues to cross the cloud, or vice versa; and (3) an expansion phase when, both shocks having crossed the cloud and shell, the shocked fluid expands. Temporally evolving spectral energy distributions (SEDs) are calculated for the problem of the interaction of a blast-wave shell with clouds that subtend large and small angles compared with the Doppler (-cone) angle θ0 = 1/Γ0, where Γ0 is the coasting Lorentz factor. The Lorentz factor evolution of the shell/cloud collision is treated in the adiabatic limit. The behavior of the light curves and SEDs on, e.g., Γ0, shell-width parameter η, where Δ0 + η_x_/Γ20 is the blast-wave shell width, and properties and locations of the cloud is examined. Short-timescale variability in GRB light curves, including ~100 keV γ-ray pulses observed with BATSE and delayed ~1 keV X-ray flares found with Swift, can be explained by emissions from an external shock formed by the GRB blast wave colliding with small density inhomogeneities in the "frozen-pulse" approximation (η → 0), and perhaps in the thin-shell approximation (η ≈ 1/Γ0), but not when η ≈ 1. If the frozen-pulse approximation is valid, then external shock processes could make the dominant prompt and afterglow emissions in GRB light curves consistent with short-delay two-step collapse models for GRBs.
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A. S. Almgren, J. B. Bell, A. Nonaka, and M. Zingale
We continue the description of a low Mach number hydrodynamics algorithm for reacting, full star flows. Here we demonstrate how to accurately incorporate reactions using a second-order accurate Strang-splitting technique. We also improve the fidelity of the model by allowing the base state to evolve in response to large-scale convection as well as large-scale heating, taking care to account for the compositional changes to the base state as well. The new algorithm is tested via comparisons with a fully compressible code and shown to be in good agreement. The resulting code, MAESTRO, once extended to incorporate a spherically symmetric base state, will be used to study the convection and ignition phases of Type Ia supernovae.
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E. Di Carlo, C. Corsi, A. A. Arkharov, F. Massi, V. M. Larionov, N. V. Efimova, M. Dolci, N. Napoleone, and A. Di Paola
In the framework of a program for the monitoring of supernovae in the near-infrared (NIR) carried out by the Teramo, Rome, and Pulkovo observatories with the AZT-24 telescope, we observed the supernova SN 2006jc in the J, H, and K photometric bands during a period of 7 months, starting ~36 days after its discovery. Our observations evidence a NIR rebrightening, peaking ~70 days after discovery, along with a reddening of (H − K) and (J − H) colors until 120 days after discovery. After that date, (J − H) seems to evolve toward bluer colors. Our data, complemented with IR and optical observations found in the literature, show that the rebrightening is produced by hot, newly formed dust surrounding the supernova.
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C.-Y. Ng, B. M. Gaensler, L. Staveley-Smith, R. N. Manchester, M. J. Kesteven, L. Ball, and A. K. Tzioumis
We present detailed Fourier modeling of the radio remnant of SN 1987A using high-resolution 9 and 18 GHz data taken with the Australia Telescope Compact Array over the period 1992-2008. We develop a parameterized three-dimensional torus model for the expanding radio shell, in which the emission is confined to an inclined equatorial belt; our model also incorporates both a correction for light-travel time effects and an overall east-west gradient in the radio emissivity. By deriving an analytic expression for the two-dimensional Fourier transform of the projected three-dimensional brightness distribution, we can fit our spatial model directly to the interferometric visibility data. This provides robust estimates of the radio morphology at each epoch. The best-fit results suggest a constant remnant expansion at 4000 ± 400 km s−1 over the 16 yr period covered by the observations. The model fits also indicate substantial midlatitude emission, extending to ±40° on either side of the equatorial plane. This likely corresponds to the extraplanar structure seen in Hα and Lyα emission from the supernova reverse shock, and broadly supports hydrodynamic models in which the complex circumstellar environment was produced by a progression of interacting winds from the progenitor. Our model quantifies the clear asymmetry seen in the radio images: we find that the eastern half of the radio remnant is consistently ~40% brighter than the western half at all epochs, which may result from an asymmetry in the ejecta distribution between these two hemispheres.
498
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Martin E. Pessah and Chi-kwan Chan
We carry out a comprehensive analysis of the behavior of the magnetorotational instability (MRI) in viscous, resistive plasmas. We find exact, nonlinear solutions of the nonideal magnetohydrodynamic (MHD) equations describing the local dynamics of an incompressible, differentially rotating background threaded by a vertical magnetic field when disturbances with wavenumbers perpendicular to the shear are considered. We provide a geometrical description of these viscous, resistive MRI modes and show how their physical structure is modified as a function of the Reynolds and magnetic Reynolds numbers. We demonstrate that when finite dissipative effects are considered, velocity and magnetic field disturbances are no longer orthogonal (as is the case in the ideal MHD limit) unless the magnetic Prandtl number is unity. We generalize previous results found in the ideal limit and show that a series of key properties of the mean Reynolds and Maxwell stresses also hold for the viscous, resistive MRI. In particular, we show that the Reynolds stress is always positive and the Maxwell stress is always negative. Therefore, even in the presence of viscosity and resistivity, the total mean angular momentum transport is always directed outward. We also find that, for any combination of the Reynolds and magnetic Reynolds numbers, magnetic disturbances dominate both the energetics and the transport of angular momentum and that the total mean energy density is an upper bound for the total mean stress responsible for angular momentum transport. The ratios between the Maxwell and Reynolds stresses and between magnetic and kinetic energy densities increase with decreasing Reynolds numbers for any magnetic Reynolds number; the lowest limit of both ratios is reached in the ideal MHD regime. The analytical results presented here provide new benchmarks for the various algorithms employed to solve the viscous, resistive MHD equations in the shearing box approximation.
515
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Wei Liu
In preparation for an experimental study of magnetorotational instability (MRI) in liquid metal, we present nonideal axisymmetric magnetohydrodynamic simulations of the nonlinear evolution of MRI in the experimental geometry. The simulations adopt fully insulating boundary conditions. No-slip conditions are imposed at all boundaries. A clear linear phase is observed with reduced linear growth rate. MRI results in an inflowing "jet" near the midplane and enhances the angular momentum transport at saturation.
525
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Randall L. Cooper
I construct a simple model of the convective burning layer during a type I X-ray burst to investigate the effects convection has on the stability of the layer to nonradial oscillations. A linear perturbation analysis demonstrates that the region is stable to nonradial oscillations when energy transport is convection-dominated, but it is unstable when energy transport is radiation-dominated. Thus, efficient convection always dampens oscillations. These results may explain the nondetection of oscillations during the peak of some X-ray bursts.
532
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Samar Safi-Harb and Harsha S. Kumar
PSR J1119–6127 is a high magnetic field (B = 4.1 × 1013 G), young (≤1700 year old), and slow (P = 408 ms) radio pulsar associated with the supernova remnant (SNR) G292.2–0.5. In 2003, Chandra allowed the detection of the X-ray counterpart of the radio pulsar and provided the first evidence for a compact and faint pulsar wind nebula (PWN). We here present new Chandra observations that allowed for the first time an imaging and spectroscopic study of the pulsar and PWN independently of each other. The PWN is only evident in the hard band (above ~2 keV) and consists of jetlike structures extending at least 7'' from the pulsar, with the southern "jet" being longer than the northern "jet." The spectrum of the PWN is described by a power law with a photon index Γ ∼ 1.1 for the compact PWN and ~1.4 for the southern long jet (at a column density _N_H = 1.8 × 1022 cm−2), and a total luminosity _L_X(0.5–7.0 keV) ∼ 4 × 1032 ergs s−1, at a distance of 8.4 kpc. We rule out a single blackbody model for the pulsar and present the first evidence of nonthermal emission that dominates above ~3 keV. A two-component model consisting of a power-law component (with photon index Γ ∼ 1.5–2.0) plus a thermal component provides the best fit. The thermal component can be fit by either a blackbody model with a temperature kT ∼ 0.21 keV, or a neutron star atmospheric model with a temperature kT ∼ 0.14 keV. The efficiency of the pulsar in converting its rotational power, , into nonthermal X-ray emission from the pulsar and PWN is ≈5 × 10−4, comparable to other rotation-powered pulsars with a similar . We discuss our results in the context of the X-ray manifestation of high magnetic field radio pulsars in comparison with rotation-powered pulsars and magnetars.
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O. Kargaltsev, Z. Misanovic, G. G. Pavlov, J. A. Wong, and G. P. Garmire
Chandra and XMM-Newton resolved extremely long tails behind two middle-aged pulsars, J1509–5850 and J1740+1000. The tail of PSR J1509–5850 is discernible up to 5.6' from the pulsar, which corresponds to the projected length _l_⊥ = 6.5_d_4 pc, where d = 4_d_4 kpc is the distance to the pulsar. The observed tail flux is 2 × 10−13 ergs s−1 cm−2 in the 0.5-8 keV band. The tail spectrum fits an absorbed power law (PL) with the photon index Γ = 2.3 ± 0.2 and 0.5-8 keV luminosity of 1 × 1033_d_42 ergs s−1, for _n_H = 2.1 × 1022 cm−2. The tail of PSR J1740+1000 is firmly detected up to 5' (_l_⊥ ∼ 2_d_1.4 pc), with a flux of 6 × 10−14 ergs cm−2 s−1 in the 0.4-10 keV band. The PL fit yields Γ = 1.4-1.5, _n_H ≈ 1 × 1021 cm−2, and an 0.4-10 keV luminosity of ~2 × 1031_d_1.42 ergs s−1. The large extent of the tails suggests that the bulk flow in the tails starts as mildly relativistic downstream of the termination shock and then gradually decelerates. Within the observed extent of the J1509–5850 tail, the average flow speed exceeds 5000 km s−1, and the equipartition magnetic field is a few × 10−5 G. For the J1740+1000 tail, the equipartition field is a factor of a few lower. For the high-latitude PSR J1740+1000, the orientation of the tail suggests that the pulsar was born from a halo-star progenitor. The X-ray efficiencies of the ram pressure-confined pulsar wind nebulae (PWNe) correlate poorly with the pulsar spin-down luminosities or ages. The efficiencies are systematically higher than those of PWNe around slowly moving pulsars with similar spin-down parameters.
558
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V. Piirola, T. Vornanen, A. Berdyugin, and G. V. Coyne, S.J.
Our simultaneous multicolor (UBVRI) circular polarimetry has revealed nearly sinusoidal variation over the WD spin cycle, and almost symmetric positive and negative polarization excursions. Maximum amplitudes are observed in the B and V bands (±3%). This is the first time that polarization peaking in the blue has been discovered in an intermediate polar (IP), and suggests that V405 Aur is the highest magnetic field IP found so far. The polarized flux spectrum is similar to those found in polars with magnetic fields in the range B ∼ 25-50 MG. Our low-resolution circular spectropolarimetry has given evidence of transient features which can be fitted by cyclotron harmonics n = 6, 7, and 8, at a field of B = 31.5 ± 0.8 MG, consistent with the broadband polarized flux spectrum. Timings of the circular polarization zero crossovers put strict upper limits on WD spin period changes, and indicate that the WD in V405 Aur is currently accreting closely at the spin equilibrium rate, with very long synchronization timescales, T s > 109 yr. For the observed spin to orbital period ratio, _P_spin/_P_orb = 0.0365, and _P_orb ∼ 4.15 hr, existing numerical accretion models predict spin equilibrium condition with B ∼ 30 MG if the mass ratio of the binary components is _q_1 ∼ 0.4. The high magnetic field makes V405 Aur a likely candidate as a progenitor of a polar.
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Hilding R. Neilson and John B. Lester
An analytical derivation is presented for computing mass-loss rates of Cepheids by using the method of Castor, Abbott, and Klein modified to include a term for momentum input from pulsation and shocks generated in the atmosphere. Using this derivation, mass-loss rates of Cepheids are determined as a function of stellar parameters. When applied to a set of known Cepheids, the calculated mass-loss rates range from 10−10 to 10−7_M_☉ yr−1, larger than if the winds were driven by radiation alone. Infrared excesses based on the predicted mass-loss rates are compared to observations from optical interferometry and IRAS, and predictions are made for Spitzer observations. The mass-loss rates are consistent with the observations, within the uncertainties of each. The rate of period change of Cepheids is discussed and shown to relate to mass loss, albeit the dependence is very weak. There is also a correlation between the large mass-loss rates and the Cepheids with slowest absolute rate of period change due to evolution through the instability strip. The enhanced mass loss helps illuminate the issue of infrared excess and the mass discrepancy found in Cepheids.
588
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Anna Frebel, Remo Collet, Kjell Eriksson, Norbert Christlieb, and Wako Aoki
We present a new abundance analysis of HE 1327–2326, which is currently the most iron-poor star, based on observational data obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES). We correct the one-dimensional (1D) LTE abundances for three-dimensional (3D) effects to provide an abundance pattern that supersedes previous works and should be used to observationally test current models of the chemical yields of the first-generation supernovae (SNe). Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D–1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 3360 Å is greatly improved due to the application of the 3D–1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected; the new Li upper limit is extremely low, A(Li) < 0.62, and in stark contrast with results not only from the Wilkinson Microwave Anisotropy Probe (WMAP) but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327–2326 is being reproduced by individual and integrated yields of SNe.
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L. D. Matthews, Y. Libert, E. Gérard, T. Le Bertre, and M. J. Reid
We report the detection of an H I counterpart to the extended, far-ultraviolet-emitting tail associated with the asymptotic giant branch star Mira (o Ceti). Using the Nançay Radio Telescope (NRT), we have detected emission as far as 88' north of the star, confirming that the tail contains a significant atomic component (_M_H I ~ 4 × 10−3_M_☉). The NRT spectra reveal a deceleration of the tail gas caused by interaction with the local interstellar medium. We estimate an age for the tail of ~1.2 × 105 yr, suggesting that the mass-loss history of Mira has been more prolonged than previous observational estimates. Using the Very Large Array we have also imaged the H I tail out to ~12' (0.4 pc) from the star. The detected emission shows a "head-tail" morphology, but with complex substructure. Regions with detected H I emission correlate with far-ultraviolet-luminous regions on large scales, but the two tracers are not closely correlated on smaller scales (≲1'). We propose that detectable tails of H I are likely to be a common feature of red giants undergoing mass loss.
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Christian Vollmer, Peter Hoppe, and Frank E. Brenker
We have measured the silicon isotopic compositions of 38 presolar silicate grains from the carbonaceous chondrite Acfer 094, which have been previously studied for oxygen isotopes. The goals of this study are to further disentangle stellar sources of the 18O-enriched (Group IV) and the most 17O-enriched (Group I) silicate grains and to put further constraints on the Galactic chemical evolution (GCE) of the Si isotopes. Our results show that the majority (6 out of 8) of the 18O enriched silicates have enhanced 28Si, in qualitative agreement with the signature of presolar silicon carbide (SiC) X grains from supernovae. Three highly 17O-enriched grains (17O/16O > 3 × 10−3) have close to solar 29Si/28Si but enhanced 30Si/28Si, possibly indicating an origin in binary systems. Alternative stellar sources are 3.5-4 _M_☉ asymptotic giant branch (AGB) stars. Si isotopic compositions of the majority of presolar silicates fall along the SiC mainstream line, although most grains plot to the 30Si-poor side of this line. Most presolar silicate grains therefore formed in red giant branch (RGB)/AGB stars of roughly solar metallicity, and incorporated less (or no) Si processed by slow neutron capture reactions ("_s_-process") than presolar SiC grains, because silicates form before large amounts of 12C and _s_-processed material has been dredged up to the surface. The inferred shift of the Si isotopes due to the different dredge-up of matter from the He intershell in C-rich AGB stars is Δ29Si = 3‰-21‰ and Δ30Si = 17‰-30‰, which is compatible with predictions for 1.5-3 _M_☉ AGB stars of solar metallicity when C/O > 1.
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S. N. Milam, D. T. Halfen, E. D. Tenenbaum, A. J. Apponi, N. J. Woolf, and L. M. Ziurys
Millimeter-wave observations of PN, CP, and HCP have been carried out toward circumstellar envelopes of evolved stars using the Arizona Radio Observatory (ARO). HCP and PN have been identified in the carbon-rich source CRL 2688 via observations at 1 mm using the Submillimeter Telescope (SMT) and 2-3 mm with the Kitt Peak 12 m. An identical set of measurements were carried out toward IRC +10216, as well as observations of CP at 1 mm. PN was also observed toward VY Canis Majoris (VY CMa), an oxygen-rich supergiant star. The PN and HCP line profiles in CRL 2688 and IRC +10216 are roughly flat topped, indicating unresolved, optically thin emission; CP, in contrast, has a distinct "U" shape in IRC +10216. Modeling of the line profiles suggests abundances, relative to H2, of f(PN) ∼ (3–5) × 10−9 and f(HCP) ∼ 2 × 10−7 in CRL 2688, about an order of magnitude higher than in IRC +10216. In VY CMa, f(PN) is ~4 × 10−8. The data in CRL 2688 and IRC +10216 are consistent with LTE formation of HCP and PN in the inner envelope, as predicted by theoretical calculations, with CP a photodissociation product at larger radii. The observed abundance of PN in VY CMa is a factor of 100 higher than LTE predictions. In IRC +10216, the chemistry of HCP/CP mimics that of HCN/CN and suggests an N2 abundance of f ∼ 1 × 10−7. The chemistry of phosphorus appears active in both carbon- and oxygen-rich envelopes of evolved stars.
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Pavel A. Denissenkov and Marc Pinsonneault
We examine the stability and observational consequences of mixing induced by 3He burning in the envelopes of first ascent red giants. We demonstrate that there are two unstable modes: a rapid, nearly adiabatic mode that we cannot identify with an underlying physical mechanism, and a slow, nearly radiative mode that can be identified with thermohaline convection. We present observational constraints that make the operation of the rapid mode unlikely to occur in real stars. Thermohaline convection turns out to be fast enough only if fluid elements have fingerlike structures with a length-to-diameter ratio l/d ≳ 10. We identify some potentially serious obstacles for thermohaline convection as the predominant mixing mechanism for giants. We show that rotation-induced horizontal turbulent diffusion may suppress the 3He-driven thermohaline convection. Another potentially serious problem for it is to explain observational evidence of enhanced extra mixing. The 3He exhaustion in stars approaching the red giant branch (RGB) tip should make the 3He mixing inefficient on the asymptotic giant branch (AGB). In spite of this, there are observational data indicating the presence of extra mixing in low-mass AGB stars similar to that operating on the RGB. Overmixing may also occur in carbon-enhanced metal-poor stars.
635
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Cullen H. Blake, Guillermo Torres, Joshua S. Bloom, and B. Scott Gaudi
We present observations of a new low-mass, double-lined eclipsing binary system discovered using repeat observations of the celestial equator from the Sloan Digital Sky Survey II. Using near-infrared photometry and optical spectroscopy we have measured the properties of this short-period [P = 0.407037(14) days] system and its two components. We find the following parameters for the two components: _M_1 = 0.272 ± 0.020 _M_☉, _R_1 = 0.268 ± 0.010 _R_☉, _M_2 = 0.240 ± 0.022 _M_☉, _R_2 = 0.248 ± 0.0090 _R_☉, _T_1 = 3320 ± 130 K, and _T_2 = 3300 ± 130 K. The masses and radii of the two components of this system agree well with theoretical expectations based on models of low-mass stars, within the admittedly large errors. Future synoptic surveys like Pan-STARRS and LSST will produce a wealth of information about low-mass eclipsing systems and should make it possible, with an increased reliance on follow-up observations, to detect many systems with low-mass and substellar companions. With the large numbers of objects for which these surveys will produce high-quality photometry, we suggest that it becomes possible to identify such systems even with sparse time sampling and a relatively small number of individual observations.
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G. Hallinan, A. Antonova, J. G. Doyle, S. Bourke, C. Lane, and A. Golden
We report on radio observations of the M8.5 dwarf LSR J1835+3259 and the L3.5 dwarf 2MASS J00361617+1821104, which provide the strongest evidence to date that the electron cyclotron maser instability is the dominant mechanism producing radio emission in the magnetospheres of ultracool dwarfs. As has previously been reported for the M9 dwarf TVLM 513–46546, periodic pulses of 100% circularly polarized, coherent radio emission are detected from both dwarfs with periods of 2.84 ± 0.01 and 3.08 ± 0.05 hr, respectively, for LSR J1835+3259 and 2MASS J00361617+1821104. Importantly, periodic unpolarized radio emission is also detected from 2MASS J00361617+1821104, and brightness temperature limitations rule out gyrosynchrotron radiation as a source of this radio emission. The unpolarized emission from this and other ultracool dwarfs is also attributed to electron cyclotron maser emission, which has become depolarized on traversing the ultracool dwarf magnetosphere, possibly due to propagations effects such as scattering. Based on available _v_sin i data in the literature and rotation periods derived from the periodic radio data for the three confirmed sources of electron cyclotron maser emission, TVLM 513–46546, LSR J1835+3259, and 2MASS J00361617+1821104, we determine that the rotation axes of all three dwarfs are close to perpendicular to our line of sight. This suggests a possible geometrical selection effect due to the inherent directivity of electron cyclotron maser emission, that may account for the previously reported relationship between radio activity and _v_sin i observed for ultracool dwarfs. We also determine the radius of the dwarf LSR J1835+3259 to be ≥0.117 ± 0.012 _R_☉. The implied size of the radius, together with the bolometric luminosity of the dwarf, suggests that either LSR J1835 is a young- or intermediate-age brown dwarf, or that current theoretical models underestimate the radii of ultracool dwarfs.
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K. L. Luhman and A. A. Muench
We have used images obtained with the Infrared Array Camera and the Multiband Imaging Photometer on board the Spitzer Space Telescope to search for low-mass stars and brown dwarfs with circumstellar disks in the Chamaeleon I star-forming region. Through optical spectroscopy of sources with red colors in these data, we have identified seven new disk-bearing members of the cluster. Three of these objects are probably brown dwarfs, according to their spectral types (M8, M8.5, M8-L0). Three of the other new members may have edge-on disks, based on the shapes of their infrared spectral energy distributions. One of the possible edge-on systems has a steeply rising slope from 4.5 to 24 μm, indicating that it could be a Class I source (star+disk+envelope) rather than a Class II source (star+disk). If so, then it would be one of the least massive known Class I protostars (M5.75, M ∼ 0.1 _M_☉).
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D. P. Bennett, I. A. Bond, A. Udalski, T. Sumi, F. Abe, A. Fukui, K. Furusawa, J. B. Hearnshaw, S. Holderness, Y. Itow et al
We report the detection of an extrasolar planet of mass ratio q ∼ 2 × 10−4 in microlensing event MOA-2007-BLG-192. The best-fit microlensing model shows both the microlensing parallax and finite source effects, and these can be combined to obtain the lens masses of M = 0.060+ 0.028−0.021_M_☉ for the primary and m = 3.3+ 4.9−1.6_M_⊕ for the planet. However, the observational coverage of the planetary deviation is sparse and incomplete, and the radius of the source was estimated without the benefit of a source star color measurement. As a result, the 2 σ limits on the mass ratio and finite source measurements are weak. Nevertheless, the microlensing parallax signal clearly favors a substellar mass planetary host, and the measurement of finite source effects in the light curve supports this conclusion. Adaptive optics images taken with the Very Large Telescope (VLT) NACO instrument are consistent with a lens star that is either a brown dwarf or a star at the bottom of the main sequence. Follow-up VLT and/or Hubble Space Telescope (HST) observations will either confirm that the primary is a brown dwarf or detect the low-mass lens star and enable a precise determination of its mass. In either case, the lens star, MOA-2007-BLG-192L, is the lowest mass primary known to have a companion with a planetary mass ratio, and the planet, MOA-2007-BLG-192Lb, is probably the lowest mass exoplanet found to date, aside from the lowest mass pulsar planet.
684
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Cheongho Han
We investigate the characteristic of microlensing signals of Earth-like moons orbiting ice-giant planets. From this, we find that nonnegligible satellite signals occur when the planet-moon separation is similar to or greater than the Einstein radius of the planet. We find that the satellite signal does not diminish with the increase of the planet-moon separation beyond the Einstein radius of the planet unlike the planetary signal which vanishes when the planet is located well beyond the Einstein radius of the star. We also find that the satellite signal tends to have the same sign as that of the planetary signal. These tendencies are caused by the lensing effect of the star on the moon in addition to the effect of the planet. We determine the range of satellite separations where the microlensing technique is optimized for the detections of moons. By setting an upper limit as the angle-average of the projected Hill radius and a lower limit as the half of the Einstein radius of the planet, we find that the microlensing method would be sensitive to moons with projected separations from the planet of 0.05 AU ≲ d _p_≲ 0.24 AU for a Jupiter-mass planet, 0.03 AU ≲ d _p_≲ 0.17 AU for a Saturn-mass planet, and 0.01 AU ≲ d _p_≲ 0.08 AU for a Uranus-mass planet. We compare the characteristics of the moons to be detected by the microlensing and transit techniques.
691
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José A. Robles, Charles H. Lineweaver, Daniel Grether, Chris Flynn, Chas A. Egan, Michael B. Pracy, Johan Holmberg, and Esko Gardner
If the origin of life and the evolution of observers on a planet is favored by atypical properties of a planet's host star, we would expect our Sun to be atypical with respect to such properties. The Sun has been described by previous studies as both typical and atypical. In an effort to reduce this ambiguity and quantify how typical the Sun is, we identify 11 maximally independent properties that have plausible correlations with habitability and that have been observed by, or can be derived from, sufficiently large, currently available, and representative stellar surveys. By comparing solar values for the 11 properties to the resultant stellar distributions, we make the most comprehensive comparison of the Sun to other stars. The two most atypical properties of the Sun are its mass and orbit. The Sun is more massive than 95% ± 2% of nearby stars, and its orbit around the Galaxy is less eccentric than 93% ± 1% of FGK stars within 40 pc. Despite these apparently atypical properties, a χ2 analysis of the Sun's values for 11 properties, taken together, yields a solar χ2☉ = 8.39 ± 0.96. If a star is chosen at random, the probability that it will have a lower value (i.e., be more typical) than the Sun, with respect to the 11 properties analyzed here, is only 29% ± 11% . These values quantify, and are consistent with, the idea that the Sun is a typical star. If we have sampled all reasonable properties associated with habitability, our result suggests that there are no special requirements for a star to host a planet with life.
707
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Uri Feldman, Yuri Ralchenko, and Enrico Landi
The spectral range of the Hinode EIS (EUV Imaging Spectrometer) instrument is 170-211 Å and 245-291 Å; lines emitted by low-energy levels in highly ionized Mn, Fe, Co, and Ni, as well as the very energetic 1s2s3_S_1–1s2p3_P_2 transition in Fe XXV, appear in the EIS range. In this paper, we investigate the use of these lines for detecting the presence of nonthermal, high-energy electrons in flares. We first calculate line fluxes among the various lines expected in the EIS range, assuming that the electron velocity distribution is strictly Maxwellian. Then, we calculate line fluxes using a velocity distribution composed of a Maxwellian distribution plus an additional population of electrons with a temperature of 10 keV (1.2 × 108 K) providing 1%, 2%, 4%, 7%, and 10% of the total free electrons. The calculations indicate that flux ratios between the highly excited Fe XXV line and lines originating in low-lying levels of other highly ionized ions in the EIS range could shed light on the electron velocity distribution in hot flare plasmas.
715
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Fabio Reale and Salvatore Orlando
Impulsive nanoflares are expected to transiently heat the plasma confined in coronal loops to temperatures of the order of 10 MK. Such hot plasma is hardly detected in quiet and active regions, outside flares. During rapid and short heat pulses in rarefied loops, the plasma can be highly out of equilibrium in ionization. Here we investigate the effects of the nonequilibrium of ionization (NEI) on the detection of hot plasma in coronal loops. Time-dependent loop hydrodynamic simulations are specifically devoted to this task, including saturated thermal conduction, and coupled to the detailed solution of the equations of the ionization rate for several abundant elements. In our simulations, initially cool and rarefied magnetic flux tubes are heated to 10 MK by nanoflares deposited either at the footpoints or at the loop apex. We test for different pulse durations and find that, due to NEI effects, the loop plasma may never be detected at temperatures above ~5 MK for heat pulses shorter than about 1 minute. We discuss some implications in the framework of multistranded nanoflare-heated coronal loops.
725
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R. Soler, R. Oliver, and J. L. Ballester
High-resolution observations show that oscillations and waves in prominence threads are common and that they are attenuated in a few periods. In addition, observers have also reported the presence of material flows in such prominence fine-structures. Here we investigate the time damping of nonleaky oscillations supported by a homogeneous cylindrical prominence thread embedded in an unbounded corona and with a steady mass flow. Thermal conduction and radiative losses are taken into account as damping mechanisms, and the effect of these nonideal effects and the steady flow on the attenuation of oscillations is assessed. We solve the general dispersion relation for linear, nonadiabatic magnetoacoustic and thermal waves supported by the model and find that slow and thermal modes are efficiently attenuated by nonadiabatic mechanisms. On the contrary, fast kink modes are much less affected and their damping times are much larger than those observed. The presence of flow has no effect on the damping of slow and thermal waves, whereas fast kink waves are more (less) attenuated when they propagate parallel (antiparallel) to the flow direction. Although the presence of steady mass flows improves the efficiency of nonadiabatic mechanisms on the attenuation of transverse, kink oscillations for propagation parallel to the flow, its effect is still not enough to obtain damping times compatible with observations.
736
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H. Watanabe, R. Kitai, K. Okamoto, K. Nishida, J. Kiyohara, S. Ueno, M. Hagino, T. T. Ishii, and K. Shibata
A high spatial resolution observation of an emerging flux region (EFR) was made using a vector magnetograph and a Hα Lyot filtergraph with the Domeless Solar Telescope at Hida Observatory on 2006 October 22. In Hα wing images, we could see many Ellerman bombs (EBs) in the EFR. Observations in two modes, slit scan and slit fixed, were performed with the vector magnetograph, along with the Hα filtergraph. Using the Hα wing images, we detected 12 EBs during the slit scan observation period and 9 EBs during the slit fixed observation period. With the slit scan observation, we found that all the EBs were distributed in the area where the spatial gradient of vertical field intensity was large, which indicates the possibility of rapid topological change in the magnetic field in the area of EBs. With the slit fixed observation, we found that EBs were distributed in the areas of undulatory magnetic fields, in both the vertical and horizontal components. This paper is the first to report the undulatory pattern in the horizontal components of the magnetic field, which is also evidence for emerging magnetic flux triggered by the Parker instability. These results allow us to confirm the association between EBs and emerging flux tubes. Three triggering mechanisms for EBs are discussed with respect to emerging flux tubes: 9 out of 21 EBs occurred at the footpoints of emerging flux tubes, 8 occurred at the top of emerging flux tubes, and 4 occurred in the unipolar region. Each case can be explained by magnetic reconnection in the low chromosphere.
747
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Lirong Tian, David Alexander, and Richard Nightingale
Exploring the origins of coronal helicity and energy, as well as determining the mechanisms that lead to coronal energy release, is a fundamental topic in solar physics. Using MDI 96 minute line-of-sight and HSOS vector magnetograms in conjunction with TRACE white-light and UV (1600 Å) images and BBSO Hα and SOHO EIT (195 Å) images, we find in active region NOAA 10030 that a large positive polarity sunspot, located in the center of the region, exhibited significant counterclockwise rotation, which continued for 6 days during the period 2002 July 12-18. This rotating sunspot was related to the formation of inverse--shaped filaments, left-handed twist of the vector magnetic fields, and the production of strong negative vertical current, but exhibited little emergence of magnetic flux. In all, five M-class and two X-class flares were produced around this rotating sunspot over the 6 day period. The observed characteristics of the strongly rotating sunspot suggest that sunspots can undergo strong intrinsic rotation, the source of which may originate below the photosphere and can play a significant role in helicity production and injection and energy buildup in the corona. A sunspot with negative magnetic polarity showed fast and significant emergence in the eastern portion of the active region, and moved northeastward over several days, but exhibited little rotation. The moving sunspot also exhibited the formation of inverse--shaped filaments, left-handed twist of vector magnetic fields and coronal structure, and the production of stronger positive current. The observed characteristics of the emerging sunspot suggest that significant emergence of twisted magnetic fields may not always result in the rotation of the associated sunspots, but they do play a very important role in the coronal helicity accumulation and free-energy build-up.
757
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Pavel A. Denissenkov, Marc Pinsonneault, and Keith B. MacGregor
Internal gravity waves (IGWs) are naturally produced by convection in stellar envelopes, and they could be an important mechanism for transporting angular momentum in the radiative interiors of stars. Prior work has established that they could operate over a short enough timescale to explain the internal solar rotation as a function of depth. We demonstrate that the natural action of IGWs is to produce large-scale oscillations in the solar rotation as a function of depth, which is in marked contrast to the nearly uniform rotation in the outer radiative envelope of the Sun. An additional angular momentum transport mechanism is therefore required, and neither molecular nor shear-induced turbulent viscosity is sufficient to smooth out the profile. Magnetic processes, such as the Tayler-Spruit dynamo, could flatten the rotation profile. We therefore conclude that IGWs must operate in conjunction with magnetic angular momentum transport processes if they operate at all. Furthermore, both classes of mechanisms must be inhibited to some degree by mean molecular weight gradients in order to explain the recent evidence for a rapidly rotating embedded core in the Sun, should it be confirmed by a further analysis of solar _g_-mode oscillations.
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Carmelita Carbone, Licia Verde, and Sabino Matarrese
We forecast constraints on primordial non-Gaussianity achievable from forthcoming surveys by exploiting the scale-dependent halo bias introduced on large scales by non-Gaussian initial conditions. We explore the performance of exploiting both the shape of the galaxy power spectrum on large scales and the cross-correlation of galaxies with cosmic microwave background maps through the integrated Sachs-Wolfe effect. We find that future surveys can detect primordial non-Gaussianity of the local form with a non-Gaussianity parameter | _f_NL| of order unity. This is particularly exciting because, while the simplest single-field slow-roll models of inflation predict a primordial | _f_NL| ≪ 1, this signal sources extra contributions to the effective _f_NL of large-scale structures that are expected to be above our predicted detection threshold.
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Noam Soker
I propose an explanation for the finding that star formation and visible filaments strong in Hα emission in cooling flow clusters occur only if the minimum specific entropy and the radiative cooling time of the intracluster medium (ICM) are below a specific threshold. The explanation is based on the cold feedback mechanism. In this mechanism, the mass accreted by the central black hole originates in nonlinear overdense blobs of gas residing in an extended region of the cooling flow region. I use the criterion that the feedback cycle period must be longer than the radiative cooling time of dense blobs, for large quantities of gas to cool to low temperatures. The falling time of the dense blobs is parameterized by the ratio of the infall velocity to the sound speed. Another parameter is the ratio of the blobs' density to that of the surrounding ICM. By taking the values of the parameters as in previous papers on the cold feedback model, I derive an expression that gives the right value of the entropy threshold. Future studies will have to examine in more detail the role these parameters play, and will have to show that the observed sharp change in the behavior of clusters across the entropy, or radiative cooling time, threshold can be reproduced by the model.
L9
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Stephanie Tonnesen and Greg L. Bryan
Cluster galaxies moving through the intracluster medium (ICM) are expected to lose some of their interstellar medium (ISM) through ram pressure stripping and related ISM-ICM interactions. Using high-resolution cosmological simulations of a large galaxy cluster including star formation, we show that the ram pressure a galaxy experiences at a fixed distance from the cluster center can vary by well over an order of magnitude We find that this variation in ram pressure is due in almost equal parts to variation in the ICM density and in the relative velocity between the galaxy and the ICM. We also find that the ICM and galaxy velocities are weakly correlated for infalling galaxies.
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Devdeep Sarkar, Alexandre Amblard, Asantha Cooray, and Daniel E. Holz
Recent work suggests that Type Ia supernovae (SNe) are composed of two distinct populations: prompt and delayed. By explicitly incorporating properties of host galaxies, it may be possible to target and eliminate systematic differences between these two putative populations. However, any resulting _post_-calibration shift in luminosity between the components will cause a redshift-dependent systematic shift in the Hubble diagram. Utilizing an existing sample of 192 SNe Ia, we find that the average luminosity difference between prompt and delayed SNe is constrained to be (4.5 ± 8.9)%. If the absolute difference between the two populations is 0.025 mag, and this is ignored when fitting for cosmological parameters, then the dark energy equation of state (EOS) determined from a sample of 2300 SNe Ia is biased at ~1 σ. By incorporating the possibility of a two-population systematic, this bias can be eliminated. However, assuming no prior on the strength of the two-population effect, the uncertainty in the best-fit EOS is increased by a factor of 2.5, when compared to the equivalent sample with no underlying two-population systematic. To avoid introducing a bias in the EOS parameters, or significantly degrading the measurement accuracy, it is necessary to control the postcalibration luminosity difference between prompt and delayed SN populations to better than 0.025 mag.
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Pieter Buyle, Sven De Rijcke, and Herwig Dejonghe
We present high spatial resolution 21 cm H I observations of EA01A and EA01B, a pair of interacting poststarburst, or E+A, galaxies at z = 0.0746. Based on optical HST/WFPC2 images, both galaxies are known to display disturbed morphologies. They also appear to be linked by a bridge of stars. Previous H I observations by Chang et al. in 2001 had already uncovered sizable quantities of neutral gas in or near these galaxies, but they lacked the spatial resolution to locate the gas with any precision within this galactic binary system. We have analyzed deep, high-resolution archival VLA observations of the couple. We find evidence for three gaseous tidal tails: one connected to EA01A and two emanating from EA01B. These findings confirm, independently from the optical imaging, that (i) EA01A and EA01B are actively interacting, and that, as a consequence, the starbursts that occurred in these galaxies were most likely triggered by this interaction, and that (ii) 6.6 ± 0.9 × 109_M_☉ of neutral gas are still present in the immediate vicinity of the optical bodies of both galaxies. The H I column density is lowest at the optical positions of the galaxies, suggesting that most of the neutral gas that is visible in our maps is associated with the tidal arms and not with the galaxies themselves. This might provide an explanation for the apparent lack of ongoing star formation in these galaxies.
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Eva Schinnerer, Torsten Böker, David S. Meier, and Daniela Calzetti
Using new, high-resolution interferometric observations of the CO and HCN molecules, we directly compare the molecular and ionized components of the interstellar medium in the center of the nearby spiral galaxy IC 342, on spatial scales of ≈10 pc. The morphology of the tracers suggests that the molecular gas flow caused by a large-scale stellar bar has been strongly affected by the mechanical feedback from recent star formation activity within the central 100 pc in the nucleus of the galaxy. Possibly, stellar winds and/or supernova shocks originating in the nuclear star cluster have compressed, and likely pushed outward, the infalling molecular gas, thus significantly reducing the gas supply to the central 10 pc. Although our analysis currently lacks kinematic confirmation due to the face-on orientation of IC 342, the described scenario is supported by the generally observed repetitive nature of star formation in the nuclear star clusters of late-type spiral galaxies.
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Vito Mennella
We present the results of an experimental study of the formation of HD molecules during exposure of hydrogenated/deuterated nano-sized carbon grains to D/H atoms. The effects of H/D irradiation have been analyzed with infrared spectroscopy and mass spectroscopy. The formation process has been investigated for grain and atom temperatures relevant to astrophysical environments such as diffuse clouds and photodissociation regions. The results indicate that HD formation occurs by abstraction of hydrogen atoms chemisorbed in the aliphatic CH2,3 groups of carbon grains. The formation process does not depend on the atom energy (activationless process), while a dependence of the formation rate on grain temperature is observed.
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J. Nordhaus, M. Busso, G. J. Wasserburg, E. G. Blackman, and S. Palmerini
The available information on isotopic abundances in the atmospheres of low-mass red giant branch (RGB) and asymptotic giant branch (AGB) stars requires that episodes of extensive mixing occur below the convective envelope, reaching down to layers close to the hydrogen burning shell (cool bottom processing). Recently, it was suggested that dynamo-produced buoyant magnetic flux tubes could provide the necessary physical mechanisms and also supply sufficient transport rates. Here, we present an α-Ω dynamo in the envelope of an RGB/AGB star in which shear and rotation drain via turbulent dissipation and Poynting flux. In this context, if the dynamo is to be sustained throughout either phase, convection must resupply shear. Under this condition, volume-averaged peak toroidal field strengths of ⟨ _B_ϕ⟩ ≃ 3 × 103 G (RGB) and ⟨ _B_ϕ⟩ ≃ 5 × 103 G (AGB) are possible at the base of the convection zone. If the magnetic fields are concentrated in flux tubes, the corresponding field strengths are comparable to those required by cool bottom processing.
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Sébastien Muller, Dinh-V-Trung, Jin-Hua He, and Jeremy Lim
We report high angular resolution observations of the HCN (3-2) line emission in the circumstellar envelope of the O-rich star W Hya with the Submillimeter Array. The proximity of this star allows us to image its molecular envelope with a spatial resolution of just ~40 AU, corresponding to about 10 times the stellar diameter. We resolve the HCN (3-2) emission and find that it is centrally peaked and has a roughly spherically symmetrical distribution. This shows that HCN is formed in the innermost region of the envelope (within ~10 stellar radii), which is consistent with predictions from pulsation-driven shock chemistry models, and rules out the scenario in which HCN forms through photochemical reactions in the outer envelope. Our model suggests that the envelope decreases steeply in temperature and increases smoothly in velocity with radius, inconsistent with the standard model for mass-loss driven by radiative pressure on dust grains. We detect a velocity gradient of ~5 km s−1 in the northwest-southeast direction over the central 40 AU. This velocity gradient is reminiscent of that seen in OH maser lines, and could be caused by the rotation of the envelope or by a weak bipolar outflow.
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Peter Plavchan, Alan H. Gee, Karl Stapelfeldt, and Andrew Becker
We present the discovery of 130.87 day periodic near-infrared flux variability for the Class II T Tauri star WL 4 (=2MASS J16271848–2429059, ISO-Oph 128). Our data are from the 2MASS Calibration Point Source Working Database and constitute 1580 observations in J, H, and K s of a field in ρ Ophiuchus used to calibrate the 2MASS All-Sky Survey. We identify a light curve for WL 4 with eclipse amplitudes of ~0.4 mag lasting more than one-quarter the period and color variations in J–H and H_–_K s of ~0.1 mag. The long period cannot be explained by stellar rotation. We propose that WL 4 is a triple YSO system, with an inner binary orbital period of 130.87 days. We posulate that we are observing each component of the inner binary alternately being eclipsed by a circumbinary disk with respect to our line of sight. This system will be useful in investigating terrestrial-zone YSO disk properties and dynamics at ~1 Myr.
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John H. Debes, Alycia J. Weinberger, and Inseok Song
We report HST NICMOS coronagraphic images of the HD 15115 circumstellar disk at 1.1 μm. We find a similar morphology to that seen in the visible and at H band—an edge-on disk that is asymmetric in surface brightness. Several aspects of the 1.1 μm data are different, highlighting the need for multiwavelength images of each circumstellar disk. We find a flattening to the western surface brightness profile at 1.1 μm interior to 2'' (90 AU) and a warp in the western half of the disk. We measure the surface brightness profiles of the two disk lobes and create a measure of the dust scattering efficiency between 0.55 and 1.65 μm at 1'', 2'', and 3''. At 2'' the western lobe has a neutral spectrum up to 1.1 μm and a strong absorption or blue spectrum >1.1 μm, while a blue trend is seen in the eastern lobe. At 1'' the disk has a red F 110W − H color in both lobes.
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Noriyuki Narukage, Takako T. Ishii, Shin'ichi Nagata, Satoru UeNo, Reizaburo Kitai, Hiroki Kurokawa, Maki Akioka, and Kazunari Shibata
We discovered three successive Moreton waves generated by a single solar flare on 2005 August 3. Although this flare was not special in magnitude or configuration, Moreton waves (shock waves) successively occurred three times. Multiple shock waves generated during a single flare have not been reported before. Furthermore, the faster second-generated Moreton wave caught up and merged with the slower first-generated one. This is the first report of shock-shock interaction associated with a solar flare. The shock-plasma interaction was also detected. When the third-generated Moreton wave passed through an erupting filament, the filament was accelerated by the Moreton wave. In this event, filaments also erupted three times. On the basis of this observation, we consider that filament eruption is indispensable to the generation of Moreton waves.
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L. Jacoutot, A. G. Kosovichev, A. Wray, and N. N. Mansour
We have used 3D, compressible, nonlinear radiative magnetohydrodynamics simulation to study the influence of magnetic fields of various strengths on convective cells and on the excitation mechanisms of acoustic oscillations by calculating the spectral properties of the convective motions and oscillations. The results reveal substantial changes of the granulation structure with increased magnetic field and a frequency-dependent reduction in the oscillation power in a good agreement with solar observations. These simulations suggest that the enhanced high-frequency acoustic emission at the boundaries of active regions ("acoustic halo" phenomenon) is caused by changes of the spatial-temporal spectrum of turbulent convection in a magnetic field, resulting in turbulent motions of smaller scales and higher frequencies than in quiet-Sun regions.
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F. Moreno, J. L. Ortiz, P. Santos-Sanz, N. Morales, M. J. Vidal-Núñez, L. M. Lara, and P. J. Gutiérrez