Tabetha Hole | Weber State University (original) (raw)

Papers by Tabetha Hole

Observations of polarization in supernovae (SNe) at early times have indicated the presence of as... more Observations of polarization in supernovae (SNe) at early times have indicated the presence of asymmetries that vary in both magnitude and orientation for different elements in the ejecta. One explanation for these observations is local chemical inhomogeneities ("clumps") in the ejecta above the region which forms the continuum. To understand the effects of clumpiness on spectropolarimetric observations, we have used a fast, flexible, approximate semi-analytic code for modeling polarized line radiative transfer within three-dimensional inhomogeneous rapidly expanding atmospheres to model a wide range of clump parameter space. The results of the model make predictions that constrain the size and number of clumps that could lead to a given polarimetric signature. This talk will present a test of the model and the resulting predictions for VLT spectropolarimetric observations of SN2006X. This project was funded by the National Science Foundation grant AST-0807664 and through the Research Experiences for Undergraduates (REU) program grant NSF AST-1004872.

Circular orbits prevail among the shortest period binary stars as a consequence of tidal interact... more Circular orbits prevail among the shortest period binary stars as a consequence of tidal interactions between the stars. The longest period circular orbit in a coeval binary population, known as the "tidal cutoff period", is a measure of the tidal circularization rate integrated over the lifetime of the population. As such, determination of tidal cutoff periods as a function of age provides a critical test for the theories of tidal circularization. As part of the WIYN Open Cluster Study (WOCS), we have used the WIYN 3.5m telescope and Hydra multiobject spectrograph since 1996 to obtain precise radial velocities for late-type main sequence stars in the open clusters NGC 2168 (age ~ 0.15 Gyr) and NGC188 (age ~ 7 Gyr). Well defined tidal circularization cutoff periods are found at periods of 10.3 days and 14.9 days for NGC 2168 and NGC 188, respectively. Combined with previously determined tidal circularization cutoff periods of 7.7 days among pre-main-sequence stars, of 8.5 days in the Hyades at an age of ~ 0.6 Gyr and of 12.4 days in NGC 2682 (M67) at an age of ~ 4 Gyr, a progression of tidal circularization cutoff period with age is clear for ages greater than ~ 1 Gyr. That this trend does not continue to younger binary populations suggests that pre-main-sequence tidal circularization processes may set an initial cutoff period at 8-10 days. We are also nearing completion of a radial-velocity survey of NGC6819 (age ~ 1.5 Gyr). Our first results also will be reported in this paper. This work is supported by NSF AST 9731302.

The quality and quantity of polarimetric data being collected for stellar sources creates new opp... more The quality and quantity of polarimetric data being collected for stellar sources creates new opportunities for studying stellar properties and evolution, and also leads to new challenges for modeling and interpreting such data. Inspired by fresh prospects for detecting the Hanle effect to study photospheric magnetic fields, we have focused attention on purely geometrical aspects for polarimetric variability in the example of oblique magnetic rotators. In the case of axisymmetric fields, we highlight two key facts: (a) polarimetric lightcurves necessarily exhibit a certain time symmetry with rotation phase, and (b) variations in the polarization position angle can be modeled based on geometrical projection effects, independent of the photospheric magnetic field. These conclusions also have general applicability, such as to Thomson scattering and the transverse Zeeman effect. The authors gratefully acknowledge that funding for this work was provided by the National Science Foundation, grant AST-0807664.

We present results of a radial-velocity study of the 2.5 Gyr old open cluster NGC 6819. We have 5... more We present results of a radial-velocity study of the 2.5 Gyr old open cluster NGC 6819. We have 5140 radial velocity measurements of 1140 stars, obtained over the course of approximately 5 years with the WIYN Multiple-Object Spectrograph. The standard deviation of the measurements for a single (formally, non-variable) star is 0.5 km/s. We have measured velocities for the brighter evolved stars and main sequence stars to V=16.5. We are very nearly complete for stars above the main sequence and approximately 50 on the main sequence. We find the cluster mean velocity to be 2.3 km/s with an observed dispersion of 1.5 km/s; we anticipate that the true cluster velocity dispersion is smaller. We discuss the distribution of velocities in the cluster field of view, and related issues of membership selection. We present a CMD of all data, and a "clean" CMD of cluster member stars. Our later measurements have emphasized velocity-variable stars, and we have orbital solutions for 35 binaries. Our solutions to the orbits of these binaries will allow us to investigate the evolution of dynamical properties of the systems. Of particular interest is the relationship of eccentricity to period, which indicates the tidal circularization cutoff for the cluster. This research was funded in part by NSF AST 9731302, Wisconsin Space Grant Consortium, and Oberlin College.

Recent studies have established a correlation between stars with extrasolar planets and enhanced ... more Recent studies have established a correlation between stars with extrasolar planets and enhanced observed metallicity as compared with the general population. One explanation for the enhancement, suggested by the close orbits of the planets so far discovered, is the pollution of convection zones of host stars by the infall of planetary material. The process of planet destruction in the outer layers of the star makes a significant impact on the observed stellar metallicity resulting from the consumption. We present results of modeling the destruction of both terrestrial and gaseous planets by shock heating and ablation in stars from 0.8 to 1.6 Msun, as well as the predicted metallicity enhancements. Our results show that higher mass stars are more efficiently polluted than those with lower masses, primarily due to slower orbital decay, higher temperatures and the smaller mass of the surface mixing layer. We conclude that the consumption model does not well reproduce the observed distribution of metallicity enhancement of the extrasolar planet host stars. Support provided by NASA contract NAS8-39073.

Polarization has been detected at early times for all types of supernova, indicating that all suc... more Polarization has been detected at early times for all types of supernova, indicating that all such systems result from or quickly develop some form of asymmetry. In addition, the detection of strong line polarization in supernovae is suggestive of chemical inhomogeneities ("clumps") in the layers above the photosphere, which may reflect hydrodynamical instabilities during the explosion. We have developed a fast, flexible, approximate semi-analytic code for modeling polarized line radiative transfer within 3-D inhomogeneous rapidly-expanding atmospheres. Given a range of model parameters, the code randomly generates sets of clumps in the expanding ejecta and calculates the emergent line profile and Stokes parameters for each configuration. The ensemble of these configurations represents both the effects of various host geometries and of different viewing angles. We present results for the first part of our survey of model geometries, specifically the effects of the number of clumps, size and distance from the photosphere (and the related effect of filling factor) on the emergent spectrum and Stokes parameters. We have also developed a method to connect the results of our simulations to robust observational parameters such as maximum degree of polarization and polarized flux throughout the line. Our models, in connection with spectropolarimetric observations, will be useful in constraining the 3-D structure of supernova ejecta, which may offer important insight into the supernova explosion physics or the nature of the progenitor system.

Tidal circularization in a population of solar-type binary stars causes a transition from eccentr... more Tidal circularization in a population of solar-type binary stars causes a transition from eccentric to circular orbits for the closest binaries (shortest orbital period). This characteristic fingerprint is found on all populations of late-type binaries. Determining the stellar separation (orbital period) at which this transition occurs as a function of age is a powerful observational constraint on the theory of tidal circularization. Proper determination of this transition between eccentric and circular orbits requires large samples of binary orbits from homogeneous stellar populations. A comprehensive radial-velocity survey using the WIYN 3.5m telescope and the HYDRA Multi-Object Spectrograph has produced such large samples of 30-50 binary orbits in the open clusters M35 (150 Myr), NGC6819 (2.5 Gyr), and NGC188 (7 Gyr). We propose a new robust method for determination of the transition between eccentric and circular orbits, and use it to determine the transition period for the established binary populations of the Pleiades, Hyades/Praesepe and M67 as well as new binary populations in M35, NGC6819, and NGC188. An estimate of the uncertainty on transition period is obtained through Monte Carlo experiments. Finally, we compare the new distribution of derived transition periods vs. population ages to the theories of evolution of tidal circularization. This work is supported by NSF AST 9731302 and the Danish Research Agency.

We are conducting a comprehensive stellar radial-velocity survey in NGC 6819, a rich, intermediat... more We are conducting a comprehensive stellar radial-velocity survey in NGC 6819, a rich, intermediate age ( 2.4 Gyr) open cluster with [Fe/H] -0.05. As of October 2006, we have obtained 7065 radial-velocity measurements of 1409 stars using the WIYN Hydra Multi-Object Spectrograph, with typical velocity measurement precisions of 0.4 km/s. Using an E/I criterion of 3, we have identified 282 velocity variables. In the past year we have expanded the number of final orbital solutions by 45 to a total of more than 80 solutions. In coeval stellar populations, circular binaries tend to have the shortest orbital periods, while longer period binaries show a distribution of non-zero eccentricities. The circularization of the shortest period orbits is the result of an exchange of stellar and orbital angular momentum due to tidal interactions. We defined a population’s tidal circularization period as the longest orbital period at which a binary of typical initial eccentricity has become circularized (e.g., has evolved to an eccentricity e = 0.01) over the lifetime of the cluster (Meibom & Mathieu, 2005, ApJ, 620, 970). We are studying the trend of increasing tidal circularization periods with population age. Preliminary results in NGC 6819 indicate a tidal circularization period of 7.5 days, which is consistent with this overall trend. We will recalculate the tidal circularization period in order to include the latest sample of orbital solutions. This comprehensive survey also allows us to investigate the relative spatial distributions of spectroscopic binaries and other constant-velocity cluster members of similar mass. We find the spectroscopic binaries to be more centrally concentrated at a statistically significant level, which we attribute to energy equipartition processes. MM was supported by REU NSF grant AST-0453442. RDM, SK, KTH, and SM were supported by NSF grant AST-0406615.

Astrophysical Journal, 2010

Polarization has been detected at early times for all types of supernovae (SNe), indicating that ... more Polarization has been detected at early times for all types of supernovae (SNe), indicating that all such systems result from or quickly develop some form of asymmetry. In addition, the detection of strong line polarization in SNe is suggestive of chemical inhomogeneities ("clumps") in the layers above the photosphere, which may reflect hydrodynamical instabilities during the explosion. We have developed a fast, flexible, approximate semi-analytic code for modeling polarized line radiative transfer within three-dimensional inhomogeneous rapidly expanding atmospheres. Given a range of model parameters, the code generates random sets of clumps in the expanding ejecta and calculates the emergent line profile and Stokes parameters for each configuration. The ensemble of these configurations represents the effects both of various host geometries and of different viewing angles. We present results for the first part of our survey of model geometries, specifically the effects of the number and size of clumps (and the related effect of filling factor) on the emergent spectrum and Stokes parameters. Our simulations show that random clumpiness can produce line polarization in the range observed in SNe Ia, as well as the Q-U loops that are frequently seen in all SNe. We have also developed a method to connect the results of our simulations to robust observational parameters such as maximum polarization and polarized equivalent width in the line. Our models, in connection with spectropolarimetric observations, can constrain the three-dimensional structure of SN ejecta and offer important insight into the SN explosion physics and the nature of their progenitor systems.

Astrophysical Journal, 2010

Polarization has been detected at early times for all types of supernova, indicating that such sy... more Polarization has been detected at early times for all types of supernova, indicating that such systems result from or quickly develop some form of asymmetry. In addition, the detection of strong line polarization in supernovae is suggestive of chemical inhomogeneities ("clumps") in the layers above the photosphere, which may reflect hydrodynamical instabilities during the explosion. We have developed a fast, flexible, approximate semi-analytic code for modeling polarized line radiative transfer within 3-D inhomogeneous rapidly-expanding atmospheres. Given a range of model parameters, the code generates random sets of clumps in the expanding ejecta and calculates the emergent line profile and Stokes parameters for each configuration. The ensemble of these configurations represents both the effects of various host geometries and of different viewing angles. We present results for the first part of our survey of model geometries, specifically the effects of the number and size of clumps (and the related effect of filling factor) on the emergent spectrum and Stokes parameters. Our simulations show that random clumpiness can produce line polarization in the range observed in SNe Ia (~1-2%), as well as the Q-U loops that are frequently seen in all SNe. We have also developed a method to connect the results of our simulations to robust observational parameters such as maximum polarization and polarized equivalent width in the line. Our models, in connection with spectropolarimetric observations, can constrain the 3-D structure of supernova ejecta and offer important insight into the SN explosion physics and the nature of their progenitor systems.

Astrophysical Journal, 2006

ABSTRACT Observations suggest that stars lose appreciable angular momentum prior to reaching the ... more ABSTRACT Observations suggest that stars lose appreciable angular momentum prior to reaching the main sequence. Two principal spin-down mechanisms have been proposed. One is removal of angular momentum by magnetized winds or jets; the other is transfer of angular momentum from the star to its accretion disk through the effects of magnetic fields. In the latter case, spin evolution occurs due to both mass accretion along field lines and torques resulting from coupling of the stellar magnetic field to the disk. In this paper we study the latter torques in the context of a magnetic field varying in time. We find that magnetic variability reduces the efficiency with which the field can wind up, somewhat widening the region of magnetic coupling. Nonetheless, the steady state result—that magnetic torques can be applied only within a thin annulus around the corotation radius—is little changed for what we believe to be realistic physical conditions. These results are generally applicable to disk accretion onto magnetized bodies.

In this paper we present and compare flight results with the latest results of the ground calibra... more In this paper we present and compare flight results with the latest results of the ground calibration for the HRC-I detector. In particular we will compare ground and in flight data on detector background, effective area, quantum efficiency and point spread response function.

There are some supernovae that display emission line profiles that are asymmetric in shape. One c... more There are some supernovae that display emission line profiles that are asymmetric in shape. One cause for asymmetry could be an in-homogeneous density distribution, or "clumps". We explore the effects of clumps on the emission line profiles of forbidden lines. Our model assumes the ejecta shell is spherically symmetric in velocity, with a central cavity. The model assigns density perturbations to conical sections in the ejecta. To model the emission profile for a forbidden line, we use Sobolev theory. Our model gives asymmetric profiles when the clumping is introduced. The amount of asymmetry varies with the range of density perturbations allowed, and the relative asymmetry evolves in time. This project was funded by a partnership between the National Science Foundation (NSF AST-0552798), Research Experiences for Undergraduates (REU), and the Department of Defense (DoD) ASSURE (Awards to Stimulate and Support Undergraduate Research Experiences) programs.

Astronomical Journal, 2009

We present the current results from our ongoing radial-velocity survey of the intermediate-age (2... more We present the current results from our ongoing radial-velocity survey of the intermediate-age (2.4 Gyr) open cluster NGC 6819. Using both newly observed and other available photometry and astrometry we define a primary target sample of 1454 stars that includes main-sequence, subgiant, giant, and blue straggler stars, spanning a magnitude range of 11<=V<=16.5 and an approximate mass range of 1.1 to 1.6 Msun. Our sample covers a 23 arcminute (13 pc) square field of view centered on the cluster. We have measured 6571 radial velocities for an unbiased sample of 1207 stars in the direction of the open cluster NGC 6819, with a single-measurement precision of 0.4 km/s for most narrow-lined stars. We use our radial-velocity data to calculate membership probabilities for stars with >= 3 measurements, providing the first comprehensive membership study of the cluster core that includes stars from the giant branch through the upper main sequence. We identify 480 cluster members. Additionally, we identify velocity-variable systems, all of which are likely hard binaries that dynamically power the cluster. Using our single cluster members, we find a cluster average radial velocity of 2.34 +/- 0.05 km/s . We use our kinematic cluster members to construct a cleaned color- magnitude diagram from which we identify rich giant, subgiant, and blue straggler populations and a well-defined red clump. The cluster displays a morphology near the cluster turnoff clearly indicative of core convective overshoot. Finally, we discuss a few stars of note, one of which is a short-period red-clump binary that we suggest may be the product of a dynamical encounter.

Astronomical Journal, 2009

A tensor-operator technique footnote D.G. Ellis, Bull. Am. Phys. Soc. 39, 1069 (1994) is used to ... more A tensor-operator technique footnote D.G. Ellis, Bull. Am. Phys. Soc. 39, 1069 (1994) is used to compute the electron pair distribution function (PDF) for the configuration 2s^22p3d for various ions in the carbon isoelectronic sequence. Wavefunctions generated by MCHF footnote C. Froese Fischer, Comp. Phys. Comm. 64, 369 (1991) and MCDF footnote K.G. Dyall et al., Comp. Phys. Comm. 55, 425 (1989) programs are compared. The behavior of the PDF near zero interelectron separation (electron coalescence) corresponds roughly to the energy intervals between LS-coupled terms within a configuration. For a Hartree-Fock solution this behavior is dominated by the symmetry of the wavefunction (the Fermi hole). As the amount of configuration mixing in the solution is increased, the dynamic correlation (Coulomb hole) becomes more important. These effects, plus the influence of the spin-orbit and Breit interactions, are studied as a function of nuclear charge.

We present unique time-series spectra of the young solar-type, planet-bearing star ɛ Eridani in t... more We present unique time-series spectra of the young solar-type, planet-bearing star ɛ Eridani in the Lyman-ß region. These data were taken with the HIRS aperture of FUSE, which leads to a reduction in the effect of both airglow and "detector walk" on the line profile. Our time series observations show the line to be variable on the scale of days to weeks. The short timescale of these fluctuations indicates they are likely due to changes in the active stellar chromosphere of ɛ Eridani, rather than its giant planet, which has an orbital period of 6.9 years. We briefly consider other possible sources of variability that could be associated with the planetary system, such as comets, and conclude these are unlikely to contribute to the short term variability seen in our data.Observations obtained through proposal C165 to the FUSE Guest Investigator program; Research based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.

Astronomy & Astrophysics, 2011

Aims: This paper extends previous studies in modeling time varying linear polarization due to axi... more Aims: This paper extends previous studies in modeling time varying linear polarization due to axisymmetric magnetic fields in rotating stars. We use the Hanle effect to predict variations in net line polarization, and use geometric arguments to generalize these results to linear polarization due to other mechanisms. Methods: Building on the work of Lopez Ariste et al. (2011, A&A, 527, A120), we use simple analytic models of rotating stars that are symmetric except for an axisymmetric magnetic field to predict the polarization lightcurve due to the Hanle effect. We highlight the effects for the variable line polarization as a function of viewing inclination and field axis obliquity. Finally, we use geometric arguments to generalize our results to linear polarization from the weak transverse Zeeman effect. Results: We derive analytic expressions to demonstrate that the variable polarization lightcurve for an oblique magnetic rotator is symmetric. This holds for any axisymmetric field distribution and arbitrary viewing inclination to the rotation axis. Conclusions: For the situation under consideration, the amplitude of the polarization variation is set by the Hanle effect, but the shape of the variation in polarization with phase depends largely on geometrical projection effects. Our work generalizes the applicability of results described in Lopez Ariste et al., inasmuch as the assumptions of a spherical star and an axisymmetric field are true, and provides a strategy for separating the effects of perspective from the Hanle effect itself for interpreting polarimetric lightcurves.

The Chandra X-ray Observatory (CXO) High Resolution Camera (HRC) is a microchannel plate (MCP) ba... more The Chandra X-ray Observatory (CXO) High Resolution Camera (HRC) is a microchannel plate (MCP) based X-ray detector with heritage from similar detectors flown on the Einstein and ROSAT missions. The HRC consists of two detectors in a common housing. Improvements from the previous instruments include: fabricating the MCP from 'low-noise' glass (glass that contains a reduced level of radioactive isotopes) and surrounding the detector housing on five sides with an active coincidence detector. Both of these improvements help to maximize the X-ray signal to background noise ratio. The on-orbit background is dominated by cosmic ray and solar-wind particles. The temporal behavior of the background has two parts: a quiescent level and a flaring component. The quiescent level slowly changes with time and is correlated with the high-energy particle flux as measured by the Electron Proton Helium Instrument (EPHIN), the CXO radiation detector. The flaring component is associated with times of elevated low-energy particle flux, primarily from the Sun. A combination of on-board vetoing and filtering during ground processing provides a substantial rejection of the non-X-ray background. This work was supported by NASA contract NAS8-39073 to the Chandra X-ray Center.

Astrophysical Journal, 2010

Astrophysical Journal, 2006

Astronomical Journal, 2009

Astronomy & Astrophysics, 2011