Michael Montgomery - Profile on Academia.edu (original) (raw)

Papers by Michael Montgomery

Astronomy & Astrophysics, 2019

Context.Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light... more Context.Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky.Aims.As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3.Methods.We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequ...

Astronomy & Astrophysics, 2018

Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well und... more Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo. Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class. Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signa...

Astrophysical Journal, 2014

We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more... more We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating WD. These results mark some of the first science to come from a two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.

The Astronomical Journal, 2013

Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired ... more Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired from 2005 October to 2006 January, revealed high-amplitude variability in the range 1166-1290 s. This accreting white dwarf underwent an outburst in 2006 October, during which its brightness increased by at least five magnitudes, and it started exhibiting superhumps in its light curve. Upon cooling to quiescence, the superhumps disappeared and it displayed the same periods in 2010 February as prior to the outburst within the uncertainties of a couple of seconds. This behavior suggests that the observed variability is likely due to nonradial pulsations in the white dwarf star, whose core structure has not been significantly affected by the outburst. The enigmatic observations begin with an absence of pulsational variability during a multi-site campaign conducted in 2011 January-February without any evidence of a new outburst; the light curve is instead dominated by superhumps with periods in the range of 83-87 minutes. Ultraviolet Hubble Space Telescope time-series spectroscopy acquired in 2011 March reveals an effective temperature of 15,400 K, placing EQ Lyn within the broad instability strip of 10,500-16,000 K for accreting pulsators. The ultraviolet light curve with 90% flux from the white dwarf shows no evidence of any pulsations. Optical photometry acquired during 2011 and Spring 2012 continues to reflect the presence of superhumps and an absence of pulsations. Subsequent observations acquired in 2012 December and 2013 January finally indicate the disappearance of superhumps and the return of pulsational variability with similar periods as previous data. However, our most recent data from 2013 March to May reveal superhumps yet again with no sign of pulsations. We speculate that this enigmatic post-outburst behavior of the frequent disappearance of pulsational variability in EQ Lyn is caused either by heating the white dwarf beyond the instability strip due to an elevated accretion rate, disrupting pulsations associated with the He ii instability strip by lowering the He abundance of the convection zone, free geometric precession of the entire system, or appearing and disappearing disk pulsations.

Journal of Physics: Conference Series, 2009

EC14012-1446 is a hydrogen atmosphere (DA) white dwarf pulsator. Its rich pulsation spectrum disp... more EC14012-1446 is a hydrogen atmosphere (DA) white dwarf pulsator. Its rich pulsation spectrum displays a range of excited modes with complex multiplet structure, in addition to numerous combination frequencies. In April 2008, EC14012-1446 was the primary target of XCOV26. We obtained over 300 hrs of nearly continuous high speed photometry with the goal of using the nonlinear pulse shapes to empirically determine the parameters of the convection zone. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 µHz, with the dominant peak at 1234 µHz. We find 13 independent frequencies distributed in 8 modes, as well as a myriad of combination frequencies. In the following, we present preliminary results and lay the groundwork for future investigation leading to light curve fitting of EC14012-1446.

The Astrophysical Journal, 2020

We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative... more We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative of circumstellar planetary debris. Using Zwicky Transient Facility DR2 photometry of ZTF J013906.17+524536.89 and follow-up observations from the Las Cumbres Observatory, we identify multiple transit events that recur every ≈ 107.2 d, much longer than the 4.5-4.9 h orbital periods observed in WD 1145+017, the only other white dwarf known with transiting planetary debris. The transits vary in both depth and duration, lasting 15-25 d and reaching 20-45 % dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with T eff = 10,530 ± 140 K and log(g) = 7.86 ± 0.06. A Ca ii K absorption feature is present in all spectra both in and out of transit. Spectra obtained during one night at roughly 15 % transit depth show increased Ca ii K absorption with a model atmospheric fit suggesting [Ca/H] = -4.6 ± 0.3, whereas spectra taken on three nights out of transit have [Ca/H] of -5.5, -5.3, and -4.9 with similar uncertainties. While the Ca ii K line strength varies by only 2-sigma, we consider a predominantly interstellar origin for Ca absorption unlikely. We suggest a larger column density of circumstellar metallic gas along the line of site or increased accretion of material onto the white dwarf's surface are responsible for the Ca absorption, but further spectroscopic studies are required. In addition, high-speed time series photometry out of transit reveals variability with periods of 900 and 1030 s, consistent with ZZ Ceti pulsations.

Frontiers in Astronomy and Space Sciences, 2022

For isolated white dwarf (WD) stars, fits to their observed spectra provide the most precise esti... more For isolated white dwarf (WD) stars, fits to their observed spectra provide the most precise estimates of their effective temperatures and surface gravities. Even so, recent studies have shown that systematic offsets exist between such spectroscopic parameter determinations and those based on broadband photometry. These large discrepancies (10% inTeff, 0.1 M⊙in mass) provide scientific motivation for reconsidering the atomic physics employed in the model atmospheres of these stars. Recent simulation work of ours suggests that the most important remaining uncertainties in simulation-based calculations of line shapes are the treatment of 1) the electric field distribution and 2) the occupation probability (OP) prescription. We review the work that has been done in these areas and outline possible avenues for progress.

arXiv: Solar and Stellar Astrophysics, 2016

Accurately measuring the masses of white dwarf stars is crucial in many astrophysical contexts (e... more Accurately measuring the masses of white dwarf stars is crucial in many astrophysical contexts (e.g., asteroseismology and cosmochronology). These masses are most commonly determined by fitting a model atmosphere to an observed spectrum; this is known as the spectroscopic method. However, for cases in which more than one method may be employed, there are well known discrepancies between masses determined by the spectroscopic method and those determined by astrometric, dynamical, and/or gravitational-redshift methods. In an effort to resolve these discrepancies, we are developing a new model of hydrogen in a dense plasma that is a significant departure from previous models. Experiments at Sandia National Laboratories are currently underway to validate these new models, and we have begun modifications to incorporate these models into stellar-atmosphere codes.

arXiv: Solar and Stellar Astrophysics, 2016

As part of our laboratory investigation of the theoretical line profiles used in white dwarf atmo... more As part of our laboratory investigation of the theoretical line profiles used in white dwarf atmosphere models, we extend the electron-density ($n_{\rm e}$) range measured by our experiments to higher densities (up to nesim80times1016n_{e}\sim80\times10^{16}nesim80times1016 cm$^{-3}$). Whereas inferred parameters using the hydrogen-$\beta$ spectral line agree among different line-shape models for nrmelesssim30times1016n_{\rm e}\lesssim30\times10^{16}nrmelesssim30times1016 cm$^{-3}$, we now see divergence between models. These are densities beyond the range previously benchmarked in the laboratory, meaning theoretical profiles in this regime have not been fully validated. Experimentally exploring these higher densities enables us to test and constrain different line-profile models, as the differences in their relative H-Balmer line shapes are more pronounced at such conditions. These experiments also aid in our study of occupation probabilities because we can measure these from relative line strengths.

Bulletin of the American Physical Society, 2015

Barbara-Among the first stars to form in the Universe may be "dark stars," i.e. stars of primordi... more Barbara-Among the first stars to form in the Universe may be "dark stars," i.e. stars of primordial composition, but powered by the heating released in the process of dark matter (DM) particle self-annihilation, which also gives the correct relic density of DM today. It has been shown in the past that a DM-powered stellar phase is feasible, due to the high DM densities in the centers of primordial minihalos and the efficiency of DM annihilation. DM could thereby be responsible for an entirely new class of stellar objects, while possible detection of the latter would provide a smoking gun for DM. We have used the stellar evolution code MESA in order to improve upon previous stellar models, which were limited to polytropes. Our more accurate models confirm earlier results which found that dark stars can be very massive (M > 10 5 M ⊙), bright, cool and puffy objects. Once these supermassive dark stars run out of DM fuel, they collapse and could be forming the seeds for the supermassive black holes which are observed in nearby and high-redshift galaxies. I will present our results on the evolution and properties of dark stars on their way of becoming supermassive, as well as new results on possible pulsations of dark stars and predicted observational signatures.

The Astrophysical Journal, 2015

We use the stellar evolution code MESA to study dark stars. Dark stars (DSs), which are powered b... more We use the stellar evolution code MESA to study dark stars. Dark stars (DSs), which are powered by dark matter (DM) self-annihilation rather than by nuclear fusion, may be the first stars to form in the Universe. We compute stellar models for accreting DSs with masses up to 10 6 M ⊙ . The heating due to DM annihilation is self-consistently included, assuming extended adiabatic contraction of DM within the minihalos in which DSs form. We find remarkably good overall agreement with previous models, which assumed polytropic interiors. There are some differences in the details, with positive implications for observability. We found that, in the mass range of 10 4 -10 5 M ⊙ , our DSs are hotter by a factor of 1.5 than those in Freese et al.( ), are smaller in radius by a factor of 0.6, denser by a factor of 3 -4, and more luminous by a factor of 2. Our models also confirm previous results, according to which supermassive DSs are very well approximated by (n = 3)-polytropes. We also perform a first study of dark star pulsations. Our DS models have pulsation modes with timescales ranging from less than a day to more than two years in their rest frames, at z ∼ 15, depending on DM particle mass and overtone number. Such pulsations may someday be used to identify bright, cool objects uniquely as DSs; if properly calibrated, they might, in principle, also supply novel standard candles for cosmological studies.

High Energy Density Physics, 2013

We present an experimental platform for measuring hydrogen Balmer emission and absorption line pr... more We present an experimental platform for measuring hydrogen Balmer emission and absorption line profiles for plasmas with white dwarf (WD) photospheric conditions (T e ∼ 1 eV, n e ∼ 10 17 cm −3). These profiles will be used to benchmark WD atmosphere models, which, used with the spectroscopic method, are responsible for determining fundamental parameters (e.g., effective temperature, mass) for tens of thousands of WDs. Our experiment, performed at the Z Pulsed Power Facility at Sandia National Laboratories, uses the large amount of x-rays generated from a z-pinch dynamic hohlraum to drive plasma formation in a gas cell. The platform is unique compared to past hydrogen line profile experiments in that the plasma is radiation-driven. This decouples the heating source from the plasma to be studied in the sense that the radiation temperature causing the photoionization is independent of the initial conditions of the gas. For the first time we measure hydrogen Balmer lines in absorption at these conditions in the laboratory for the purpose of benchmarking Stark-broadened line shapes. The platform can be used to study other plasma species and to explore non-LTE, time-dependent collisional-radiative atomic kinetics.

Monthly Notices of the Royal Astronomical Society, 2017

We present results from a search for short timescale white dwarf variability using gPhoton, a tim... more We present results from a search for short timescale white dwarf variability using gPhoton, a time-tagged data base of GALEX photon events and associated software package. We conducted a survey of 320 white dwarf stars in the McCook-Sion catalogue, inspecting each for photometric variability with particular emphasis on variability over timescales less than ∼30 min. From that survey, we present the discovery of a new pulsating white dwarf: WD 2246-069. A Ca II K line is found in archival ESO spectra and an IR excess is seen in WISE W1 and W2 bands. Its independent modes are identified in follow-up optical photometry and used to model its interior structure. Additionally, we detect UV pulsations in four previously known pulsating ZZ Ceti-type (DAVs). Included in this group is the simultaneous fitting of the pulsations of WD 1401-147 in optical, near-ultraviolet and far-ultraviolet bands using nearly concurrent Whole Earth Telescope and GALEX data, providing observational insight into the wavelength dependence of white dwarf pulsation amplitudes.

Recent Progress Characterizing Convection Using White Dwarf Light Curves from the Whole Earth Telescope

Open Astronomy, 1998

BPM 37093 is a ZZ Ceti type star of the mass ~ 1.1 M Q. Its temperature 11 000 K) and high mass i... more BPM 37093 is a ZZ Ceti type star of the mass ~ 1.1 M Q. Its temperature 11 000 K) and high mass imply that it should be crystallized throughout most of its core, the exact fraction depending on the core composition. BPM 37093 is the first object where the results of crystallization theory can be probed observationally. If the star is crystallized, its pulsation spectrum should differ significantly from non-crystallized pulsating stars. Very high signal-to-noise high speed photometry will enable us to detect very low amplitude (<lmmag) modes. The finding of additional pulsation modes will enable us to better disentangle competing physical processes such as the surface layer masses from the effects of crystallization itself. In this paper we report the observations of BPM 37093 from 1991 till 1997 and present the plan for WET observations in 1998.

White Dwarf and Pre-White Dwarf Pulsations

AIP Conference Proceedings, 2009

AIP Conference Proceedings, 2007

I present an overview of the close relationship which convection and stellar pulsation share. Thi... more I present an overview of the close relationship which convection and stellar pulsation share. This includes the driving, damping, and modulation of oscillations in stars such as the Sun, other solar-type stars, the "classical" pulsators (e.g., Cepheids, RR Lyrae), as well as more recent results from the realm of pulsating white dwarf stars.

The Astrophysical Journal, 2015

We present the Kepler light curve of KIC 4552982, the first ZZ Ceti (hydrogen-atmosphere pulsatin... more We present the Kepler light curve of KIC 4552982, the first ZZ Ceti (hydrogen-atmosphere pulsating white dwarf star) discovered in the Kepler field of view. Our data span more than 1.5 years, with a 86% duty cycle, making it the longest pseudo-continuous light curve ever recorded for a ZZ Ceti. This extensive data set provides the most complete coverage to date of amplitude and frequency variations in a cool ZZ Ceti. We detect 20 independent frequencies of variability in the data that we compare with asteroseismic models to demonstrate that this star has a mass M * 0.6 > M . We identify a rotationally split pulsation mode and derive a probable rotation period for this star of 17.47 ± 0.04 hr. In addition to pulsation signatures, the Kepler light curve exhibits sporadic, energetic outbursts that increase the star's relative flux by 2%-17%, last 4-25 hr, and recur on an average timescale of 2.7 days. These are the first detections of a new dynamic white dwarf phenomenon that may be related to the pulsations of this relatively cool (T eff 10,860 120 =  K) ZZ Ceti star near the red edge of the instability strip.

Table of Tables xvi List of Figures xvii Chapter 1. Overview Bibliography 193 Vita 203 xv 157 sub... more Table of Tables xvi List of Figures xvii Chapter 1. Overview Bibliography 193 Vita 203 xv 157 subroutine phasep2 implicit double precision (a-h, o-z) common/shells/ sa(400),ra(400),ba(400),pa(400),ta(400), ea(400),xca(400),fca(400),s(400),r(400),b(400), p(400),t(400),e(400),xc(400),sk(400), rk(400),bk(400),pk(400),tk(400) common/contrl/ds,g,sm,wc,it,nite,ja,jb,j,k,l common/xbnd/amxc,amxo,gold common/crystal/jxlast,jx,firstcall,iphasepflag, ixtalyes,iconvyes common/crystal2/xcinit(400),oradlimit c common/compold/xcold(400),xcn(400) common/density/dens(400),cv(400) common/phasecalc/iphcalc common/xenergy2/phengy(400),dphdt(0:400),dphdp(0:400),totph(400), enxtal2(0:400),dphdm(400),damxdt,damxdp common/epflag/epflag1 common/debug/debug common/temp3/temp3(2,12) common/jxhist/jxold c common/amxchist/amxcold,amxcvold common/amxchist/amxcold c common/dsave/xx,yy,yy2,sprof,lprof,sprof2,lprof2,dmass common/dsave/xx,yy,yy2,xcp,dmass,massx,massf common/startx/istartxtal common/xcxtal/xcxtal(400),enxtal2a(400),fnxtal common/xistat/en2spline(400),xmass(400),en2dd(400) common/nxistat/nxsp common/blum/bl,bla common/once2/once2 logical debug,once2 dimension fluid(400), solid (400) c common/prep/ aa(19,800),ecv(400),ext(400),exr(400),iprep double precision cprof(400),xcprof(0:400,400), phengy,dmass(400),dudx(400),dphdm,norm,damxdt,damxdp,erelease, xx(400),yy(400),yy2(400),xcomp,massx(400),massf(400),xcp(400,400), profile1(400),profile2(400),p1(400),p2(400) if (istartxtal.eq.0) then if (jx.gt.0) then istartxtal=1 else return

We explore the physics of crystallization in the deep interiors of white dwarf stars using the co... more We explore the physics of crystallization in the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of white dwarf stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn (1968). We outline how this data can be used to observationally constrain the value of Γ ≡ ECoulomb/Ethermal near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation. Subject headings: white dwarfs — dense matter — equation of state

Astronomy & Astrophysics, 2019

Context.Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light... more Context.Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky.Aims.As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3.Methods.We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequ...

Astronomy & Astrophysics, 2018

Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well und... more Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo. Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class. Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signa...

Astrophysical Journal, 2014

We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more... more We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating WD. These results mark some of the first science to come from a two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.

The Astronomical Journal, 2013

Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired ... more Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired from 2005 October to 2006 January, revealed high-amplitude variability in the range 1166-1290 s. This accreting white dwarf underwent an outburst in 2006 October, during which its brightness increased by at least five magnitudes, and it started exhibiting superhumps in its light curve. Upon cooling to quiescence, the superhumps disappeared and it displayed the same periods in 2010 February as prior to the outburst within the uncertainties of a couple of seconds. This behavior suggests that the observed variability is likely due to nonradial pulsations in the white dwarf star, whose core structure has not been significantly affected by the outburst. The enigmatic observations begin with an absence of pulsational variability during a multi-site campaign conducted in 2011 January-February without any evidence of a new outburst; the light curve is instead dominated by superhumps with periods in the range of 83-87 minutes. Ultraviolet Hubble Space Telescope time-series spectroscopy acquired in 2011 March reveals an effective temperature of 15,400 K, placing EQ Lyn within the broad instability strip of 10,500-16,000 K for accreting pulsators. The ultraviolet light curve with 90% flux from the white dwarf shows no evidence of any pulsations. Optical photometry acquired during 2011 and Spring 2012 continues to reflect the presence of superhumps and an absence of pulsations. Subsequent observations acquired in 2012 December and 2013 January finally indicate the disappearance of superhumps and the return of pulsational variability with similar periods as previous data. However, our most recent data from 2013 March to May reveal superhumps yet again with no sign of pulsations. We speculate that this enigmatic post-outburst behavior of the frequent disappearance of pulsational variability in EQ Lyn is caused either by heating the white dwarf beyond the instability strip due to an elevated accretion rate, disrupting pulsations associated with the He ii instability strip by lowering the He abundance of the convection zone, free geometric precession of the entire system, or appearing and disappearing disk pulsations.

Journal of Physics: Conference Series, 2009

EC14012-1446 is a hydrogen atmosphere (DA) white dwarf pulsator. Its rich pulsation spectrum disp... more EC14012-1446 is a hydrogen atmosphere (DA) white dwarf pulsator. Its rich pulsation spectrum displays a range of excited modes with complex multiplet structure, in addition to numerous combination frequencies. In April 2008, EC14012-1446 was the primary target of XCOV26. We obtained over 300 hrs of nearly continuous high speed photometry with the goal of using the nonlinear pulse shapes to empirically determine the parameters of the convection zone. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 µHz, with the dominant peak at 1234 µHz. We find 13 independent frequencies distributed in 8 modes, as well as a myriad of combination frequencies. In the following, we present preliminary results and lay the groundwork for future investigation leading to light curve fitting of EC14012-1446.

The Astrophysical Journal, 2020

We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative... more We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative of circumstellar planetary debris. Using Zwicky Transient Facility DR2 photometry of ZTF J013906.17+524536.89 and follow-up observations from the Las Cumbres Observatory, we identify multiple transit events that recur every ≈ 107.2 d, much longer than the 4.5-4.9 h orbital periods observed in WD 1145+017, the only other white dwarf known with transiting planetary debris. The transits vary in both depth and duration, lasting 15-25 d and reaching 20-45 % dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with T eff = 10,530 ± 140 K and log(g) = 7.86 ± 0.06. A Ca ii K absorption feature is present in all spectra both in and out of transit. Spectra obtained during one night at roughly 15 % transit depth show increased Ca ii K absorption with a model atmospheric fit suggesting [Ca/H] = -4.6 ± 0.3, whereas spectra taken on three nights out of transit have [Ca/H] of -5.5, -5.3, and -4.9 with similar uncertainties. While the Ca ii K line strength varies by only 2-sigma, we consider a predominantly interstellar origin for Ca absorption unlikely. We suggest a larger column density of circumstellar metallic gas along the line of site or increased accretion of material onto the white dwarf's surface are responsible for the Ca absorption, but further spectroscopic studies are required. In addition, high-speed time series photometry out of transit reveals variability with periods of 900 and 1030 s, consistent with ZZ Ceti pulsations.

Frontiers in Astronomy and Space Sciences, 2022

For isolated white dwarf (WD) stars, fits to their observed spectra provide the most precise esti... more For isolated white dwarf (WD) stars, fits to their observed spectra provide the most precise estimates of their effective temperatures and surface gravities. Even so, recent studies have shown that systematic offsets exist between such spectroscopic parameter determinations and those based on broadband photometry. These large discrepancies (10% inTeff, 0.1 M⊙in mass) provide scientific motivation for reconsidering the atomic physics employed in the model atmospheres of these stars. Recent simulation work of ours suggests that the most important remaining uncertainties in simulation-based calculations of line shapes are the treatment of 1) the electric field distribution and 2) the occupation probability (OP) prescription. We review the work that has been done in these areas and outline possible avenues for progress.

arXiv: Solar and Stellar Astrophysics, 2016

Accurately measuring the masses of white dwarf stars is crucial in many astrophysical contexts (e... more Accurately measuring the masses of white dwarf stars is crucial in many astrophysical contexts (e.g., asteroseismology and cosmochronology). These masses are most commonly determined by fitting a model atmosphere to an observed spectrum; this is known as the spectroscopic method. However, for cases in which more than one method may be employed, there are well known discrepancies between masses determined by the spectroscopic method and those determined by astrometric, dynamical, and/or gravitational-redshift methods. In an effort to resolve these discrepancies, we are developing a new model of hydrogen in a dense plasma that is a significant departure from previous models. Experiments at Sandia National Laboratories are currently underway to validate these new models, and we have begun modifications to incorporate these models into stellar-atmosphere codes.

arXiv: Solar and Stellar Astrophysics, 2016

As part of our laboratory investigation of the theoretical line profiles used in white dwarf atmo... more As part of our laboratory investigation of the theoretical line profiles used in white dwarf atmosphere models, we extend the electron-density ($n_{\rm e}$) range measured by our experiments to higher densities (up to nesim80times1016n_{e}\sim80\times10^{16}nesim80times1016 cm$^{-3}$). Whereas inferred parameters using the hydrogen-$\beta$ spectral line agree among different line-shape models for nrmelesssim30times1016n_{\rm e}\lesssim30\times10^{16}nrmelesssim30times1016 cm$^{-3}$, we now see divergence between models. These are densities beyond the range previously benchmarked in the laboratory, meaning theoretical profiles in this regime have not been fully validated. Experimentally exploring these higher densities enables us to test and constrain different line-profile models, as the differences in their relative H-Balmer line shapes are more pronounced at such conditions. These experiments also aid in our study of occupation probabilities because we can measure these from relative line strengths.

Bulletin of the American Physical Society, 2015

Barbara-Among the first stars to form in the Universe may be "dark stars," i.e. stars of primordi... more Barbara-Among the first stars to form in the Universe may be "dark stars," i.e. stars of primordial composition, but powered by the heating released in the process of dark matter (DM) particle self-annihilation, which also gives the correct relic density of DM today. It has been shown in the past that a DM-powered stellar phase is feasible, due to the high DM densities in the centers of primordial minihalos and the efficiency of DM annihilation. DM could thereby be responsible for an entirely new class of stellar objects, while possible detection of the latter would provide a smoking gun for DM. We have used the stellar evolution code MESA in order to improve upon previous stellar models, which were limited to polytropes. Our more accurate models confirm earlier results which found that dark stars can be very massive (M > 10 5 M ⊙), bright, cool and puffy objects. Once these supermassive dark stars run out of DM fuel, they collapse and could be forming the seeds for the supermassive black holes which are observed in nearby and high-redshift galaxies. I will present our results on the evolution and properties of dark stars on their way of becoming supermassive, as well as new results on possible pulsations of dark stars and predicted observational signatures.

The Astrophysical Journal, 2015

We use the stellar evolution code MESA to study dark stars. Dark stars (DSs), which are powered b... more We use the stellar evolution code MESA to study dark stars. Dark stars (DSs), which are powered by dark matter (DM) self-annihilation rather than by nuclear fusion, may be the first stars to form in the Universe. We compute stellar models for accreting DSs with masses up to 10 6 M ⊙ . The heating due to DM annihilation is self-consistently included, assuming extended adiabatic contraction of DM within the minihalos in which DSs form. We find remarkably good overall agreement with previous models, which assumed polytropic interiors. There are some differences in the details, with positive implications for observability. We found that, in the mass range of 10 4 -10 5 M ⊙ , our DSs are hotter by a factor of 1.5 than those in Freese et al.( ), are smaller in radius by a factor of 0.6, denser by a factor of 3 -4, and more luminous by a factor of 2. Our models also confirm previous results, according to which supermassive DSs are very well approximated by (n = 3)-polytropes. We also perform a first study of dark star pulsations. Our DS models have pulsation modes with timescales ranging from less than a day to more than two years in their rest frames, at z ∼ 15, depending on DM particle mass and overtone number. Such pulsations may someday be used to identify bright, cool objects uniquely as DSs; if properly calibrated, they might, in principle, also supply novel standard candles for cosmological studies.

High Energy Density Physics, 2013

We present an experimental platform for measuring hydrogen Balmer emission and absorption line pr... more We present an experimental platform for measuring hydrogen Balmer emission and absorption line profiles for plasmas with white dwarf (WD) photospheric conditions (T e ∼ 1 eV, n e ∼ 10 17 cm −3). These profiles will be used to benchmark WD atmosphere models, which, used with the spectroscopic method, are responsible for determining fundamental parameters (e.g., effective temperature, mass) for tens of thousands of WDs. Our experiment, performed at the Z Pulsed Power Facility at Sandia National Laboratories, uses the large amount of x-rays generated from a z-pinch dynamic hohlraum to drive plasma formation in a gas cell. The platform is unique compared to past hydrogen line profile experiments in that the plasma is radiation-driven. This decouples the heating source from the plasma to be studied in the sense that the radiation temperature causing the photoionization is independent of the initial conditions of the gas. For the first time we measure hydrogen Balmer lines in absorption at these conditions in the laboratory for the purpose of benchmarking Stark-broadened line shapes. The platform can be used to study other plasma species and to explore non-LTE, time-dependent collisional-radiative atomic kinetics.

Monthly Notices of the Royal Astronomical Society, 2017

We present results from a search for short timescale white dwarf variability using gPhoton, a tim... more We present results from a search for short timescale white dwarf variability using gPhoton, a time-tagged data base of GALEX photon events and associated software package. We conducted a survey of 320 white dwarf stars in the McCook-Sion catalogue, inspecting each for photometric variability with particular emphasis on variability over timescales less than ∼30 min. From that survey, we present the discovery of a new pulsating white dwarf: WD 2246-069. A Ca II K line is found in archival ESO spectra and an IR excess is seen in WISE W1 and W2 bands. Its independent modes are identified in follow-up optical photometry and used to model its interior structure. Additionally, we detect UV pulsations in four previously known pulsating ZZ Ceti-type (DAVs). Included in this group is the simultaneous fitting of the pulsations of WD 1401-147 in optical, near-ultraviolet and far-ultraviolet bands using nearly concurrent Whole Earth Telescope and GALEX data, providing observational insight into the wavelength dependence of white dwarf pulsation amplitudes.

Recent Progress Characterizing Convection Using White Dwarf Light Curves from the Whole Earth Telescope

Open Astronomy, 1998

BPM 37093 is a ZZ Ceti type star of the mass ~ 1.1 M Q. Its temperature 11 000 K) and high mass i... more BPM 37093 is a ZZ Ceti type star of the mass ~ 1.1 M Q. Its temperature 11 000 K) and high mass imply that it should be crystallized throughout most of its core, the exact fraction depending on the core composition. BPM 37093 is the first object where the results of crystallization theory can be probed observationally. If the star is crystallized, its pulsation spectrum should differ significantly from non-crystallized pulsating stars. Very high signal-to-noise high speed photometry will enable us to detect very low amplitude (<lmmag) modes. The finding of additional pulsation modes will enable us to better disentangle competing physical processes such as the surface layer masses from the effects of crystallization itself. In this paper we report the observations of BPM 37093 from 1991 till 1997 and present the plan for WET observations in 1998.

White Dwarf and Pre-White Dwarf Pulsations

AIP Conference Proceedings, 2009

AIP Conference Proceedings, 2007

I present an overview of the close relationship which convection and stellar pulsation share. Thi... more I present an overview of the close relationship which convection and stellar pulsation share. This includes the driving, damping, and modulation of oscillations in stars such as the Sun, other solar-type stars, the "classical" pulsators (e.g., Cepheids, RR Lyrae), as well as more recent results from the realm of pulsating white dwarf stars.

The Astrophysical Journal, 2015

We present the Kepler light curve of KIC 4552982, the first ZZ Ceti (hydrogen-atmosphere pulsatin... more We present the Kepler light curve of KIC 4552982, the first ZZ Ceti (hydrogen-atmosphere pulsating white dwarf star) discovered in the Kepler field of view. Our data span more than 1.5 years, with a 86% duty cycle, making it the longest pseudo-continuous light curve ever recorded for a ZZ Ceti. This extensive data set provides the most complete coverage to date of amplitude and frequency variations in a cool ZZ Ceti. We detect 20 independent frequencies of variability in the data that we compare with asteroseismic models to demonstrate that this star has a mass M * 0.6 > M . We identify a rotationally split pulsation mode and derive a probable rotation period for this star of 17.47 ± 0.04 hr. In addition to pulsation signatures, the Kepler light curve exhibits sporadic, energetic outbursts that increase the star's relative flux by 2%-17%, last 4-25 hr, and recur on an average timescale of 2.7 days. These are the first detections of a new dynamic white dwarf phenomenon that may be related to the pulsations of this relatively cool (T eff 10,860 120 =  K) ZZ Ceti star near the red edge of the instability strip.

Table of Tables xvi List of Figures xvii Chapter 1. Overview Bibliography 193 Vita 203 xv 157 sub... more Table of Tables xvi List of Figures xvii Chapter 1. Overview Bibliography 193 Vita 203 xv 157 subroutine phasep2 implicit double precision (a-h, o-z) common/shells/ sa(400),ra(400),ba(400),pa(400),ta(400), ea(400),xca(400),fca(400),s(400),r(400),b(400), p(400),t(400),e(400),xc(400),sk(400), rk(400),bk(400),pk(400),tk(400) common/contrl/ds,g,sm,wc,it,nite,ja,jb,j,k,l common/xbnd/amxc,amxo,gold common/crystal/jxlast,jx,firstcall,iphasepflag, ixtalyes,iconvyes common/crystal2/xcinit(400),oradlimit c common/compold/xcold(400),xcn(400) common/density/dens(400),cv(400) common/phasecalc/iphcalc common/xenergy2/phengy(400),dphdt(0:400),dphdp(0:400),totph(400), enxtal2(0:400),dphdm(400),damxdt,damxdp common/epflag/epflag1 common/debug/debug common/temp3/temp3(2,12) common/jxhist/jxold c common/amxchist/amxcold,amxcvold common/amxchist/amxcold c common/dsave/xx,yy,yy2,sprof,lprof,sprof2,lprof2,dmass common/dsave/xx,yy,yy2,xcp,dmass,massx,massf common/startx/istartxtal common/xcxtal/xcxtal(400),enxtal2a(400),fnxtal common/xistat/en2spline(400),xmass(400),en2dd(400) common/nxistat/nxsp common/blum/bl,bla common/once2/once2 logical debug,once2 dimension fluid(400), solid (400) c common/prep/ aa(19,800),ecv(400),ext(400),exr(400),iprep double precision cprof(400),xcprof(0:400,400), phengy,dmass(400),dudx(400),dphdm,norm,damxdt,damxdp,erelease, xx(400),yy(400),yy2(400),xcomp,massx(400),massf(400),xcp(400,400), profile1(400),profile2(400),p1(400),p2(400) if (istartxtal.eq.0) then if (jx.gt.0) then istartxtal=1 else return

We explore the physics of crystallization in the deep interiors of white dwarf stars using the co... more We explore the physics of crystallization in the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with the theory of crystallization of the ions in the interior of white dwarf stars and provide the first empirical evidence that the phase transition is first order: latent heat is released in the process of crystallization as predicted by van Horn (1968). We outline how this data can be used to observationally constrain the value of Γ ≡ ECoulomb/Ethermal near the onset of crystallization, the central carbon/oxygen abundance, and the importance of phase separation. Subject headings: white dwarfs — dense matter — equation of state