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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.
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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.
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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.
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Bulletin of the American Physical Society, 2015
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The Astrophysical Journal, 2015
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High Energy Density Physics, 2013
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Monthly Notices of the Royal Astronomical Society, 2017
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Open Astronomy, 1998
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AIP Conference Proceedings, 2009
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AIP Conference Proceedings, 2007
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The Astrophysical Journal, 2015
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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
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The Astrophysical Journal, 2013
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The Astrophysical Journal, 2012
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The Astrophysical Journal, 2004
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The Astrophysical Journal, 2010
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The Astrophysical Journal, 2010
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The Astrophysical Journal, 2006
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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.
Bookmarks Related papers MentionsView impact
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.
Bookmarks Related papers MentionsView impact
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.
Bookmarks Related papers MentionsView impact
Bulletin of the American Physical Society, 2015
Bookmarks Related papers MentionsView impact
The Astrophysical Journal, 2015
Bookmarks Related papers MentionsView impact
High Energy Density Physics, 2013
Bookmarks Related papers MentionsView impact
Monthly Notices of the Royal Astronomical Society, 2017
Bookmarks Related papers MentionsView impact
Bookmarks Related papers MentionsView impact
Open Astronomy, 1998
Bookmarks Related papers MentionsView impact
AIP Conference Proceedings, 2009
Bookmarks Related papers MentionsView impact
AIP Conference Proceedings, 2007
Bookmarks Related papers MentionsView impact
The Astrophysical Journal, 2015
Bookmarks Related papers MentionsView impact
Bookmarks Related papers MentionsView impact
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
Bookmarks Related papers MentionsView impact
The Astrophysical Journal, 2013
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The Astrophysical Journal, 2012
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The Astrophysical Journal, 2004
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The Astrophysical Journal, 2010
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The Astrophysical Journal, 2010
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The Astrophysical Journal, 2006
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