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Register a new userA study of the Lyman-$alpha$ line profile in DBA white dwarfs
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Cynthia Genest-Beaulieu
Publication date
2016
fields
Physics
and research's language is
English
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The hydrogen abundances in DBA white dwarfs determined from optical or UV spectra have been reported to differ significantly in some studies. We revisit this problem using our own model atmospheres and synthetic spectra, and present a theoretical investigation of the Lyman-$alpha$ line profile as a function of effective temperature and hydrogen abundance. We identify one possible solution to this discrepancy and show considerable improvement from a detailed analysis of optical and UV spectra of DBA stars.
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Traces of photospheric hydrogen are detected in at least half of all white dwarfs with helium-dominated atmospheres through the presence of H alpha in high-quality optical spectroscopy. Previous studies have noted significant discrepancies between the hydrogen abundances derived from H alpha and Ly alpha for a number of stars where ultraviolet spectroscopy is also available. We demonstrate that this discrepancy is caused by inadequate treatment of the broadening of Ly alpha by neutral helium. When fitting Hubble Space Telescope COS spectroscopy of 17 DB white dwarfs using our new line profile calculations, we find good agreement between log(H/He) measured from Ly alpha and H alpha. Larger values of log(H/He) based on Ly alpha are still found for three stars, which are among the most distant in our sample, and we show that a small amount of interstellar absorption from neutral hydrogen can account for this discrepancy.
We present a modification of a model of solar cycle evolution of the solar Lyman-alpha line profile, along with a sensitivity study of interstellar neutral H hydrogen to uncertainties in radiation pressure level. The line profile model, originally developed by Kowalska-Leszczynska et al. 2018a, is parametrized by the composite solar Lyman-alpha flux, which recently was revised Machol et al. 2019. We present modified parameters of the previously-developed model of solar radiation pressure for neutral hydrogen and deuterium atoms in the heliosphere. The mathematical function used in the model, as well as the fitting procedure, remain unchanged. We show selected effects of the model modification on ISN H properties in the heliosphere and we discuss the sensitivity of these quantities to uncertainties in the calibration of the composite Lyman-alpha series.
Recent studies of interstellar neutral (ISN) hydrogen observed by the Interstellar Boundary Explorer (IBEX) suggested that the present understanding of the radiation pressure acting on hydrogen atoms in the heliosphere should be revised. There is a significant discrepancy between theoretical predictions of the ISN H signal using the currently used model of the solar Lyman-alpha profile by Tarnopolski et al. 2009 (TB09) and the signal due to ISN H observed by IBEX-Lo. We developed a new model of evolution of the solar Lyman-alpha profile that takes into account all available observations of the full-disk solar Lyman-alpha profiles from SUMER/SOHO, provided by Lemaire et al. 2015 (L15), covering practically the entire 23rd solar cycle. The model has three components that reproduce different features of the profile. The main shape of the emission line that is produced in the chromosphere is modeled by the kappa function; the central reversal due to absorption in the transition region is modeled by the Gauss function; the spectral background is represented by the linear function. The coefficients of all those components are linear functions of the line-integrated full-disk Lyman-alpha irradiance, which is the only free parameter of the model. The new model features potentially important differences in comparison with the model by TB09, which was based on a limited set of observations. This change in the understanding of radiation pressure, especially during low solar activity, may significantly affect the interstellar H and D distributions in the inner heliosphere and their derivative populations.
We revisit the problem of the formation of DB white dwarfs, as well as the origin of hydrogen in DBA stars, using a new set of envelope model calculations with stratified and mixed hydrogen/helium compositions. We first describe an approximate model to simulate the so-called convective dilution process, where a thin, superficial hydrogen radiative layer is gradually eroded by the underlying and more massive convective helium envelope, thus transforming a DA white dwarf into a DB star. We show that this convective dilution process is able to account for the large increase in the number of DB white dwarfs below Teff ~ 20,000 K, but that the residual hydrogen abundances expected from this process are still orders of magnitude lower than those observed in DBA white dwarfs. Scenarios involving the accretion of hydrogen from the interstellar medium or other external bodies have often been invoked to explain these overabundances of hydrogen. In this paper, we describe a new paradigm where hydrogen, initially diluted within the thick stellar envelope, is still present and slowly diffusing upward in the deeper layers of a Teff ~ 20,000 K white dwarf. When the convective dilution process occurs, the bottom of the mixed H/He convection zone sinks deep into the star, resulting in large amounts of hydrogen being dredged-up to the stellar surface, a phenomenon similar to that invoked in the context of DQ white dwarfs.
Arguably, the best method for determining the effective temperature ($T_{mathrm{eff}}$) and surface gravity (log $g$) of a DA white dwarf is by fitting the Hydrogen Lyman and Balmer absorption features. However, as has been shown for white dwarfs with $T_{mathrm{eff}}$>50,000K, the calculated value from the Lyman and Balmer lines are discrepant, which worsens with increasing temperature. Many different solutions have been suggested, ranging from the input physics used to calculate the models, to interstellar reddening. We will focus on the former, and consider three variables. The first is the atomic data used, namely the number of transitions included in line blanketing treatments and the photoionization cross sections. The second is the stark broadening treatment used to synthesise the Lyman and Balmer line profiles, namely the calculations performed by Lemke (1997) and Tremblay & Bergeron (2009). Finally, the third is the atmospheric content. The model grids are calculated with a pure H composition, and a metal polluted composition using the abundances of Preval et al. (2013). We present the preliminary results of our analysis, whereby we have determined the $T_{mathrm{eff}}$ for a small selection of white dwarfs. We plan to extend our analysis by allowing metallicity to vary in future model grids.
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