On the Spectral Evolution of Cool, Helium-Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars

By Patrick Dufour, Pierre Bergeron, J. Liebert, H. Harris, G. Knapp, S. Anderson, P. Hall, M. Strauss, M. Collinge, M. Edwards. 2007

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We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the SDSS. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log(Ca/He) = -12 to -8. We also find that fits of the coolest objects show peculiarities, suggesting that our physical models may not correctly describe the conditions of high atmospheric pressure encountered in the coolest DZ stars. We find that the mean mass of the 11 DZ stars with trigonometric parallaxes, ⟨M⟩ = 0.63 M☉, is significantly lower than that obtained from pure helium models, ⟨M⟩ = 0.78 M☉, and in much better agreement with the mean mass of other types of white dwarfs. We determine hydrogen abundances for 27% of the DZ stars in our sample, while only upper limits are obtained for objects with low-S/N spectroscopic data. We confirm with a high level of confidence that the accretion rate of hydrogen is at least 2 orders of magnitude smaller than that of metals (and up to 5 in some cases) to be compatible with the observations. We find a correlation between the hydrogen abundance and the effective temperature, suggesting for the first time empirical evidence of a lower temperature boundary for the hydrogen screening mechanism. Finally, we speculate on the possibility that the DZA white dwarfs could be the result of the convective mixing of thin hydrogen-rich atmospheres with the underlying helium convection zone.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/9254415d46e8324fa452ebaa10c9669929994d2d

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Citations in the corpus

POLLUTION OF SINGLE WHITE DWARFS BY ACCRETION OF MANY SMALL ASTEROIDS (2008, background): “Although interstellar accretion could supply the metals in the atmospheres of contaminated white dwarfs (see, for example, Dupuis et al. 1993, Koester & Wilken 2006, Zuckerman et al. 2003, Dufour et al. 2007), there are difficulties with this model.”
SDSS DR7 WHITE DWARF CATALOG (2012, methodology): “We ﬁnd that these objects are best explained as helium-rich DAs, as conﬁrmed by ﬁts with a grid of helium-rich white dwarf stars with traces of hydrogen (see Dufour et al. 2007a).”
AGaiaData Release 2 catalogue of white dwarfs and a comparison with SDSS (2018, background): “Finally the bottom right panel shows a distinction in the distribution of DQ (Dufour et al. 2005) and hot DQ white dwarfs (Dufour et al. 2007a).”
A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS (2011, methodology): “We can explore this problem more quantitatively by converting the hydrogen abundances from Figure 25 into total hydrogen masses by assuming that hydrogen is indeed homogeneously mixed in the helium convection zone (see also Dufour et al. 2007a and Voss et al. 2007).”
The Chemical Composition of an Extrasolar Minor Planet (2007, background): “…the discovery of metals, notably calcium, in the spectra of numerous white dwarf helium atmospheres (Sion et al. 1990a, 1990b;Koester et al. 2005b;Dufour et al. 2007; and references therein) and hydrogen atmospheres (Zuckerman et al. 2003; Koester et al. 2005b), stimulated a suite of explanatory…”
KNOW YOUR NEIGHBORHOOD: A DETAILED MODEL ATMOSPHERE ANALYSIS OF NEARBY WHITE DWARFS (2012, methodology): “The fitting procedure for DZ stars is similar to that described above for DQ stars with the exception that spectroscopic observations of the Ca ii H & K doublet are used to determine the metal abundance (see also Dufour et al. 2007).”
Circumstellar Debris and Pollution at White Dwarf Stars (2016, background): “…at the lowest temperatures introduces some difficulty in re-producing extraordinarily broad and complex metal absorption features (Koester et al., 2011), and the highest (approaching liquid) densities found in helium atmospheres leads to uncertainties in the equation of state (Dufour et al., 2007).”
ANCIENT PLANETARY SYSTEMS ARE ORBITING A LARGE FRACTION OF WHITE DWARF STARS (2010, background): “…(but non-zero) pollution, were the observed high-Z elements initially carried by multiple rocky objects of only modest size that originate in an ancient planetary system (Jura 2008), or do these elements originate from an alternative source, for example the interstellar medium (Dufour et al. 2007)?”