New white dwarf envelope models and diffusion

Abstract

Context. Recent studies of the atmospheres of carbon-rich (DQ) white dwarfs have demonstrated the existence of two different populations that are distinguished by the temperature range, but more importantly, by the extremely high masses of the hotter group. The classical DQ below 10 000 K are well understood as the result of dredge-up of carbon by the expanding helium convection zone. The high-mass group poses several problems regarding their origin and also an unexpected correlation of effective temperature with mass.Aims. We propose to study the envelopes of these objects to determine the total hydrogen and helium masses as possible clues to their evolution. Methods. We developed new codes for envelope integration and diffusive equilibrium that are adapted to the unusual chemical composition, which is not necessarily dominated by hydrogen and helium. Results. Using the new results for the atmospheric parameters, in particular, the masses obtained using Gaia parallaxes, we confirm that the narrow sequence of carbon abundances with T eff in the cool classical DQ is indeed caused by an almost constant helium to total mass fraction, as found in earlier studies. This mass fraction is smaller than predicted by stellar evolution calculations. For the warm DQ above 10 000 K, which are thought to originate from double white dwarf mergers, we obtain extremely low hydrogen and helium masses. The correlation of mass with T eff remains unexplained, but another possible correlation of helium layer masses with T eff as well as the gravitational redshifts casts doubt on the reality of both and suggests possible shortcomings of current models.

Planar and non-planar nucleus-acoustic solitary waves in warm degenerate multi-nucleus plasmas

“…We have obtained the direct numerical solutions of (3.13) using this initial pulse, and using typical plasma parameters corresponding to hot white dwarfs, e.g. DQ white dwarfs (Dufour et al 2008(Dufour et al , 2011Koester et al 2020), white dwarf H1504+65 (Werner & Rauch 2015), white dwarf PG 0948+534 (Werner et al 2019), etc. The direct numerical solutions of (3.13) are displayed in figure 4.…”

“…2008, 2011; Koester et al. 2020), white dwarf H1504+65 (Werner & Rauch 2015), white dwarf PG 0948+534 (Werner et al. 2019), etc.…”

“…The works highlighted above are valid only for absolutely cold degenerate plasma species, but are not valid for degenerate plasma species at finite temperature, particularly for hot white dwarfs (Dufour et al 2008(Dufour et al , 2011Werner & Rauch 2015;Werner, Rauch & Reindl 2019;Koester, Kepler & Irwin 2020). To overcome this limitation in previously published works (Mamun et al 2016(Mamun et al , 2017Zaman et al 2017Zaman et al , 2018Chowdhury et al 2018;Jannat & Mamun 2018a,b;Das & Karmakar 2019), the equation of state for a degenerate plasma at finite temperature (Manfredi 2005;Vavrukh & Smerechynskyi 2012;Hossain & Mandal 2019) is derived by quantum field theory (Hossain & Mandal 2019) and by introducing two parameters, namely σ j = m j c 2 /E Fj and σ Tj = k B T j /E Fj , where E Fj (T j ) is the the Fermi energy (temperature) of species j with k B being the Boltzmann constant.…”

“…The WDP model of our present consideration is applicable not only to cold white dwarfs, but also to hot white dwarfs like DQ white dwarfs, white dwarf H1504+65, white dwarf PG 0948+534, etc. (Dufour et al 2008(Dufour et al , 2011Werner & Rauch 2015;Werner et al 2019;Koester et al 2020).…”

“…The works highlighted above are valid only for absolutely cold degenerate plasma species, but are not valid for degenerate plasma species at finite temperature, particularly for hot white dwarfs (Dufour et al. 2008, 2011; Werner & Rauch 2015; Werner, Rauch & Reindl 2019; Koester, Kepler & Irwin 2020). To overcome this limitation in previously published works (Mamun et al.…”