The Ratio of Helium‐ to Hydrogen‐Atmosphere White Dwarfs: Direct Evidence for Convective Mixing

Abstract

We determine the ratio of helium-to hydrogen-atmosphere white dwarf stars as a function of effective temperature from a model atmosphere analysis of the infrared photometric data from the Two Micron All Sky Survey combined with available visual magnitudes. Our study surpasses any previous analysis of this kind both in terms of the accuracy of the T eff determinations as well as the size of the sample. We observe that the ratio of helium-to hydrogen-atmosphere white dwarfs increases gradually from a constant value of ∼ 0.25 between T eff = 15, 000 K and 10,000 K to a value twice as large in the range 10, 000 > T eff > 8000 K, suggesting that convective mixing, which occurs when the bottom of the hydrogen convection zone reaches the underlying convective helium envelope, is responsible for this gradual transition. The comparison of our results with an approximate model used to describe the outcome of this convective mixing process implies hydrogen mass layers in the range M H /M tot = 10 −10 to 10 −8 for about 15% of the DA stars that survived the DA to DB transition near T eff ∼ 30, 000 K, the remainder having presumably more massive layers above M H /M tot ∼ 10 −6 .

Spectroscopic analysis of DA white dwarfs with 3D model atmospheres

“…Finally, it is worth mentionning that our assumption of thick H layers for all DA white dwarfs also has a slight effect on our results. Considering that most ( > ∼ 80%) DAs appear to have thick layers (Tremblay & Bergeron 2008), we expect an offset below 1% on the derived mean masses.…”

X-Ray and Infrared Observations of Two Externally Polluted White Dwarfs

“…The relative amount of hydrogen and helium in the atmospheres of helium-dominated white dwarfs is not fully understood (Tremblay & Bergeron 2008; E. Y. Chen & B. Hansen 2009, in preparation. Regardless, it appears that GD 362 is unusual.…”

“…Zuckerman et al (2007) measured n(He)/n(H) = 14 in GD 362's photosphere, and there are two possible scenarios to explain this measurement. First, the star may have been a hydrogen-dominated white dwarf until it cooled to an effective temperature less than 11,000 K. At this temperature, the mass of the convective envelope increased, and it is possible that some interior helium was dredged up to the surface (Tremblay & Bergeron 2008). However, even if this scenario correctly describes GD 362's history, the star still may be unusual.…”

DISCOVERY OF A POSSIBLE COOL WHITE DWARF COMPANION FROM THE AllWISE MOTION SURVEY

“…However, the effects of these two possibilities on CIA have to be taken into account. Borysow et al (1997) showed that CIA decreases with increasing effective temperature, and increases with surface gravity, and that CIA is pronounced for low metallicity (Z<0.1  Z ) and effective temperature up to 4000 K. Bergeron et al (2005) and Tremblay & Bergeron (2008) computed pure-H atmospheric models of cool white dwarfs. In these models, the color V−H becomes redder by ∼0.10-0.17 mag when the effective temperature increases from 4200 K (as in models for WD J0205−053) to 4500 K (the temperature at which the above reddening effect, due to reduced CIA, is reversed).…”