Numerical simulation of DA white dwarf surface convection
Abstract
White dwarfs are compact objects with masses comparable to our Sun, but a radius similar to our Earth. They are the final evolutionary stage for about 95% of all stars in the Galaxy, i.e., for all stars that have a final mass less than the Chandrasekhar mass (about 1.4 times the solar mass), the upper mass limit for which hydrostatic equilibrium can be maintained by the degeneracy pressure of electrons at very high densities. The outermost shell of most white dwarfs contains a convective layer. Even if the latter is very thin (≲ 10 km), it is important for mixing properties, observed radiation, and pulsational stability of the whole object. During a long phase white dwarfs have effective temperatures Teff of about 10000K ∼ 14000K, since the time scale to reach such temperatures by cooling is already ≈ 109 years. Here, we focus on DA (hydrogen-rich) white dwarfs with Teff ≈ 12000K. This is at the transition from shallow to deep convection zones. Due to very high gravitational acceleration (∼ 106 g at the surface) the material is overturned about five times per second over the distance of a few kilometers. Numerical simulations of such objects have to be done for a compressible flow and feature highly turbulent granules at the surface, which are qualitatively comparable to the convection cells observed at the surface of the Sun. For this study we compare three white dwarf surface simulations with realistic microphysical properties and full 3D radiative transport. The simulations differ in effective temperature, namely, Teff = 11800K, 12100K, and 12400K. A statistical analysis of the convective processes as function of Teff is presented.
- Publication:
-
Journal of Physics Conference Series
- Pub Date:
- May 2018
- DOI:
- 10.1088/1742-6596/1031/1/012013
- Bibcode:
- 2018JPhCS1031a2013Z