Numerical simulations of convection at the surface of a ZZ Ceti white dwarf.
Abstract
We applied twodimensional hydrodynamics and nongrey radiative transfer calculations to the surface layers of a hydrogenrich white dwarf (spectral type DA) with T_eff_=12600K and log(g)=8.0, corresponding to a position in the HRdiagram slightly cooler than the hot boundary of the ZZ Ceti instability strip. In our simulations the entire convection zone including the overshoot layers is embedded in the computational box so that we obtain a complete and detailed model of convection for this representative object. We address the important question to what extent models based on mixing length theory (MLT) are able to predict the physical properties of convection. We find a rapidly (timescale ~100ms) evolving flow pattern with fast concentrated downdrafts surrounded by slow broad upflows of warmer material. Convection carries up to 30% of the total flux and excites internal gravity waves by dynamical processes associated with the merging of downdrafts. The mean entropy gradient is reversed with respect to MLT predictions in the deeper layers of the convection zone. Strong overshoot occurs at its upper and lower boundary. A synthetic spectrum calculated from the mean photospheric temperature stratification can be fitted satisfactorily with a MLT model adopting α=1.5. At greater depth the temperature profile approaches a model with α=4. The total depth of the convective layers is rather small compared to values suggested by studies of the excitation mechanism for the pulsations of DAs.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 April 1994
 Bibcode:
 1994A&A...284..105L
 Keywords:

 Computerized Simulation;
 Convective Flow;
 Numerical Integration;
 Stellar Atmospheres;
 Stellar Models;
 Variable Stars;
 White Dwarf Stars;
 Equations Of State;
 Hydrodynamics;
 Mixing Length Flow Theory;
 Stellar Temperature;
 Astrophysics;
 STARS: WHITE DWARFS;
 STARS: VARIABLES:;
 CONVECTION;
 HYDRODYNAMICS