The effects of differences in composition, equation of state, and mixing length upon the structure of white-dwarf convection zones

The effects of differences in composition, equation of state, and convective mixing length upon the structure of white-dwarf convection zones are studied by comparing six different helium-rich envelope model sequences with g = 10 cm 2 and spanning the temperature range 6000 Te < 28,000 K. In the hotter models, the structure of the convection zone is quite sensitive to the choice of mixing length, but the core temperature is not affected. Composition differences have moderate effects throughout the temperature range and are understandable in terms of their effects upon the atmospheric opacities. Equation-of-state differences also are discernible at all Te, but are relatively unimportant above Te 7500 K. At low temperatures, however, the equation-of-state effects dominate, and a calculation for the envelope of Van Maanen 2 (Te = 5500 K, g = 5.65 x 10 cm) yields a temperature at the base of the convection zone of Tb = 1.12 x 100 K, compared with the value Tb = 2.15 x 100 K found previously for this temperature and surface gravity. In addition, the use of an improved equation of state appears to remove the previously reported sensitivity of Tb to changes ingand Te. Subject headings: abundances, stellar - interiors, stellar - white-dwarf stars