August 11-15, 2014


Calibration of the Mixing-Length Theory for White Dwarf Structures

Pier-Emmanuel Tremblay (Space Telescope Science Institute)

H.-G. Ludwig (Zentrum fur Astronomie der Universitat Heidelberg, Landessternwarte), B. Freytag (Astronomical Observatory, Uppsala University), G. Fontaine (Universite de Montreal), M. Steffen (Leibniz-Institut fur Astrophysik Potsdam), and P. Brassard (Universite de Montreal)

We present a comparison of our grid of 3D radiation-hydrodynamical simulations for 70 pure-hydrogen DA white dwarfs, in the range 7.0 < log g < 9.0, with 1D envelope models based on the mixing-length theory (MLT) for convection. We perform a calibration of the MLT for the lower part of the convective zone, which can be applied to 1D structure calculations, and in particular for pulsation, diffusion, and convective mixing studies. This calibration represents the integrated effect of the convective zone on the structures, or in other words, the difference in temperature and pressure between the surface layers and the bottom of the convective zone, which is different to the MLT calibration for spectroscopic applications. In many instances, the 3D simulations are restricted to the upper part of the convective zone, and in those cases, we rely on the resolved asymptotic entropy value of the adiabatic 3D upflows to calibrate 1D envelopes. We also demonstrate that while the 1D MLT only provides a bottom boundary of the convective zone based on the stability criterion, the 3D stratifications are more elaborate. In the non-local picture, convective bubbles are still accelerated at the unstable boundary, and it is essential to account for the significant overshoot layer below the convective zone in diffusion and convective mixing applications.

Mode of presentation: oral