Diffusion and accreting pollution of DBZ or DZ white dwarfs

Abstract

Context. Over 1500 DBZ or DZ white dwarfs (WDs) have been observed so far, and polluted atmospheres with metal elements have been found among these WDs. The surface heavy element abundances of known DBZ or DZ WDs show an evolutionary sequence. The cooling, diffusion and accretion are important physical processes in the WD evolution which can alter the element abundances of the WD surface. Aims. Using the stellar evolutionary code, we investigate the DB WD formation and the effects of input parameters−mixing length parameter (αMLT), thermohaline mixing efficiency (αth) and the metallicity (Z)−on the structures of these DB WDs. The impacts of convective zone mass (Mcvz), cooling timescales, diffusive timescales (τdiff), and mass-accretion rate (Ṁa) on the element abundances of the WDs’ surfaces are discussed. By comparing the theoretical model results with observations, we try to understand the evolutionary sequence of the heavy element abundance on DBZ WD surfaces. Methods. By using Modules for Experiments in Stellar Evolution, we create DB WDs, and simulate the element diffusion due to high gravitational fields and the metal-rich material accretion coming from the planet disrupted by the WD. Then, we calculate the element abundances of these DB WDs for the further comparison with observations. Results. In our models, the input parameters (αMLT, αth and Z) have very weak effect on DB WD structures including interior temperatures, chemical profiles and convective zones. They hardly affect the evolution of the heavy elements on the surface of DB WDs. The mass-accretion rate and the effective temperature of DB WDs determine the abundances of heavy elements. The evolutionary sequence of Ca element for about 1500 observed DB or DBZ WDs cannot be explained by the model with a constant mass-accretion rate, but is consistent well with the model in which the mass-accretion rate decreases by one power law when Teff > 10 kK and slightly increases by another power law when Teff < 10 kK. Conclusions. The observed DB WD evolutionary sequence of heavy element abundances originates from WD cooling and the change of mass-accretion rate.