A study of metal abundance patterns in cool white dwarfs. II - Simulations of accretion episodes

Time-dependent simulations of episodic accretion of metals onto cool white dwarfs are presented. Calculations are made for Mg, Si, Ca, and Fe in H-rich and He-rich white dwarf models having a mass of 0.6 M ○ . in the range 25,000 K≥T eff ≥5000 K. It is first shown that under the assumption of continuous accretion the metal distributions rapidly reach a steady state in the outer layers of the models. The surface steady-state abundance of a metal directly scales on the assumed accretion rate. Using estimates of accretion rates based on a simplified understanding of the interstellar medium, a two-phase accretion model is next developed