Increases to Inferred Rates of Planetesimal Accretion Due to Thermohaline Mixing in Metal Accreting White Dwarfs

Abstract

Many isolated, old white dwarfs (WDs) show surprising evidence of metals in their photospheres. Given that the timescale for gravitational sedimentation is astronomically short, this is taken as evidence for ongoing accretion, likely of tidally disrupted planetesimals. The rate of such accretion, ${\dot{M}}_{\mathrm{acc}}$, is important to constrain, and most modeling of this process relies on assuming an equilibrium between diffusive sedimentation and metal accretion supplied to the WD's surface convective envelope. Building on earlier work of Deal and collaborators, we show that high ${\dot{M}}_{\mathrm{acc}}$ models with only diffusive sedimentation are unstable to thermohaline mixing and that models which account for the enhanced mixing from the active thermohaline instability require larger accretion rates, sometimes reaching ${\dot{M}}_{\mathrm{acc}}\approx 10^{13}\mathrm{g}{\mathrm{s}}^{-1}$ to explain observed Calcium abundances. We present results from a grid of MESA models that include both diffusion and thermohaline mixing. These results demonstrate that both mechanisms are essential for understanding metal pollution across the range of polluted WDs with hydrogen atmospheres. Another consequence of active thermohaline mixing is that the observed metal abundance ratios are identical to accreted material.

arXiv comment7 pages, 4 figures; Accepted for publication in ApJ Letters

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