Modeling the Evolution of Silicate Volatile Accretion Discs around White Dwarfs by Ayaka Okuya et al. on Wednesday 30 November

A growing number of debris discs have been detected around metal-polluted

white dwarfs. They are thought to be originated from tidally disrupted

exoplanetary bodies and responsible for metal accretion onto host WDs. To

explain (1) the observationally inferred accretion rate higher than that

induced by Poynting-Robertson drag, $\dot{M}_{\rm PR}$, and (2) refractory-rich

photosphere composition indicating the accretion of terrestrial rocky

materials, previous studies proposed runaway accretion of silicate particles

due to gas drag by the increasing silicate vapor produced by the sublimation of

the particles. Because re-condensation of the vapor diffused beyond the

sublimation line was neglected, we revisit this problem by one-dimensional

advection/diffusion simulation that consistently incorporates silicate

sublimation/condensation and back-reaction to particle drift due to gas drag in

the solid-rich disc. We find that the silicate vapor density in the region

overlapping the solid particles follows the saturating vapor pressure and that

no runaway accretion occurs if the re-condensation is included. This always

limits the accretion rate from mono-compositional silicate discs to

$\dot{M}_{\rm PR}$ in the equilibrium state. Alternatively, by performing

additional simulations that couple the volatile gas (e.g., water vapor), we

demonstrate that the volatile gas enhances the silicate accretion to

$>\dot{M}_{\rm PR}$ through gas drag. The refractory-rich accretion is

simultaneously reproduced when the initial volatile fraction of disc is

$\lesssim 10$ wt\% because of the suppression of volatile accretion due to the

efficient back-reaction of solid to gas. The discs originating from C-type

asteroid analogs might be a possible clue to the high-$\dot{M}$ puzzle.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.16797v1