Old pre-main-sequence Stars: Disc reformation by Bondi-Hoyle accretion

TL;DR

This paper proposes that Bondi-Hoyle accretion during star passage through molecular clouds can explain the presence of seemingly old (>30 Myr) stars with accretion discs, suggesting a potential second epoch of planet formation.

Contribution

It introduces a Monte Carlo and viscous evolution model demonstrating how old stars can rebuild discs via Bondi-Hoyle accretion, a novel explanation for observed old accretors.

Findings

A significant fraction of old stars can rebuild discs through Bondi-Hoyle accretion.

Up to 10% of such stars may have observable accretion levels.

This mechanism could enable a second epoch of planet formation.

Abstract

Young stars show evidence of accretion discs which evolve quickly and disperse with an e-folding time of $\sim$ 3Myr. This is in striking contrast with recent observations that suggest evidence for numerous $>30$ Myr old stars with an accretion disc in large star-forming complexes. We consider whether these observations of apparently old accretors could be explained by invoking Bondi-Hoyle accretion to rebuild a new disc around these stars during passage through a clumpy molecular cloud. We combine a simple Monte Carlo model to explore the capture of mass by such systems with a viscous evolution model to infer the levels of accretion that would be observed. We find that a significant fraction of stars may capture enough material via the Bondi-Hoyle mechanism to rebuild a disc of mass $\gtrsim$ 1 minimum-mass solar nebula, and $\lesssim 10\%$ accrete at observable levels at any given time. A significant fraction of the observed old accretors may be explained with our proposed mechanism. Such accretion may provide a chance for a second epoch of planet formation, and have unpredictable consequences for planetary evolution.