Letter to the Editor Old pre-main-sequence stars Disc reformation by Bondi-Hoyle accretion

Young stars show evidence of accretion discs which evolve quickly and disperse with an e-folding time of ∼ 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 & 1 minimum-mass solar nebula, and. 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. Key words. Accretion, accretion discs – circumstellar matter – Stars:formation – Stars: pre-main sequence – Protoplanetary discs 1