Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-10-09
Astronomy and Astrophysics
Astrophysics
Accepted by MNRAS, high-res (recommended) can be download at http://www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications/
Scientific paper
10.1111/j.1365-2966.2008.14069.x
We suggest that a high proportion of brown dwarfs are formed by gravitational fragmentation of massive extended discs around Sun-like stars. Such discs should arise frequently, but should be observed infrequently, precisely because they fragment rapidly. By performing an ensemble of radiation-hydrodynamic simulations, we show that such discs fragment within a few thousand years, and produce mainlybrown dwarf (BDs) stars, but also planetary mass (PM) stars and very low-mass hydrogen-burning (HB) stars. Most of the the PM stars and BDs are ejected by mutual interactions. We analyse the statistical properties of these stars, and compare them with observations. After a few hundred thousand years the Sun-like primary is typically left with a close low-mass HB companion, and two much wider companions: a low-mass HB star and a BD star, or a BD-BD binary. There is a BD desert extending out to at least ~100 AU; this is because BDs tend to be formed further out than low-mass HB stars, and then they tend to be scattered even further out, or even into the field. BDs form with discs of a few Mj and radii of a few tens of AU, and they are more likely to retain these discs if they remain bound to the primary star. Binaries form by pairing of the newly-formed stars in the disc, giving a low-mass binary fraction of ~0.16. These binaries include close and wide BD/BD binaries and BD/PM binaries. BDs that remain as companions to Sun-like stars are more likely to be in BD/BD binaries than are BDs ejected into the field. Disc fragmentation is a robust mechanism; even if only a small fraction of Sun-like stars host the required massive extended discs,this mechanism can produce all the PM stars observed, most of the BD stars, and a significant proportion of the very low-mass HB stars.
Stamatellos Dimitris
Whitworth Anthony P.
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