Brown dwarf formation by gravitational fragmentation of massive, extended protostellar discs

TL;DR

This paper demonstrates through radiation hydrodynamic simulations that massive, extended protostellar discs around low-mass stars can fragment to form brown dwarfs and other low-mass objects, explaining their origins and properties.

Contribution

It introduces a new formation mechanism for brown dwarfs via gravitational fragmentation of massive protostellar discs, addressing limitations of previous models.

Findings

Outer disc regions (R>100 AU) are prone to fragmentation.

Formed brown dwarfs often retain small discs with infrared excess.

Some brown dwarfs form close binaries and survive ejection.

Abstract

We suggest that low-mass hydrogen-burning stars like the Sun should sometimes form with massive extended discs; and we show, by means of radiation hydrodynamic simulations, that the outer parts of such discs (R>100 AU) are likely to fragment on a dynamical timescale (10^3 to $10^4 yr), forming low-mass companions: principally brown dwarfs (BDs), but also very low-mass hydrogen-burning stars and planetary-mass objects. A few of the BDs formed in this way remain attached to the primary star, orbiting at large radii. The majority are released into the field, by interactions amongst themselves; in so doing they acquire only a low velocity dispersion (<2 km/s), and therefore they usually retain small discs, capable of registering an infrared excess and sustaining accretion. Some BDs form close BD/BD binaries, and these binaries can survive ejection into the field. This BD formation mechanism appears to avoid some of the problems associated with the `embryo ejection' scenario, and to answer some of the questions not yet answered by the `turbulent fragmentation' scenario.