Formation mechanism of brown dwarf binaries orbiting stars

Determine the formation mechanisms of brown dwarf binaries that orbit stars, with particular attention to systems exhibiting very tight separations (less than 1 astronomical unit) such as Gliese 229 Ba–Bb, in order to reconcile observations and simulations of substellar multiplicity at small separations.

Background

Gliese 229 B is resolved into a close binary brown dwarf (Gliese 229 Ba and Bb) with a 12.1-day period and a separation of ~0.042 AU, making it among the tightest known brown dwarf binaries in triple systems. Previous studies indicate brown dwarf binaries tend to have separations of 1–3 AU and near-equal mass ratios, leaving the origin of such exceptionally tight binaries uncertain.

The paper notes that observations and simulations are highly incomplete for sub-AU separations, and discusses several proposed pathways including opacity-limited fragmentation and disk fragmentation. Any viable mechanism must also account for the observed eccentric, misaligned outer orbit of Gliese 229 A–Bab.