Forming isolated brown dwarfs by turbulent fragmentation

TL;DR

This paper investigates the turbulent conditions needed to form isolated brown dwarfs and reanalyzes a candidate core, suggesting such objects are rare and often transient, challenging the idea that turbulence alone explains their formation.

Contribution

It combines hydrodynamical simulations with Bayesian analysis to assess the likelihood of brown dwarf formation from low-mass cores, highlighting the unlikelihood of turbulence-driven formation pathways.

Findings

Strongly convergent flows are required for brown dwarf formation, which are unlikely.

Reanalysis shows a high probability that Oph-B11 is below the hydrogen burning limit.

Many low-mass cores may be transient or misidentified, complicating formation theories.

Abstract

We use Smoothed Particle Hydrodynamics to explore the circumstances under which an isolated very-low-mass prestellar core can be formed by colliding turbulent flows and collapse to form a brown-dwarf. Our simulations suggest that the flows need not be very fast, but do need to be very strongly convergent, i.e. the gas must flow in at comparable speeds from all sides, which seems rather unlikely. We therefore revisit the object Oph-B11, which Andre, Ward-Thompson and Greaves (2012) have identified as a prestellar core with mass between $\sim 0.020\,\mathrm{M_\odot}$ and $\sim 0.030\,\mathrm{M_\odot}$. We reanalyse the observations using a Markov-chain Monte Carlo method that allows us (i) to include the uncertainties on the distance, temperature and dust mass opacity, and (ii) to consider different Bayesian prior distributions of the mass. We estimate that the posterior probability that Oph-B11 has a mass below the hydrogen burning limit at $\sim 0.075\,\mathrm{M_\odot}$, is between 0.66 and 0.86. We conclude that, if Oph-B11 is destined to collapse, it probably will form a brown dwarf. However, the flows required to trigger this appear to be so contrived that it is difficult to envisage this being the only way, or even a major way, of forming isolated brown dwarfs. Moreover, Oph-B11 could easily be a transient, bouncing, prolate core, seen end-on; there could, indeed should, be many such objects masquerading as very low-mass prestellar cores.