Forming isolated brown dwarfs by turbulent fragmentation
  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.
