CO Outflow Survey of 68 Very Low Luminosity Objects: a Search for
  Proto-Brown Dwarf Candidates
  We present the results of a systematic search for molecular outflows in 68
Very Low Luminosity Objects (VeLLOs) from single-dish observations in CO
isotopologues which find 16 VeLLOs showing clear outflow signatures in the CO
maps. With additional three VeLLOs from the literature, we analyzed the outflow
properties for 19 VeLLOs, identifying 15 VeLLOs as proto-Brown Dwarf (BD)
candidates and four VeLLOs as likely faint protostar candidates. The proto-BD
candidates are found to have a mass accretion rate ($\sim 10^{-8} - 10^{-7}$
$\rm M_{\odot}$ yr$^{-1}$) lower than that of the protostar candidates
($\gtrsim 10^{-6}$ $\rm M_{\odot}$ yr$^{-1}$). Their accretion luminosities are
similar to or smaller than their internal luminosities, implying that many
proto-BD candidates might have had either small accretion activity in a
quiescent manner throughout their lifetime, or be currently exhibiting a
relatively higher (or episodic) mass accretion than the past. There are strong
trends that outflows of many proto-BDs are less active if they are fainter or
have less massive envelopes. The outflow forces and internal luminosities for
more than half of the proto-BD candidates seem to follow an evolutionary track
of a protostar with its initial envelope mass of $\sim$0.08 $\rm M_{\odot}$,
indicating that some BDs may form in less massive dense cores in a way similar
to normal stars. But, because there also exists a significant fraction (about
40%) of proto-BDs with much weaker outflow force than expected by the relations
for protostars, we should not rule out the possibility of other formation
mechanism for the BDs.
