A search for water maser emission from brown dwarfs and low-luminosity
  young stellar objects
  We present a survey for water maser emission toward a sample of 44
low-luminosity young objects, comprising (proto-)brown dwarfs, first
hydrostatic cores (FHCs), and other young stellar objects (YSOs) with
bolometric luminosities lower than 0.4 L$_\odot$. Water maser emission is a
good tracer of energetic processes, such as mass-loss and/or accretion, and is
a useful tool to study this processes with very high angular resolution. This
type of emission has been confirmed in objects with L$_{\rm bol}\ge 1$
L$_\odot$. Objects with lower luminosities also undergo mass-loss and
accretion, and thus, are prospective sites of maser emission. Our sensitive
single-dish observations provided a single detection when pointing toward the
FHC L1448 IRS 2E. However, follow-up interferometric observations showed water
maser emission associated with the nearby YSO L1448 IRS 2 { (a Class 0
protostar of L$_{\rm bol}\simeq 3.6-5.3$ L$_\odot$)}, and did not find any
emission toward L1448 IRS 2E. The upper limits for water maser emission
determined by our observations are one order of magnitude lower than expected
from the correlation between water maser luminosities and bolometric
luminosities found for YSOs. This suggests that this correlation does not hold
at the lower end of the (sub)stellar mass spectrum. Possible reasons are that
the slope of this correlation is steeper at L$_{\rm bol}\le 1$ L$_\odot$, or
that there is an absolute luminosity threshold below which water maser emission
cannot be produced. Alternatively, if the correlation still stands at low
luminosity, the detection rates of masers would be significantly lower than the
values obtained in higher-luminosity Class 0 protostars.
