Chemical tracers in proto-brown dwarfs: CN, HCN, and HNC observations
  We present results from a study of nitrogen chemistry in Class 0/I
proto-brown dwarfs (proto-BDs). We have used the IRAM 30 m telescope to observe
the CN (2-1), HCN (3-2), and HNC (3-2) lines in 7 proto-BDs. All proto-BDs show
a large CN/HCN abundance ratio of >20, and a HNC/HCN abundance ratio close to
or larger than unity. The enhanced CN/HCN ratios can be explained by high UV
flux originating from an active accretion zone in the proto-BDs. The larger
than unity HNC/HCN ratio for the proto-BDs is likely caused by a combination of
low temperature and high density. Both CN and HNC show a flat distribution with
CO, indicating that these species can survive in regions where CO is depleted.
We have investigated the correlations in the molecular abundances of these
species for the proto-BDs with Class 0/I protostars. We find tentative trends
of CN (HCN) abundances being about an order of magnitude higher (lower) in the
proto-BDs compared to protostars. HNC for the proto-BDs shows a nearly constant
abundance unlike the large spread of ~2 orders of magnitude seen for the
protostars. Also notable is a rise in the HNC/HCN abundance ratio for the
lowest luminosity objects, suggesting that this ratio is higher under
low-temperature environments. None of the relatively evolved Class Flat/Class
II brown dwarfs in our sample show emission in HNC. The HNC molecule can be
considered as an efficient tracer to search and identify early stage
sub-stellar mass objects.
