Tracing H2 column density with atomic carbon (CI) and CO isotopologues
  We present first results of neutral carbon ([CI], 3P1 - 3P0 at 492 GHz) and
carbon monoxide (13CO, J = 1 - 0) mapping in the Vela Molecular Ridge cloud C
(VMR-C) and G333 giant molecular cloud complexes with the NANTEN2 and Mopra
telescopes. For the four regions mapped in this work, we find that [CI] has
very similar spectral emission profiles to 13CO, with comparable line widths.
We find that [CI] has opacity of 0.1 - 1.3 across the mapped region while the
[CI]/13CO peak brightness temperature ratio is between 0.2 to 0.8. The [CI]
column density is an order of magnitude lower than that of 13CO. The H2 column
density derived from [CI] is comparable to values obtained from 12CO. Our maps
show CI is preferentially detected in gas with low temperatures (below 20 K),
which possibly explains the comparable H2 column density calculated from both
tracers (both CI and 12CO underestimate column density), as a significant
amount of the CI in the warmer gas is likely in the higher energy state
transition ([CI], 3P2 - 3P1 at 810 GHz), and thus it is likely that
observations of both the above [CI] transitions are needed in order to recover
the total H2 column density.
