Tracing H2 column density with atomic carbon (C i) and CO isotopologs.
LO N., CUNNINGHAM M.R., JONES P.A., BRONFMAN L., CORTES P.C., SIMON R., LOWE V., FISSEL L. and NOVAK G.
Abstract (from CDS):
We present the first results of neutral carbon ([C I] 3 P1-3 P0 at 492 GHz) and carbon monoxide (13CO, J = 1-0) mapping in the Vela Molecular Ridge cloud C (VMR-C) and the G333 giant molecular cloud complexes with the NANTEN2 and Mopra telescopes. For the four regions mapped in this work, we find that [C I] has very similar spectral emission profiles to 13CO, with comparable line widths. We find that [C I] has an opacity of 0.1-1.3 across the mapped region while the [C I]/13CO peak brightness temperature ratio is between 0.2 and 0.8. The [C I] column density is an order of magnitude lower than that of 13CO. The H2column density derived from [C I] is comparable to values obtained from 12CO. Our maps show that C I is preferentially detected in gas with low temperatures (below 20 K), which possibly explains the comparable H2column density calculated from both tracers (both C I and 12CO underestimate column density), as a significant amount of the C I in the warmer gas is likely in the higher energy state transition ([C I] 3 P2-3 P1 at 810 GHz), and thus it is likely that observations of both the above [C I] transitions are needed in order to recover the total H2column density.