Tracing H2 Column density with atomic carbon (C I) and Co isotopologs

Abstract

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 H2 column 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 H2 column 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 H2 column density.