2011A&A...527A..45L

We wish to understand the processes whereby the dominant state of free carbon shifts from C⁺ to C I and CO in progressively denser and/or darker diffuse and translucent clouds. We discuss recent compilations and observations of C I, H I, H₂ and CO measured in uv absorption and compare the observations with models of the thermal and ionization equilibrium including and excluding grain-assisted neutralization of atomic ions such as C⁺. There are significant disparities in N(C I) and divergent behaviour with respect to H I and especially H₂ and CO in two recent discussions of the C I abundance in diffuse and translucent gas. If the older data tabulated by Wolfire et al. (2008ApJ...680..384W) are considered, the run of N(C I) with N(H I) and N(H₂) is comfortably explained only by models embodying grain-assisted atomic-ion neutralization, much as those authors noted. If the newer data of Burgh et al. (2010ApJ...708..334B) are considered, either lower density models with grain-assisted atomic-ion neutralization or much denser models without it may suffice. In either case N(CO) increases from 10¹⁴cm^–2 to 10¹⁶cm^–2 with little change in N(C I) in either dataset, presenting a real challenge to models of C⁺ recombination and CO formation in the C⁺->CI->CO transition. N(CO) exceeds N(C I) even at N(CO)>3x10¹⁵cm^–2, well within the regime of diffuse gas where the dominant form of free gas phase carbon is C⁺; one of the supposed signatures of the translucent regime, that C I is the dominant form of free carbon, is not found on the sky. However, the C I data clearly need to be put on a firmer basis before the C⁺->CI->CO transition may be understood. Ambiguities in the C I column densities determined in uv absorption may perhaps be resolved by sub-mm observations with Herschel or ALMA.