[CII] observations of H$_2$ molecular layers in transition clouds

TL;DR

This study uses [CII] line emission observations to analyze diffuse molecular clouds in transition regions, revealing the structure, evolution, and 'dark gas' components of H$_2$ layers around CO cores in the Galaxy.

Contribution

First detailed analysis of [CII] emission in transition clouds, linking [CII] intensities to molecular cloud structure and evolution, including dark gas estimation.

Findings

Approximately 30% of clouds have H$_2$ 'dark gas' exceeding 50% of CO-traced H$_2$.

On average, 25% of H$_2$ in clouds is not traced by CO.

Detected excess [CII] indicates thick H$_2$ layers around CO cores.

Abstract

We present the first results on the diffuse transition clouds observed in [CII] line emission at 158 microns (1.9 THz) towards Galactic longitudes near 340deg (5 LOSs) and 20deg (11 LOSs) as part of the GOT C+ survey. Out of the total 146 [CII] velocity components detected by profile fitting we identify 53 as diffuse molecular clouds with associated $^{12}$CO emission but without $^{13}$CO emission and characterized by A$_V$ < 5 mag. We estimate the fraction of the [CII] emission in the diffuse HI layer in each cloud and then determine the [CII] emitted from the molecular layers in the cloud. We show that the excess [CII] intensities detected in a few clouds is indicative of a thick H$_2$ layer around the CO core. The wide range of clouds in our sample with thin to thick H$_2$ layers suggests that these are at various evolutionary states characterized by the formation of H$_2$ and CO layers from HI and C$^+$, respectively. In about 30% of the clouds the H$_2$ column densities (''dark gas'') traced by the [CII] is 50% or more than that traced by $^{12}$CO emission. On the average about 25% of the total H$_2$ in these clouds is in an H$_2$ layer which is not traced by CO. We use the HI, [CII], and $^{12}$CO intensities in each cloud along with simple chemical models to obtain constraints on the FUV fields and cosmic ray ionization rates.