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arXiv · 1007.3338

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

T. Velusamy·W. D. Langer·J. L. Pineda·P. F. Goldsmith·D. Li.·H. W. Yorke
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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.

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T. Velusamy, W. D. Langer, J. L. Pineda, P. F. Goldsmith, D. Li., H. W. Yorke. 2010-07-20. [CII] observations of H$_2$ molecular layers in transition clouds. https://doi.org/10.1051/0004-6361/201015091

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