[CII] observations of H$_2$ molecular layers in transition clouds
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 CO emission but without CO emission and characterized by A < 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 layer around the CO core. The wide range of clouds in our sample with thin to thick H layers suggests that these are at various evolutionary states characterized by the formation of H and CO layers from HI and C, respectively. In about 30% of the clouds the H column densities (''dark gas'') traced by the [CII] is 50% or more than that traced by CO emission. On the average about 25% of the total H in these clouds is in an H layer which is not traced by CO. We use the HI, [CII], and CO intensities in each cloud along with simple chemical models to obtain constraints on the FUV fields and cosmic ray ionization rates.
Keywords
Cite
@article{arxiv.1007.3338,
title = {[CII] observations of H$_2$ molecular layers in transition clouds},
author = {T. Velusamy and W. D. Langer and J. L. Pineda and P. F. Goldsmith and D. Li. and H. W. Yorke},
journal= {arXiv preprint arXiv:1007.3338},
year = {2015}
}
Comments
To be published in HIFI A&A special edition