Cold Atomic Hydrogen in the Perseus Molecular Cloud
American Astronomical Society, AAS Meeting #221, #349.12
Published in Jan 2013
A recent theoretical model of the transition from atomic to molecular hydrogen in a large molecular cloud predicts that a certain HI column density is required to shield H2 against photodissociation. In this shielding model, once the minimum HI column density is achieved, all additional HI is converted into H2, resulting in a uniform HI column density distribution. HI observations of the Perseus Molecular Cloud (Lee et al. 2012) show a remarkably uniform HI column density distribution of ~8*10^20 cm^-2, slightly under the prediction. However, a uniform HI column density distribution could also be due to the presence of a large amount of cold, high optical depth HI gas. By looking at 27 background radio sources, we obtain absorption and emission spectra of the 21-cm hydrogen line for each source. We decompose these spectra into Gaussian components, from which we calculate the maximum kinetic temperature and spin temperature of each component. Total column density of HI is then derived from these temperatures. Comparing this total column density for 27 sources to the detected column density, we derive the correction factor for cold gas. We applied the correction to the Perseus Molecular Cloud and found that the median of the corrected HI column density is now ~10^21 cm^-2, which is closer to what the theoretical model predicts for the shielding HI column density. Our study, therefore, suggests the need for high optical depth correction in other molecular clouds. For a more systematic test of the theoretical model, we plan to expand our study by deriving HI column density images to study spatial variation and applying the high optical depth correction for other molecular clouds in a wide range of environments. This work was partially supported by the National Science Foundation’s REU program through NSF Award AST-1004881.
(c) 2013: American Astronomical Society