A High-resolution Study of the H I-H2 Transition across the Perseus Molecular Cloud

Type Journal Article
Names Min-Young Lee, Snežana Stanimirović, Kevin A. Douglas, Lewis B. G. Knee, James Di Francesco, Steven J. Gibson, Ayesha Begum, Jana Grcevich, Carl Heiles, Eric J. Korpela, Adam K. Leroy, J. E. G. Peek, Nickolas M. Pingel, Mary E. Putman, Destry Saul
Publication The Astrophysical Journal
Volume 748
Issue 2
Pages 75
Date April 1, 2012
URL http://adsabs.harvard.edu/abs/2012ApJ...748...75L
Library Catalog NASA ADS
Abstract To investigate the fundamental principles of H2 formation in a giant molecular cloud, we derive the H I and H2 surface density (ΣH I and ΣH2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the ΣH2 distribution across Perseus. We find a relatively uniform ΣH I ~ 6-8 M &sun; pc-2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between ΣH2/ΣH I and ΣH I + ΣH2. The transition between the H I- and H2-dominated regions occurs at N(H I) + 2N(H2) ~ (8-14) × 1020 cm-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, f DG ~ 0.3. While significant spatial variations of f DG are found, we do not find a clear correlation with the mean V-band extinction.
Tags DUST, ISM: MOLECULES, ISM: individual objects: Perseus, Radio Lines: ISM, extinction, infrared: ISM
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