A HIGH-RESOLUTION STUDY OF THE HI-H2 TRANSITION ACROSS THE PERSEUS MOLECULAR CLOUD

To investigate the fundamental principles of H-2 formation in a giant molecular cloud, we derive the HI and H-2 surface density (Sigma(HI) and Sigma(H2)) images of the Perseus molecular cloud on sub-pc scales (similar to 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 HI data from the Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local dust-to-gas ratio, we then derive the Sigma(H2) distribution across Perseus. We find a relatively uniform Sigma(HI) similar to 6-8 M-circle dot pc(-2) for both dark and star-forming regions, suggesting a minimum HI surface density required to shield H-2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Sigma(H2)/Sigma(HI) and Sigma(HI) + Sigma(H2). The transition between the HI- and H-2-dominated regions occurs at N(HI) + 2N(H-2) similar to (8-14) x 10(20) cm(-2). Our findings are consistent with predictions for H-2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H-2 formation. However, the importance of a warm neutral medium for H-2 shielding, an internal radiation field, and the timescale of H-2 formation still remain as open questions. We also compare H-2 and CO distributions and estimate the fraction of CO-dark gas, f(DG) similar to 0.3. While significant spatial variations of f(DG) are found, we do not find a clear correlation with the mean V-band extinction.