HI-to-H2 Transition Layers in the Star-Forming Region W43
Abstract
The process of atomic-to-molecular (HI-to-H2) gas conversion is fundamental for molecular-cloud formation and star formation. 21 cm observations of the star-forming region W43 revealed extremely high HI column densities, of 120-180 M pc-2, a factor of 10-20 larger than predicted by HI-to-H2 transition theories. We analyze the observed HI with an HI-to-H2 transition theoretical model, and show that the theory-observation discrepancy cannot be explained by the intense radiation in W43, nor by variations of the assumed volume density or H2 formation-rate coefficient. We show that the large observed HI columns are naturally explained by several (9-22) HI-to-H2 transition layers, superimposed along the sightlines of W43. We discuss other possible interpretations such as a non-steady-state scenario, and inefficient dust absorption. The case of W43 suggests that HI thresholds reported in extra-galactic observations are probably not associated with a single HI-to-H2 transition, but are rather a result of several transition layers (clouds) along the sightlines, beam-diluted with diffuse inter-cloud gas.