H I-to-H₂ Transition Layers in the Star-forming Region W43

The process of atomic-to-molecular (H i-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 H i column densities, of 120-180 , a factor of 10-20 larger than predicted by H i-to-H₂ transition theories. We analyze the observed H i with a theoretical model of the H i-to-H₂ transition, and show that the discrepancy between theory and observation cannot be explained by the intense radiation in W43, nor be explained by variations of the assumed volume density or H₂ formation rate coefficient. We show that the large observed H i columns are naturally explained by several (9-22) H i-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 H i thresholds reported in extragalactic observations are probably not associated with a single H i-to-H₂ transition, but are rather a result of several transition layers (clouds) along the sightlines, beam-diluted with diffuse intercloud gas.