HI-to-H2 Transition Layers in the Star-Forming Region W43
  The process of atomic-to-molecular (HI-to-H$_2$) 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$_{\odot}$ pc$^{-2}$, a factor of 10-20 larger than
predicted by HI-to-H$_2$ transition theories. We analyze the observed HI with
an HI-to-H$_2$ 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 H$_2$ formation-rate
coefficient. We show that the large observed HI columns are naturally explained
by several ($9-22$) HI-to-H$_2$ 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-H$_2$ transition, but are rather a
result of several transition layers (clouds) along the sightlines, beam-diluted
with diffuse inter-cloud gas.
