HI-to-H2 Transition Layers in the Star-Forming Region W43

TL;DR

This study investigates the discrepancy between observed high HI column densities in W43 and existing HI-to-H2 transition theories, revealing multiple transition layers along sightlines as a key factor.

Contribution

The paper demonstrates that multiple HI-to-H2 transition layers explain high HI columns in W43, challenging the notion of a single transition threshold in molecular cloud formation.

Findings

High HI columns in W43 are due to multiple transition layers.

Discrepancies cannot be explained by radiation or density variations.

HI thresholds in extragalactic observations may result from multiple layered transitions.

Abstract

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.