HI-to-H2 Transitions in the Perseus Molecular Cloud

TL;DR

This study applies a theoretical model to analyze HI-to-H2 transitions in five regions of the Perseus molecular cloud, revealing insights into the shielding layers and gas phases involved in star formation.

Contribution

It demonstrates the use of the Sternberg et al. (2014) theory to interpret HI-to-H2 transitions in specific star-forming regions, constraining key physical parameters and suggesting a multiphased shielding layer.

Findings

HI surface densities are consistent with dust shielding dominated HI-to-H2 transitions.

Derived parameters indicate a range of atomic hydrogen densities and UV-to-gas ratios.

The shielding layers are likely multiphased, containing both thermally unstable and cold neutral medium.

Abstract

We use the Sternberg et al. (2014) theory for interstellar atomic to molecular (HI-to-H$_2$) conversion to analyze HI-to-H$_2$ transitions in five (low-mass) star-forming and dark regions in the Perseus molecular cloud, B1, B1E, B5, IC348, and NGC1333. The observed HI mass surface densities of 6.3 to 9.2 M$_{\odot}$ pc$^{-2}$ are consistent with HI-to-H$_2$ transitions dominated by HI-dust shielding in predominantly atomic envelopes. For each source, we constrain the dimensionless parameter $\alpha G$, and the ratio $I_{\rm UV}/n$, of the FUV intensity to hydrogen gas density. We find $\alpha G$ values from 5.0 to 26.1, implying characteristic atomic hydrogen densities 11.8 to 1.8 cm$^{-3}$, for $I_{\rm UV} \approx 1$ appropriate for Perseus. Our analysis implies that the dusty HI shielding layers are probably multiphased, with thermally unstable UNM gas in addition to cold CNM within the 21 cm kinematic radius.