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HI-to-H2 transitions in the Perseus molecular cloud


  Shmuel Bialy [1]  ,  Amiel Sternberg [1]  ,  Min-Young Lee [2]  ,  Franck Le Petit [3]  ,  Evelyne Roueff [3]  
 [1] Raymond and Beverly Sackler School of Physics & Astronomy, Tel Aviv University, Israel
[2] Laboratoire AIM, CEA/IRFU/Service d'Astrophyque, France.
[3] LERMA, Observatoire de Paris, PSL Research University, CNRS, France.

Conversion of hydrogen gas from atomic (H I) to molecular (H2) form is of critical importance for the evolution of the interstellar medium (ISM) and for star-formation in galaxies.
Recently, Lee et al. (2015) used the H I data provided by the Galactic Arecibo L-band Feed Array H I Survey, together with far-infrared data from the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey, to derive H I and H2 surface densities for several
hundred sight-lines towards five dark and (low-mass) star-forming regions within the Perseus molecular cloud.
We use the Sternberg et al. (2014) theory for interstellar atomic to molecular conversion to analyze and fit the observed H I-to-H2 transitions in Perseus.
We constrain hydrogen densities of 11.8 to 1.8 cm-3 for the H I shielding envelopes. These H I gas densities are consistent with multiphased mixture in which the unstable neutral medium (UNM) contributes significantly to the
shielding of the H2 cores. Our analysis has important implications for the interpretation of global H I-to-H2 in galaxies and star-formation thresholds in the
Kennicutt-Schmidt relation.