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2016ApJ...829..102I - Astrophys. J., 829, 102-102 (2016/October-1)

The H I probability distribution function and the atomic-to-molecular transition in molecular clouds.

IMARA N. and BURKHART B.

Abstract (from CDS):

We characterize the column-density probability distribution functions (PDFs) of the atomic hydrogen gas, H i, associated with seven Galactic molecular clouds (MCs). We use 21 cm observations from the Leiden/Argentine/Bonn Galactic H i Survey to derive column-density maps and PDFs. We find that the peaks of the H i PDFs occur at column densities in the range ∼1-2 x 1021 cm^{-2} (equivalently, ∼0.5-1 mag). The PDFs are uniformly narrow, with a mean dispersion of σ_{HI}~{10}{20} cm{-2} (∼0.1 mag). We also investigate the H i-to-H2 transition toward the cloud complexes and estimate H i surface densities ranging from 7 to 16 {M}_☉ pc^{-2} at the transition. We propose that the H i PDF is a fitting tool for identifying the H i-to-H2 transition column in Galactic MCs.