THE H I PROBABILITY DISTRIBUTION FUNCTION AND THE ATOMIC-TO-MOLECULAR TRANSITION IN MOLECULAR CLOUDS

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. similar to 1-2 x 10(21) cm(-2) (equivalently, similar to 0.5-1 mag). The PDFs are uniformly narrow,. with a mean dispersion of sigma(Pi) (I) approximate to 10(20) cm(-2) (similar to 0.1 mag). We also investigate the H I-to-H-2. transition toward the cloud complexes and estimate H I. surface densities ranging from 7 to 16 M-circle dot pc(-2). at the transition. We propose that the H I. PDF is a fitting tool for identifying the H I-to-H-2. transition column in Galactic MCs.