2008ApJ...689..276K

The presence of atomic gas mixed with molecular species in a ``molecular'' cloud may significantly affect its chemistry, the excitation of some species, and can serve as probe of the cloud's evolution. Cold neutral atomic hydrogen (H I) in molecular clouds is revealed by its self absorption of background galactic H I 21 cm emission. The properties of this gas can be investigated quantitatively through observation of H I narrow self-absorption (HINSA). In this paper we present a new technique for measuring atomic gas physical parameters from HINSA observations that utilizes molecular tracers to guide the HINSA extraction. This technique offers a significant improvement in the precision with which H I column densities can be determined over previous methods, and it opens several new avenues of study of relevance to the field of star formation.