2020ApJ...901...50E

We infer the density of hydrogen nuclei and optical depths associated with molecular gas in front of a star-forming region and in a quiescent region of the Galactic interstellar medium (ISM) using 18 cm OH and archival 3 mm ¹²CO observations, together with a diffuse cloud model from the literature. The results indicate that the volume densities of hydrogen nuclei are greatest near the star-forming region, intermediate in the CO-bright portions of the quiescent region, and lowest in the CO-dark portions of the quiescent region. Low volume density and low column density appear to be correlated with the presence of CO-dark molecular gas, which is primarily located in the vast space outside of star-forming regions. Molecular gas containing CO may exist in similar quantities both outside of and in the vicinity of star-forming regions; however, the CO emission radiates more efficiently in regions where the ISM is locally compressed, leading to the observational phenomenon of CO-dark and CO-bright molecular gas. We suggest that processes occurring as a natural consequence of star formation such as shock waves and stellar winds lead to compression of the gas into smaller pockets of higher volume density, and such "clumpy" structures will naturally emit more efficiently in CO owing to higher rates of collisional excitation. In this picture, the apparent prominence of CO emission in and near star-forming regions is itself a by-product of the star formation process.