Probing the Transition from the Diffuse to the Molecular ISM

Dirks, Cody

doi:https://doi.org/10.21985/n2-e5gg-ky39

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Probing the Transition from the Diffuse to the Molecular ISM

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The study of the translucent interstellar medium (ISM) is important for understanding the behavior of the ISM as it transitions from hot, tenuous gas to dense, star-forming regions. This regime is know to be associated with the initial formation of persistent molecular material, and thus represents the first stages of the process that eventually forms the large molecular clouds responsible for the bulk of the star formation in the Galaxy. The Planck mission has provided a wealth of information about this regime of the ISM via the Planck Catalogue of Galactic Cold Clumps, which contains thousands of sources of cold dust emission. These sources represent a broad range of physical conditions spanning from diffuse sources with relatively little molecular material to dense, highly molecular cores embedded within larger molecular cloud complexes. This work uses high-resolution ultraviolet absorption-line spectroscopy from the Hubble Space Telescope to investigate these sources and thus study the translucent ISM. 25 spectroscopic targets whose sightlines probe the environments of a PGCC source were chosen. Neutral carbon and carbon monoxide were used to probe the gas content of these environments, while oxygen, krypton, germanium, magnesium, and nickel were used to probe the dust content. Analyses of these species reveal that a transition occurs around a CO column density of N(CO)∼10^15 cm^−2. Above this threshold, it appears that smoother, more coherent molecular envelopes begin to form out of the patchier, diffuse ISM that is present below this threshold. These molecular envelopes show evidence of being quiescent rather than turbulent, dynamic sources. Additionally, a correlation is observed between the presence of molecular gas and elemental depletion onto dust grains within these sources. Taken as a whole, these results suggest that the PGCC sources do indeed represent an important transitionary phase of the ISM.

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