Investigating the Gas within the Planck Galactic Cold Clumps

The recently released Planck Catalogue of Galactic Cold Clumps (PGCC) is a detailed survey of the cold dust content of the interstellar medium across the entire sky. It provides a new opportunity to understand the processes through which interstellar material is cycled from diffuse clouds to molecular clouds to star-forming cloud cores. By combining the data from Planck with the diagnostic capabilities of UV absorption-line spectroscopy, we aim to better characterize the physical conditions of the interstellar gas within the PGCC clouds. We have identified 35 stellar sightlines (with a total of 165 high-resolution UV spectral datasets in the HST STIS data archive) that lie close on the sky to one or more of these cold clouds. We plan to analyze the corresponding interstellar absorption lines in these datasets to study a variety of important cloud properties. Specifically, our goals in this archival study are to: (1) determine the thermal gas pressures of the PGCC clouds through measurements of the C I fine-structure excitation in the targeted sightlines; (2) compare any trends in the measured pressures to the morphology of the PGCC clouds; (3) search for evidence of shock processing in the PGCC clouds through comparisons of the gas-phase abundances of elements that are typically heavily- and lightly-depleted into dust grains; and (4) measure the isotopic CO abundance ratio in the PGCC clouds and determine the extent to which their chemical processes are a function of the cloud pressure.