Ultraviolet HST Spectroscopy of Planck Cold Clumps

We report results of the first study utilizing the ultraviolet capabilities of the Hubble Space Telescope to investigate a sample of Planck Galactic Cold Clump (PGCC) sources. We have selected high-resolution spectra toward 25 stars that contain a multitude of interstellar absorption lines associated with the interstellar medium (ISM) gas within these PGCC sources, including carbon monoxide (CO), C i and O i. By building cloud-component models of the individual absorption components present in these spectra, we can identify and isolate components associated with the PGCC sources, allowing for a more accurate investigation of the ISM behavior within these sources. Despite probing a broad range of overall sightline properties, we detect CO along each sightline. Sightlines with CO column density N(CO) > 10 ¹⁵ cm ^-2 exhibit spatial dependence in N(CO) and CO/C i, while sightlines with N(CO) < 10 ¹⁵ cm ^-2 show no such spatial dependence. Differences between N(H ₂ ) values derived from UV absorption and dust emission suggest structure in the spatial distribution of N(H ₂ ), where "CO-bright" sightlines are associated with PGCC sources embedded within smooth translucent envelopes, and "CO-dark" sightlines are associated with PGCC sources embedded in patchier environments containing more diffuse gas.