# Ultraviolet HST Spectroscopy of Planck Cold Clumps

## TL;DR

This study uses Hubble Space Telescope ultraviolet spectroscopy to analyze interstellar medium components in Planck Cold Clumps, revealing spatial variations in CO and molecular hydrogen distributions within these cold molecular cloud sources.

## Contribution

First UV spectroscopic investigation of PGCC sources with detailed cloud-component modeling to distinguish ISM components and analyze molecular gas properties.

## Key findings

- CO detected along all sightlines with varying column densities.
- Spatial dependence observed in CO and CO/C I ratios for N(CO)>10^{15} cm^{-2}.
- Differences in N(H$_2$) suggest structural variations in PGCC environments.

## Abstract

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.