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.
