A Sample of [CII] Clouds Tracing Dense Clouds in Weak FUV Fields
  observed by Herschel
  The [CII] fine--structure line at 158um is an excellent tracer of the warm
diffuse gas in the ISM and the interfaces between molecular clouds and their
surrounding atomic and ionized envelopes. Here we present the initial results
from Galactic Observations of Terahertz C+ (GOTC+), a Herschel Key Project
devoted to study the [CII] fine structure emission in the galactic plane using
the HIFI instrument. We use the [CII] emission together with observations of CO
as a probe to understand the effects of newly--formed stars on their
interstellar environment and characterize the physical and chemical state of
the star-forming gas. We collected data along 16 lines--of--sight passing near
star forming regions in the inner Galaxy near longitudes 330 degrees and 20
degrees. We identify fifty-eight [CII] components that are associated with
high--column density molecular clouds as traced by 13CO emission. We combine
[CII], 12CO, and 13CO observations to derive the physical conditions of the
[CII]--emitting regions in our sample of high--column density clouds based on
comparison with results from a grid of Photon Dominated Region (PDR) models.
From this unbiased sample, our results suggest that most of [CII] emission
originates from clouds with H2 volume densities between 10e3.5 and 10e5.5 cm^-3
and weak FUV strength (CHI_0=1-10). We find two regions where our analysis
suggests high densities >10e5 cm^-3 and strong FUV fields (CHI=10e4-10e6),
likely associated with massive star formation. We suggest that [CII] emission
in conjunction with CO isotopes is a good tool to differentiate between regions
of massive star formation (high densities/strong FUV fields) and regions that
are distant from massive stars (lower densities/weaker FUV fields) along the
line--of--sight
