CII in the Interstellar Medium: Excitation by H2 Revisited
  C$^+$ is a critical constituent of many regions of the interstellar medium,
as it can be a major reservoir of carbon and, under a wide range of conditions,
the dominant gas coolant. Emission from its 158$\mu$m fine structure line is
used to trace the structure of photon dominated regions in the Milky Way and is
often employed as a measure of the star formation rate in external galaxies.
Under most conditions, the emission from the single [CII] line is proportional
to the collisional excitation rate coefficient. We here used improved
calculations of the deexcitation rate of [CII] by collisions with H$_2$ to
calculate more accurate expressions for interstellar C$^+$ fine structure
emission, its critical density, and its cooling rate. The collision rates in
the new quantum calculation are $\sim$ 25% larger than those previously
available, and narrow the difference between rates for excitation by atomic and
molecular hydrogen. This results in [CII] excitation being quasi-independent of
the molecular fraction and thus dependent only on the total hydrogen particle
density. A convenient expression for the cooling rate at temperatures between
20 K and 400 K, assuming an LTE H$_2$ ortho to para ration is $\Lambda ({\rm
LTE~OPR}) = \left(11.5 + 4.0\,e^{-100\,\mathrm K/T^{\rm
kin}}\right)\;e^{-91.25\,\mathrm K/T^{\rm kin}}\,n ({\rm C}^{+})\,n({\rm
H}_2)\times 10^{-24}\;{\rm ergs}~{\rm cm}^{-3}~{\rm s}^{-1}$. The present work
should allow more accurate and convenient analysis of the [\CII] line emission
and its cooling.
