Photon-Dominated Region Modeling of the [C I],[C II], and CO Line
  Emission from a Boundary in the Taurus Molecular Cloud
  We present [Ci] and [Cii] observations of a linear edge region in the Taurus
molecular cloud, and model this region as a cylindrically symmetric PDR exposed
to a low-intensity UV radiation field. The sharp, long profile of the linear
edge makes it an ideal case to test PDR models and determine cloud parameters.
We compare observations of the [C i], 3P1 -> 3P0 (492 GHz), [C i] 3P2 -> 3P1
(809 GHz), and [Cii] 2P3/2 -> 2P1/2 (1900 GHz) transitions, as well as the
lowest rotational transitions of 12CO and 13CO, with line intensities produced
by the RATRAN radiative transfer code from the results of the Meudon PDR code.
We constrain the density structure of the cloud by fitting a cylindrical
density function to visual extinction data. We study the effects of variation
of the FUV field, 12C/13C isotopic abundance ratio, sulfur depletion, cosmic
ray ionization rate, and inclination of the filament relative to the sky-plane
on the chemical network of the PDR model and resulting line emission. We also
consider the role of suprathermal chemistry and density inhomogeneities. We
find good agreement between the model and observations, and that the integrated
line intensities can be explained by a PDR model with an external FUV field of
0.05 G0, a low ratio of 12C to 13C ~ 43, a highly depleted sulfur abundance (by
a factor of at least 50), a cosmic ray ionization rate (3 - 6) x 10-17 s^-1,
and without significant effects from inclination, clumping or suprathermal
chemistry.
