  We have carried out an in-depth study of the peripheral region of the
molecular cloud L1204/S140, where the far ultraviolet radiation and the density
are relatively low. Our observations test theories of photon-dominated regions
(PDRs) in a regime that has been little explored. CII 158 micron and OI 63
micron lines are detected by ISO at all 16 positions along a 1-dimensional cut
in right ascension. Emission from molecular hydrogen rotational transitions, at
28 and 17 micron, was also detected at several positions. The CII, OI, and
molecular hydrogen intensities along the cut show much less spatial variation
than do the rotational lines of CO and other CO isotopes. The average CII and
OI intensities and their ratio are consistent with models of PDRs with low FUV
radiation (Go) and density. The best-fitting model has Go about 15 and density,
n about 1000 per cubic cm. Standard PDR models underpredict the intensity in
the H2 rotational lines by up to an order of magnitude. This problem has also
been seen in bright PDRs and attributed to factors, such as geometry and
gas-grain drift, that should be much less important in the regime studied here.
The fact that we see the same problem in our data suggests that more
fundamental solutions, such as higher H2 formation rates, are needed. Also, in
this regime of low density and small line width, the OI line is sensitive to
the radiative transfer and geometry. Using the ionization structure of the
models, a quantitative analysis of timescales for ambipolar diffusion in the
peripheral regions of the S140 cloud is consistent with a theory of
photoionization-regulated star formation. Observations of CII in other galaxies
differ both from those of high Go PDRs in our galaxy and from the low Go
regions we have studied. ~
