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We have carried out an in-depth study of the peripheral region of themolecular cloud L1204/S140, where the far ultraviolet radiation and the densityare relatively low. Our observations test theories of photon-dominated regions(PDRs) in a regime that has been little explored. CII 158 micron and OI 63micron lines are detected by ISO at all 16 positions along a 1-dimensional cutin right ascension. Emission from molecular hydrogen rotational transitions, at28 and 17 micron, was also detected at several positions. The CII, OI, andmolecular hydrogen intensities along the cut show much less spatial variationthan do the rotational lines of CO and other CO isotopes. The average CII andOI intensities and their ratio are consistent with models of PDRs with low FUVradiation (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 inthe H2 rotational lines by up to an order of magnitude. This problem has alsobeen seen in bright PDRs and attributed to factors, such as geometry andgas-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 morefundamental solutions, such as higher H2 formation rates, are needed. Also, inthis regime of low density and small line width, the OI line is sensitive tothe radiative transfer and geometry. Using the ionization structure of themodels, a quantitative analysis of timescales for ambipolar diffusion in theperipheral regions of the S140 cloud is consistent with a theory ofphotoionization-regulated star formation. Observations of CII in other galaxiesdiffer both from those of high Go PDRs in our galaxy and from the low Goregions we have studied. ~Comment: 20 pagers, 6 figures, accepted to Ap. J. Uses emulate ap

Photon‐dominated Regions in Low‐Ultraviolet Fields: A Study of the Peripheral Region of L1204/S140