Atomic carbon is a temperature probe in dark clouds

We have mapped the C I P-3(1) --> P-3(0) line at 492 GHz in three molecular clouds immersed in weak ultraviolet radiation fields, TMC-1, L134N, and IC 5146. In all three clouds, the C I peak T-A* similar to 1 K, with very sma ll dispersion. The spatial C I distribution is extended and rather smooth. The J = 2 --> 1 transitions of CO isotopomers were observed at the same ang ular resolution as C I. The C I peak T-A* is typically one-third of the pea k T-A* of (CO)-C-13 J = 2,1, and the C I emission is usually more extended than emission in (CO)-C-13 Or (CO)-O-18 J = 2 --> 1. The C I line width is close to the (CO)-C-13 J = 2, 1 line width, larger than the (CO)-O-18 J = 2 ,1 line width and smaller than the (CO)-C-12 J = 2,1 line width. The shapes of these lines occasionally differ significantly, probably because of the combined effects of differing opacities and the physical separation of the line-forming regions. The uniformity of the C I peak T-A* is remarkable for a line in the Wien portion of the Planck function and indicates a very uni form excitation temperature. This uniformity is best explained if the line is opaque and thermalized. If so, the C I line probes kinetic temperature i n clouds exposed to low-ultraviolet fluxes. This conclusion has significant implications for the thermal balance in such clouds. At A, 2:2, these clou ds have a remarkably constant temperature from place to place and from clou d to cloud (7.9 +/- 0.8 K). Photodissociation region models of clouds immer sed in the mean interstellar radiation field tend to predict stronger lines than we see, but this may be an artifact of assumptions about the temperat ure.