Atomic Carbon Is a Temperature Probe in Dark Clouds

We have mapped the C I 3P1-3P0 line at 492 GHzin three molecular cloudsimmersed in weak ultraviolet radiation fields, TMC-1, L134N, and IC 5146. Inall three clouds, the CI peak TA* ~ 1 K, with very small dispersion. Thespatial C I distribution is extended and rather smooth. The J = 2-1 transitionsof CO isotopomers were observed at the same angular resolution as C I. The C Ipeak TA* is typically a third of the peak TA* of 13CO J = 2-1, and the C Iemission is usually more extended than emission in 13CO or C18O J=2-1. The C Ilinewidth is close to the 13CO J = 2-1 linewidth, larger than the C18O J = 2-1line width, and smaller than the 12CO J = 2-1 linewidth. The uniformity of theC I peak TA* is remarkable for a line in the Wien portion of the Planckfunction and indicates a very uniform excitation temperature. This uniformityis best explained if the line if opaque and thermalized. If so, the CI lineprobes kinetic temperature in clouds exposed to low ultraviolet fluxes. Thisconclusion has significant implications for the thermal balance in such clouds.At Av ~ 2, these clouds have a remarkably constant temperature from place toplace and from cloud to cloud (7.9+/-0.8 K). Photodissociation region models ofclouds immersed in the mean interstellar radiation field tend to predictstronger lines than we see, but this may be an artifact of assumptions aboutthe temperature.Comment: 2 pages, 16 PS figures, uses aaspp4.sty; Astrophys. J., in pres