  We have mapped the C I 3P1-3P0 line at 492 GHzin three molecular clouds
immersed in weak ultraviolet radiation fields, TMC-1, L134N, and IC 5146. In
all three clouds, the CI peak TA* ~ 1 K, with very small dispersion. The
spatial C I distribution is extended and rather smooth. The J = 2-1 transitions
of CO isotopomers were observed at the same angular resolution as C I. The C I
peak TA* is typically a third of the peak TA* of 13CO J = 2-1, and the C I
emission is usually more extended than emission in 13CO or C18O J=2-1. The C I
linewidth is close to the 13CO J = 2-1 linewidth, larger than the C18O J = 2-1
line width, and smaller than the 12CO J = 2-1 linewidth. The uniformity of the
C I peak TA* is remarkable for a line in the Wien portion of the Planck
function and indicates a very uniform excitation temperature. This uniformity
is best explained if the line if opaque and thermalized. If so, the CI line
probes kinetic temperature in clouds exposed to low ultraviolet fluxes. This
conclusion has significant implications for the thermal balance in such clouds.
At Av ~ 2, these clouds have a remarkably constant temperature from place to
place and from cloud to cloud (7.9+/-0.8 K). Photodissociation region models of
clouds immersed in the mean interstellar radiation field tend to predict
stronger lines than we see, but this may be an artifact of assumptions about
the temperature.
