DETECTION OF [CI] 492 GHZ EMISSION FROM A HIGH-LATITUDE TRANSLUCENT CLOUD

The [C I] P-3(1) - P-3(0) 499 GHz fine structure line has been detecte d in the high-latitude cloud toward HD 210121 using the James Clerk Ma xwell Telescope. The measured [C I] line strengths vary between T(MB) = 0.6 - 1.6 K and appear correlated with the CO J = 1 - 0 emission. Th e inferred [C I] column densities can be well explained by models with densities n(H) congruent-to 500-5000 cm-3 and a low incident radiatio n field, which causes the C+, C --> CO transition zone to be shifted t o lower A(V) values. The derived N(C)/N(CO) ratio.is about 3 - 6, sign ificantly larger than found in dense PDRs. Cooling by [C I] is larger than that by CO in this cloud and may be comparable to that by [C II] within a factor of a few.