Translucent clouds are clouds with Av = 1-5 mag which lie in the chemically interesting region where carbon is transformed from atomic to molecular form, C+ -> C -> CO. With the advent of sensitive SIS receivers at 492 GHz such as RxC2. it is now possible to search for the weak [C I] 3P1 -> 3P0 line from translucent clouds in order to test the models. Stark and van Dishoeck (both from Leiden Observatory) detected the [C I] 492 GHz line in the well- characterized high latitude translucent cloud towards HD 210121 (see figure 1). The measured line strengths TMB vary from <0.6 to 1.6 K, and are more than an order of magnitude weaker than the [C I] line commonly detected in dense photon-dominated regions (PDRs). Also, the derived abundance ratio C/CO = 6-7 in this cloud is significantly larger than the ratio C/CO = 0.1 found in dense PDRs. Such a high ratio is consistent with models in which not all carbon has yet been converted into molecular form. From the observed line strengths, it is concluded that the [C I] 492 GHz line plays a non-negligible role in the cooling of translucent clouds. Its cooling is found to be larger than that of CO, and comparable to that by [C II] 158 mm emission within a factor of a few in this cold cloud.

Ronald Stark and Ewine van Dishoeck

Leiden Observatory, Leiden, Netherlands