title: Observations of neutral atomic carbon at 809 GHz.
abstract: We have detected the 809 GHz 3P2-3P1 fine-structure line of neutral atomic carbon in four dense molecular clouds:  M17, W51, W3, and DR 21(OH).  These observations complement the published observations of the 492 GHz 3P1-3P0 line and allow the excitation temperature of the 3P levels along with the line optical depths to be determined.  The results indicate excitation temperatures Tx approximately 30-60 K and optical depths of tau 10 < or approximately 1.  This implies that the approximately 10(18) cm-2 lower limit to the C I abundance derived from 492 GHz observations is probably the actual abundance, which gives C I/CO approximately 0.1 in dense molecular clouds.
authors.affiliation: Space Sciences Laboratory, University of California, Berkeley, USA.
authors.forename: J
authors.initials: J
authors.lastname: Zmuidzinas
authors.forename: A L
authors.initials: AL
authors.lastname: Betz
authors.forename: D M
authors.initials: DM
authors.lastname: Goldhaber
mesh_terms: D000401:Aircraft; D055580:Astronomical Phenomena; D001256:Astronomy; D002244:Carbon / Q000032:analysis*; D005118:Extraterrestrial Environment; D007834:Lasers; D013696:Temperature
chemical_list: D002244:Carbon