title: The C3H2 2(20)-2(11) transition:  absorption in cold dark clouds.
abstract: The first observations of the 2(20)-2(11) transition of cyclopropenylidene (C3H2) at 21.6 GHz are described.  In all cases where it has been detected, the line appears in absorption, showing that this transition is "refrigerated" (i.e., Tex < 2.7 K) in cold dust clouds.  These results are compared with those for the 1(10)-1(01) transition of C3H2, and the consequences for the excitation of C3H2 qualitatively discussed.
authors.affiliation: Herzberg Institute of Astrophysics, National Research Council of Canada.
authors.forename: H E
authors.initials: HE
authors.lastname: Matthews
authors.forename: S C
authors.initials: SC
authors.lastname: Madden
authors.forename: L W
authors.initials: LW
authors.lastname: Avery
authors.forename: W M
authors.initials: WM
authors.lastname: Irvine
mesh_terms: D055580:Astronomical Phenomena; D001256:Astronomy; D019443:Cosmic Dust; D005118:Extraterrestrial Environment; D006844:Hydrocarbons, Cyclic / Q000032:analysis*; D008872:Microwaves; D013696:Temperature
chemical_list: D019443:Cosmic Dust; D006844:Hydrocarbons, Cyclic