Laboratory detection and astronomical identification of a new free radical, CCS 3Sigma/-/

Abstract

The linear CCS radical has been detected, for the first time, by laboratory microwave spectroscopy, and four unidentified lines including the U45379 line from TMC-1 and seven from Sgr B2 have been assigned to the transitions of this radical. The radical was produced in a free-space absorption cell by a DC glow discharge in a mixture of CS2 and He. Twenty-nine fines were observed in the frequency region up to 300 GHz and were least-squares analyzed to determine the detailed molecular constants. Two strong unidentified fines, U45379 (detected by Suzuki et al in 1984) and U22344 (detected by Kaifu et al. in 1987), were assigned to the F l components, J N = 4 3 -3 2 and respectively. The J N = 2^ transition of CC 34 S (21930.486 MHz) was also observed in TMC-1. The column densities of CCS in Sgr B2 and TMC-1 are estimated to be (6 ± 1) X 10 13 and (8 ± 2) X 10 13 cm -2 , respectively. Subject headings: interstellar: molecules — laboratory spectra — molecular processes The U45379 fine has attracted much attention of molecular spectroscopists as well as radio astronomers since Suzuki et al. (1984) reported its detection from Sgr B2, TMC-1, and TMC-2. The fine has the highest intensity among those of unidentified interstellar radio fines so far reported, T r = 4.2 K in TMC-1. This fine shows no apparent fine or hyperfine structures and has no harmonically related fines in the frequency regions surveyed at that time. On the basis of the observational facts, Suzuki et al. (1984) suggested that the U45379 fine is a transition of 6-type R or P branch of a fight asymmetric top molecule or of a linear molecule in the 3 2 ground state with a large spin splitting. Since the source molecule of the U45379 fine seemed to be one of the main constituents of cold dark molecular clouds, we considered a possibility that the fine is a transition of interstellar molecular candidates which were unknown spec-troscopically. We especially examined molecular ions, isotopic species of abundant interstellar molecules, and polyyne and carbene derivatives. When we monitored the frequency region of 45379 MHz by discharging a mixture of acetylene and nitrogen, we observed a fine at 45379.039 ± 0.010 MHz, which exactly coincides with the interstellar U45379 fine in 1 Nobeyama Radio Observatory, a branch of Tokyo Astronomical Observatory, University of Tokyo, is a cosmic radio observing facility open for astronomers.