A spectral-line survey of W51 from 17.6 to 22.0 GHz

Abstract

The complete centimeter-wave spectrum between 17.57 and 22.04 GHz of the W51 Main/South complex has been surveyed with the NRAO 140 ft (43 m) telescope with channel spacings of 0.3125 MHz and down to a 1s level between TA = 1-5 mK. Approx. 224 lines were detected, of which 94 are hydrogen or helium recombination lines, 11 are ammonia lines, 19 are coincident with known mol. transitions, and 100 are unidentified. Observations obtained with longer integration times, but over very limited frequency ranges, indicate that many of the known mols. might be detectable in this source with only a few hours of integration time. The carbon chain mols. more common in other sources (CnH, CnN, HCnN, etc.) are not strong, although HC3N, HC5N and several nonlinear mols. contg. C3 appear to have been detected. Based on the high signal-to-noise ratio obtained for C3H2 lines, and the fact that the off-axis 13C isotopomer was also detected, it was concluded that W51 is probably an excellent source in which to search for propargylene, the nearly linear version of this mol. Eleven transitions of ammonia from (4,2) to (14,11) were also detected in the survey and appear to show an increase in excitation temp. with both J and (J - K) for this mol. [on SciFinder (R)]