A SPECTRAL-LINE SURVEY OF W51 FROM 17.6 TO 22.0 GHZ

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 foot (43 m) telescope with channel spacings of 0.3125 MHz and down to a 1 sigma level between T(A) = 1-5 mK. Approximately 224 lines were detected, of which 94 are hydrogen or helium recombination lines, 11 are ammonia lines, 19 are coincident with known molecular transitions, and 100 are unidentified. Observations obtained with longer integration times, but over very limited frequency ranges, indicate that many of the known molecules might be detectable in this source with only a few hours of integration time. The carbon chain molecules more common in other sources (C(n)H, C(n)N, HC(n)N, etc.) are not strong, although HC3N, HC5N and several nonlinear molecules containing 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 C-13 isotopomer was also detected, we have concluded that W51 is probably an excellent source in which to search for propargylene, the nearly linear version of this molecule. Eleven transitions of ammonia from (4,2) to (14,11) were also detected in the survey and appear to show an increase in excitation temperature with both J and (J-K) for this molecule.