Observations of long CnH molecules in the dust cloud TMC-1

We report detection of the J = 15.5-14.5 and 17.5-16.5 transitions of C8H, a sensitive upper limit for the J = 10.5-9.5 transition of C7H, and measure ments of two low-frequency transitions of C6H in TMC-1. These results give new information on the relative abundances of long carbon-chain radicals in TMC-1 and confirm the recent laboratory-measured hyperfine-splitting const ants of C6H. We compare our results with the recent early-time, gas-phase c hemistry model of Herbst and Terzieva and with that of Millar and coworkers . We find the abundance ratios in the longer CnH chains decline much more r apidly than found for the longer HCnN chains. Although the decrease in frac tional abundance with increasing chain length from C4H to C8H is reasonably well reproduced by the models of Herbst and Terzieva, C8H is observational ly somewhat underabundant compared to these calculations. Although we searc hed for the J = 7.5-6.5 and J = 10.5-9.5 transitions of C7H, nearby interfe rence in the case of the higher transition and possible confusion with near by U-lines prevent us from claiming a detection. However, we are able to re port an upper limit to the abundance of C7H in TMC-1 that is at least a fac tor of 20 below that of C5H. The steep fall-off in abundance for the longer CnH molecules, also found in IRC+10216 by Gut lin and coworkers, suggests that the likelihood is small that a large fraction of carbon is locked up i n long CnH chains in dense dark clouds like TMC-1. If long CnH chains are n ot present in high abundance in either dark clouds or the envelopes of carb on stars, the possibility that they are present in abundance in the diffuse gas also appears less likely. The cyanopolyynes, which have been detected at relatively high abundances to and including HC11N, may then be the most abundant carbon-chain molecules in the diffuse gas. These observations and our earlier observations of HC9N in TMC-1 have allowed us to estimate the l ine density at a level of T-A*similar to 1 mK in this dust cloud to be 0.9 lines MHz(-1).