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 measurements 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 constants of C6H. We compare our results with the recent early-time, gas-phase chemistry 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 rapidly than found for the longer HCnN chains. Although the decrease in fractional abundance with increasing chain length from C4H to C8H is reasonably well reproduced by the models of Herbst and Terzieva, C8H is observationally somewhat underabundant compared to these calculations. Although we searched for the J = 7.5-6.5 and J = 10.5-9.5 transitions of C7H, nearby interference in the case of the higher transition and possible confusion with nearby U-lines prevent us from claiming a detection. However, we are able to report an upper limit to the abundance of C7H in TMC-1 that is at least a factor of 20 below that of C5H. The steep fall-off in abundance for the longer CnH molecules, also found in IRC+10216 by Guelin and coworkers, suggests that the likelihood is small that a large fraction of carbon is locked up in long CnH chains in dense dark clouds like TMC-1. If long CnH chains are not present in high abundance in either dark clouds or the envelopes of carbon 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 line density at a level of T*A~1 mK in this dust cloud to be 0.9 lines MHz-1