The excitation temperatures of HC9N and other long cyanopolyynes in TMC-1

We report observations of seven different transitions of HC9N between 9.8 and 23.3 GHz in the cold, dark cloud TMC-1. Because of its lower rotational constant, HC9N is expected to yield a rotational temperature closer to the kinetic temperature of the gas than the shorter cyanopolyynes. Assuming that HC9N in TMC-1 is optically thin, we obtain a rotational temperature T-rot = 7.5-8.7 K and a corresponding column density of N-L(HC9N) = (5.4-2.3) x 10(12) cm(-2), depending on the source size L assumed. The rate of radiative cooling is less efficient in the longer cyanopolyyne chains and is approximately proportional to (n + 1)(-6), where n is the number of carbon atoms in the chain. In steady state this is balanced by the collisional rate of excitation. The rotational temperatures of the cyanopolyynes are found to increase with the number of heavy atoms over the range HC5N-HC9N, in reasonable agreement with calculations. Concomitantly, we find evidence that the longer cyanopolyynes are located in regions that become progressively smaller with chain length. We also report newly measured values for the rotational and centrifugal distortion constants of HC9N that improve the accuracy of the calculated millimeter wave transitions.