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

We report observations of seven different transitions of HC9N between 9.8 a nd 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 tha t 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 num ber of carbon atoms in the chain. In steady state this is balanced by the c ollisional rate of excitation. The rotational temperatures of the cyanopoly ynes are found to increase with the number of heavy atoms over the range HC 5N-HC9N, in reasonable agreement with calculations. Concomitantly, we find evidence that the longer cyanopolyynes are located in regions that become p rogressively smaller with chain length. We also report newly measured values for the rotational and centrifugal dis tortion constants of HC9N that improve the accuracy of the calculated milli meter wave transitions.