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.