Methyl acetylene as a temperature probe in molecular clouds

The J = 5 - 4(K = 0 - 3) and J = 2 - 1(K = 0, 1) transitions of methyl acetylene (CH3C2H) have been observed in DR 21(OH), TMC-1, W3(OH), W 51, and Orion A. Methyl acetylene is shown to be an excellent tool for investigations of the kinetic temperature of interstellar clouds. Due to its low dipole moment a thermal population distribution exists in most cases, and a simple rotation diagram can be used to determine the kinetic temperature. This has been verified by means of a statistical equilibrium analysis including 91 energy levels, but the latter, more stringent analysis proves to be necessary at low temperatures and densities. The statistical equilibrium analysis also gives some indication of the density in the cloud. Diagrams for deriving possible n(H2)-T(k) values from observed J = 5 - 4 and J = 2 - 1 transitions are included, and have been applied to the sources observed.