On the theory of H2 rotational excitation

The equilibrium populations of the H2 rotational levels are calculated and compared with the excitation observed in interstellar clouds. The excitation processes considered include collisions with H atoms and electrons and downward radiative cascading following photon excitation or molecule formation. If the probability, p, of ultraviolet photon absorption is less than the mean value of 5 X tOlo s- , the computed excitation temperature T2,4, corresponding to the relative population of the levels J = 2 and 4, is between 200 and 40O K, for a gas temperature of 800 K, and n(H i) < 102 this is the range of observed excitation temperatures for clouds with strong H2 lines, for which p is reduced by shielding. To explain the excitation temperatures of about 10000 K observed in some weak-line clouds requires either that p is some 10 times its mean value, corresponding to a cloud position relatively close to an earlytype star, or that the cloud is dense [n(H I) > 102] and possibly also hot (T 102). Snbject headings: interstellar matter - molecules, interstellar