On the Theory of H_{2} 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