Methanol line formation in outflow sources

Abstract

We report the first calculations of the spectrum of methanol, arising in shock waves in molecular outflows. The small grid of shock wave models that we have computed incorporates the results of very recent computations of the rate coefficients for the collisional excitation of methanol by ortho- and para-H 2 and by He. The two strongest transitions, one of A- and the other of E-type methanol, are masers that have been observed in a Class I methanol maser source, which is believed to be related to a molecular outflow. The same collisional propensities that give rise to population inversion and maser action can, in other transitions, lead to population anti-inversion and the lines appearing in absorption against the cosmic background radiation. We attempted to model specifically the outflow source L1157 B1, in which transitions of methanol have been observed recently by means of the Herschel satellite. Comparison with the predictions of the shock wave models is complicated by uncertainty in the value of the beam filling factor that should be adopted.