H_2 emission from shocks in molecular outflows: the significance of departures from a stationary state

Abstract

We have computed the time dependence of the H_2 rovibrational emission spectrum from molecular outflows. This emission arises in shock waves generated by the impact of jets, associated with low-mass star formation, on molecular gas. The shocks are unlikely to have attained a state of equilibrium, and so their structure will exhibit both C- and J-type characteristics. The rotational excitation diagram is found to provide a measure of the age of the shock; in the case of the outflow observed in Cepheus A West by the ISO satellite, the shock age is found to be approximately 1.5x10^3yr. Emission by other species, such as NH_3 and SiO, is also considered, as are the intensities of the fine-structure transitions of atoms and ions.