Characterizing shock waves in molecular outflow sources: the interpretation of Herschel and Spitzer observations of NGC 1333 IRAS 4B

Abstract

We have computed C- and J-type models of shock waves in molecular outflow sources. In addition to the (optically thin) emission line spectrum of molecular hydrogen, the spectra of CO, OH, SiO, H₂O and NH₃ were computed by means of the large velocity gradient approximation. We find that the intensities of the OH lines are particularly sensitive to the character (C- or J-type) of the shock wave. The results of these computations were used to guide the interpretation of the spectrum of the outflow source NGC 1333 IRAS 4B, recently observed by Herschel/PACS and the Spitzer satellites. We find that the best overall fit to the spectrum of this object is provided by quasi-time-dependent (CJ-type) models, which have both C- and J-type characteristics; the dynamical age of the emitting region is found to be of the order of 10² yr. The principal limitation to the robustness of the predictions of the current model relate to the possible effects of dust on the dynamical and thermal profiles of the gas. Specifically, the shattering and vaporization of grains, which can enhance the total grain cross-section, have not been taken into account. Furthermore, there remain significant uncertainties relating to the rate of reformation of H₂ molecules, on dust grains, at the high gas kinetic temperatures at which this process occurs in the shock wave.