Spectroscopy of molecular hydrogen in outflows from young stars

We present new medium-resolution longslit spectra in the near-infrared and examine the excitation of molecular hydrogen across several outflows from young stars. In contrast to previous studies, in which the brightest patches in the flows were selected, we find evidence for variations in excitation with position within several sources. We present and comment on a number of H2 excitation mechanisms of outflows from the literature, and compare them with our observations. H2 position-velocity diagrams and line ratios support a picture in which the excitation variations are caused by the large-scale shock geometry. The variations are best interpreted by magnetohydrodynamic C-shocks, although some locations which demonstrate hydrodynamic J-type excitation are found. In particular, VLA 1623 is accurately modelled by a C-type bow, whereas a planar J-type model is favoured for HH 57. HH 1/2 consists of a collection of molecular bow and planar shocks. The H2 emission from the leading edge of the HH 1 bow structure is actually from a low-excitation oblique wing from one of many mini bow shocks rather than from fluorescence or a magnetic precursor. In two of our objects, the jets themselves are also traced in molecular hydrogen emission. Radial velocities in the infrared jet in HH 47C demonstrate acceleration consistent with the proper motions, and hint at ballistic motions originating in an eruptive event about 280 years ago. In this jet, as well as in the HH 1 infrared jet, we find the excitation to decline with distance from the source.