Molecular Hydrogen Outflows in W51

Abstract

We present the results of a deep search for the molecular hydrogen shock fronts associated with young stellar outflows in the giant molecular cloud and massive star-forming region W51. A total of 14 outflows were identified by comparing images in the H and K bands and in a narrowband filter centered on the H2 1-0 S(1) line at 2.122 μm. A few of the newly discovered outflows were subsequently imaged at higher spatial resolution in the S(1) filter; one outflow was also imaged in the 1.644 μm emission line of [Fe II]. For two of the outflows, high-resolution echelle spectroscopy in the H2 1-0 S(1) line was obtained using NIRSPEC at Keck. For one outflow additional high-resolution spectra were obtained in the [Fe II] line and in Brγ. The largest and best-studied outflow shock front shows a remarkably broad [Fe II] line, an unusual high-velocity component in Brγ, and comparably narrow line widths in the H2 1-0 S(1) line. A scenario involving high-velocity shocks and UV excitation of preshock material is used to explain these spectra.