VLT-ISAAC 3-5 mu m spectroscopy as a new tool for investigating H-2 emission in protostellar jets

We report 3-5 mum IR spectroscopy obtained with VLT-ISAAC on the IRS17 mole cular hydrogen jet in the Vela-D Molecular Cloud. Together with H-2 emissio n lines from the v = 1 rovibrational state, the spectra show several pure r otational lines of the fundamental state with excitation temperature up to similar to 22 000 K. We show how theoretical rotation diagrams indicate the se lines as unique both to probe the presence of collisionally excited gas in NLTE conditions and to infer the gas density.