VLT-ISAAC 3-5 μm spectroscopy as a new tool for investigating H2 emission in protostellar jets

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