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Issue A&A
Volume 474, Number 3, November II 2007
Page(s) 941 - 950
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20078260



A&A 474, 941-950 (2007)
DOI: 10.1051/0004-6361:20078260

Quantitative optical and near-infrared spectroscopy of H2 towards HH91A

R. Gredel

Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
    e-mail: gredel@mpia.de

(Received 12 July 2007 / Accepted 31 August 2007)

Abstract
Aims.Optical and near-infrared spectroscopy of molecular hydrogen in interstellar shocks provide a very powerful probe of the physical conditions that prevail in interstellar shocks.
Methods.Integral-field spectroscopy of H2 in the optical wavelength region and complementary long-slit near-infrared spectroscopy towards HH91A are used to characterize the ro-vibrational population distribution among H2 levels with excitation energies up to 30 000 cm-1.
Results.The detection of some 200 ro-vibrational lines of molecular hydrogen ranging between 7700 Å and 2.3 $\mu$m is reported. Emission lines which arise from vibrational levels up to v' = 8 are detected. The H2 emission arises from thermally excited gas where the bulk of the material is at a temperature of 2750 K and where 1% is at 6000 K. The total column density of shocked molecular hydrogen is N(H2) = 1018 cm-2. Non-thermal excitation scenarios such as UV fluorescence do not contribute to the H2 excitation observed towards HH91A.
Conclusions.The emission of molecular hydrogen towards HH91A is explained in terms of a slow J-shock that propagates into a low-density medium, that has been swept up by previous episodes of outflows that have occurred in the evolved HH90 / 91  complex.


Key words: ISM: molecules -- ISM: Herbig-Haro objects -- ISM: jets and outflows



© ESO 2007


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