Shock diagnostics in Herbig–Haro 7: evidence for H2 fluorescence

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Abstract
Near-infrared spectra have been obtained between 2 and 2.5 μm at six positions in Herbig-Haro object 7, which show emission lines of molecular hydrogen arising from upper energy levels that range from 6000 to 25000 K. Planar J- and C-type shock models fail to predict the relatively large columns of warm gas measured in the higher vibrational levels (v>2) of H2. We suggest that the excess emission observed in these high-excitation lines arises from H2 fluorescence produced by Lyα pumping of the lower density (∼ 103–104) pre-shocked gas, while the bulk of the H2 emission is excited in the hotter and denser (∼ 105–106) post-shocked layers behind a bow C-shock. By comparing the observed line ratios with the H2 emission from a composite ‘Bow C-shock+Fluorescence’ model, we derive that the shock-induced UV radiation field is about 102–103 times larger than the average interstellar field, and that the ortho-to-para ratio of the fluorescent H2 population is 1.8. The best-fitting parameters for the bow yield a shock velocity Vs = 140 km s−1, with the axis of symmetry positioned at θ=40° to the line of sight. This configuration suggests that the bow's dissociative cap (box apex) is producing a far-UV radiation field of the order of 0.16–1.6 erg s−1 cm−2.

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