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

A. J. L. Fernandes, P. W. J. L. Brand

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 V_s=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.

Monthly Notices of the Royal Astronomical Society
Volume 274, Page 639
May 1995

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