SHOCK DIAGNOSTICS IN HERBIG-HARO-7 - EVIDENCE FOR H-2 FLUORESCENCE

Near-infrared spectra have been obtained between 2 and 2.5 mu m at six positions in Herbig-Haro object 7, which show emission lines of molec ular hydrogen arising from upper energy levels that range from 6000 to 25 000 K. Planar J- and C-type shock models fail to predict the relat ively large columns of warm gas measured in the higher vibrational lev els (v > 2) of H-2. We suggest that the excess emission observed in th ese high-excitation lines arises from H-2 fluorescence produced by Ly alpha pumping of the lower density (similar to 10(3)-10(4)) pre-shocke d gas, while the bulk of the H-2 emission is excited in the hotter and denser (similar to 10(5)-10(6)) post-shocked layers behind a bow C-sh ock. By comparing the observed line ratios with the H-2 emission from a composite 'Bow C-shock + Fluorescence' model, we derive that the sho ck-induced UV radiation field is about 10(2)-10(3) times larger than t he average interstellar field, and that the ortho-to-para ratio of the fluorescent H-2 population is 1.8. The best-fitting parameters for th e bow yield a shock velocity V-s = 140 km s(-1), with the axis of symm etry positioned at theta = 40 degrees to the line of sight. This confi guration suggests that the bow's dissociative cap (bow apex) is produc ing a far-UV radiation field of the order of 0.16-1.6 erg s(-1) cm(-2) .