1-2.5 μm spectra of jets from young stars: Strong Fe II emission in HH111, HH240-241 and HH120

Abstract

As part of a 1-2.5 μm spectroscopic survey of jets and molecular outflows, we present the spectra of three Herbig Haro chains (HH111, HH240/241, HH120) characterized by strong emission from several F II transitions originating from the first 13 fine structure levels. Such emission is correlated with optical S II emission and appears to decrease moving away from the driving source. From the analysis of the Fe II lines we have derived electron densities values in the range 3 × 103-2 × 104 cm-3, which are systematically larger than those inferred from optical S II line ratios. We suggest that Fe II lines, having critical densities higher than the optical S n transitions, trace either regions of the post-shock cooling layers with higher compression, or a section of the jet axis at a higher degree of ionization. Strong H2 emission lines are also detected along the three flows and their analysis indicates that a combination of different shocks can be responsible for their excitation in the different objects. Consequently the Fe II line emission, which requires the presence of fast dissociative shocks, is completely independent from the excitation mechanism giving rise to the molecular emission. In addition to the Fe II and H2 lines, emission from other species such as C I, S II, N I as well as recombination lines from the Paschen series are detected and have been used as a reference to infer the gas-phase iron abundance in the observed HH objects. We estimate a grain destruction efficiency of about 30-60%: the highest value is found for HH240A, which also shows the highest degree of excitation among the observed objects.