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

INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy e-mail: bruni,caratti,teresa,dloren@mporzio.astro.it

Corresponding author: B. Nisini, bruni@mporzio.astro.it

Received: 4 June 2002
Accepted: 19 July 2002

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   transitions originating from the first 13 fine structure levels. Such emission is correlated with optical emission and appears to decrease moving away from the driving source. From the analysis of the   lines we have derived electron densities values in the range – cm^-3, which are systematically larger than those inferred from optical line ratios. We suggest that   lines, having critical densities higher than the optical 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   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   and H₂  lines, emission from other species such as , , 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.