1–2.5 μm spectra of jets from young stars: Strong $\ion{Fe}{ii}$ emission in HH111, HH240-241 and HH120

Abstract

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 Fe  transitions originating from the first 13 fine structure levels. Such emission is correlated with optical S  emission and appears to decrease moving away from the driving source. From the analysis of the Fe  lines we have derived electron densities values in the range 3 × 10 3 -2 × 10 4 cm −3 , which are systematically larger than those inferred from optical S  line ratios. We suggest that Fe  lines, having critical densities higher than the optical S  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 H 2 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  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  and H 2 lines, emission from other species such as C , S , N  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.

A near-infrared study of the bow shocks within the L1634 protostellar outflow

“…We measure (2, 1)/(1, 0) S(1) ratios of 0.11 and 0.15 (with errors of a few per cent) for HH 240A and HH 240C, respectively (Table 1). This result is consistent with the line strengths tabulated by Nisini et al (2002), on integration along a slit of width 1 . They found (2, 1)/(1, 0) S(1) ratios of 0.11 and 0.17 (also with errors of a few per cent) for HH 240A and HH 240C, respectively.…”

“…We estimate an extinction (A K ) of 0.28 ± 0.2 from the CDR data. This value is consistent with the low extinction found by Nisini et al (2002) from a near infrared [Fe II] line analysis. These [Fe II] lines were not detected in HH 240C so we cannot check for consistency.…”

“…C-type or J-type) and (3) limit the speeds of the bow shocks. Recent 1−2.5 µm long-slit spectra yield a high molecular excitation for HH 240A, and excitation of atomic lines consistent with a shock speed of ∼80 km s −1 (Nisini et al 2002). Our excitation imaging should reveal if the slit spectra are representative of the entire bow shock.…”

“…Many fluxes, originating from several vibrational levels, have been measured by Nisini et al (2002). We have calculated here the H 2 column densities in the upper energy levels T j of the transitions required to produce these fluxes.…”

“…A reddening factor, E J−K , can be determined from the two brightest [Fe II] forbidden transition lines at 1.257 µm and 1.644 µm which both originate from the same upper level. Using this method and adopting the Rieke & Lebofsky (1985) extinction law, Nisini et al (2002) determine a visual extinction A V = 2.5 ± 0.8 towards HH 240A. Assuming a standard gas-to-dust ratio N H = 1.9 × 10 21 cm −2 mag −1 A V (Bohlin et al 1978), and that the HH 240A bow lies at a distance of 300 (2 × 10 18 cm at the distance of 460 parsecs) from the edge of the L1634 cloud (from DSS2 image), we estimate an average hydrogen density of ∼2.5 × 10 3 cm −3 .…”