Spitzer spectral line mapping of the HH211 outflow

Abstract

Context. Jets from the youngest protostars are often detected only at mm wavelengths, by means of line emission of CO and SiO. However, it is not yet clear whether these jets are mostly molecular or atomic, nor whether they trace ejected gas or an entrained layer around an embedded atomic jet. Aims: We investigate the warm gas content of the HH211 protostellar outflow to assess the jet mass-flux in the form of H 2 and investigate the existence of an embedded atomic jet. Methods: We employ archival Spitzer slit-scan observations of the HH211 outflow over 5.2-37 μm obtained with the low resolution IRS modules. Detected molecular and atomic lines are interpreted by means of emission line diagnostics and an existing grid of molecular shock models. The physical properties of the warm gas are compared with those of other molecular jet tracers and to the results of a similar study towards the L1448-C outflow. Results: We detected and mapped the v = 0-0 S(0)-S(7) H 2 lines as well as fine-structure lines of S, Fe +, and Si +. The H 2 is detected to 5 '' from the source and is characterized by a “cool” T ~ 300 K and a “warm” T ~ 1000 ± 300 K component, with an extinction A V ~ 8 mag. The amount of cool H 2 towards the jet agrees with that estimated from CO assuming fully molecular gas. The warm component is well fitted by C-type shocks with a low beam filling factor ~0.01-0.04 and a mass-flux similar to the cool H 2. The fine-structure line emission arises from dense gas with ionization fraction ~0.5-5 × 10 -3, and is indicative of dissociative shocks. Line ratios with respect to sulfur indicate that iron and silicon are depleted relative to solar abundances by a factor ~10-50. Conclusions: Spitzer spectral mapping observations reveal for the first time a cool H 2 component towards the CO jet of HH211 consistent with the CO material being fully molecular and warm at ≃300 K. These maps detect also for the first time an embedded atomic jet in the HH211 outflow that can be traced close to the central source position. Its significant iron and silicon depletion excludes an origin from within the dust sublimation zone around the protostar. The momentum-flux seems insufficient to entrain the CO jet, although current uncertainties in jet speed and shock conditions are too large to reach a definite conclusion.