Spitzer spectral line mapping of the HH211 outflow

¹ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark e-mail: odysseas@snm.ku.dk
² INAF - Osservatorio Astronomico di Roma, via di Frascati, 33 00040 Monte Porzio Catone (RM), Italy
³ LERMA, Observatoire de Paris, UMR 8112 du CNRS, 61 avenue de l'Observatoire, 75014 Paris, France
⁴ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2300 CA Leiden, The Netherlands
⁵ Institut Astrophysique Spatiale (IAS), UMR 8617, CNRS, Université Paris-Sud 11, Bâtiment 121, 91405 Orsay Cedex, France

Received: 12 November 2010
Accepted: 3 June 2010

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 H₂ 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) H₂ lines as well as fine-structure lines of S, Fe⁺, and Si⁺. The H₂ 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 H₂ 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 H₂. 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 H₂ 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.