The shocked gas of the BHR71 outflow observed by Herschel: indirect evidence for an atomic jet^⋆

¹ INAF, Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: milena.benedettini@iaps.inaf.it
² LERMA, Observatoire de Paris, PSL Research University, CNRS, UMR 8112, 75014 Paris, France
³ Sorbonne Universités, UPMC Univ. Paris 6, UMR 8112, LERMA, 75005 Paris, France
⁴ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁶ CNRS, IPAG, 38000 Grenoble, France
⁷ Institut de Ciències de l’Espai (IEEC-CSIC), Campus UAB, Carrer de can Magrans, s/n, 08193 Barcelona, Catalunya, Spain
⁸ INAF, Osservatorio Astonomico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 1 August 2016
Accepted: 8 November 2016

Abstract

Context. In the BHR71 region, two low-mass protostars IRS1 and IRS2 drive two distinguishable outflows. They constitute an ideal laboratory to investigate both the effects of shock chemistry and the mechanisms that led to their formation.

Aims. We aim to define the global morphology of the warm gas component of the BHR71 outflow and at modelling its shocked component.

Methods. We present the first far infrared Herschel images of the BHR71 outflows system in the CO (14–13), H₂O (2₂₁–1₁₀), H₂O (2₁₂–1₀₁)  and [O i]  145 μm transitions, revealing the presence of several knots of warm, shocked gas associated with the fast outflowing gas. In two of these knots we performed a detailed study of the physical conditions by comparing a large set of transitions from several molecules to a grid of shock models.

Results. The Herschel lines ratios in the outflow knots are quite similar, showing that the excitation conditions of the fast moving gas do not change significantly within the first ~0.068 pc of the outflow, apart at the extremity of the southern blue-shifted lobe that is expanding outside the parental molecular cloud. Rotational diagram, spectral line profile and LVG analysis of the CO lines in knot A show the presence of two gas components: one extended, cold (T ~ 80 K) and dense (n(H₂) = 3 × 10⁵–4 × 10⁶ cm^-3) and another compact (18′′), warm (T = 1700–2200 K) with slightly lower density (n(H₂) = 2 × 10⁴–6 × 10⁴ cm^-3). In the two brightest knots (where we performed shock modelling) we found that H₂ and CO are well fitted with non-stationary (young) shocks. These models, however, significantly underestimate the observed fluxes of [O i]  and OH lines, but are not too far off those of H₂O, calling for an additional, possibly dissociative, J-type shock component.

Conclusions. Our modelling indirectly suggests that an additional shock component exists, possibly a remnant of the primary jet. Direct, observational evidence for such a jet must be searched for.