Letter to the Editor

PdBI sub-arcsecond study of the SiO microjet in HH212

Origin and collimation of class 0 jets

¹ LERMA, UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: sylvie.cabrit@obspm.fr
² INAF, Istituto di Radioastronomia, Sezione di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
³ IRAM, 300 rue de la Piscine, 38406 Grenoble Cedex, France
⁴ INAF-Osservatorio Astrofisico di Roma, via di Frascati 33, 00040 Monte Catone, Italy
⁵ Physics Department, The University, Durham DH1 3LE, UK
⁶ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex, France
⁷ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 1 March 2007
Accepted: 3 April 2007

Abstract

Context.The bipolar HH 212 outflow has been mapped in SiO using the extended configuration of the Plateau de Bure Interferometer (PdBI), revealing a highly collimated SiO jet closely associated with the H₂ jet component.

Aims.We study at unprecedented resolution ( across the jet axis) the properties of the innermost SiO “microjet” within 1000 AU of this young Class 0 source, to compare it with atomic microjets from more evolved sources and to constrain its origin.

Methods.The SiO channel maps are used to investigate the microjet collimation and velocity structure. A large velocity gradient analysis is applied to SiO (2-1), (5-4) and (8-7) data from the PdBI and the Submillimeter Array to constrain the SiO opacity and abundance.

Results.The HH212 Class 0 microjet shows striking similarities in collimation and energetic budget with atomic microjets from T Tauri sources. Furthermore, the SiO lines appear optically thick, unlike what is generally assumed. We infer T_k  50-500 K and an SiO/H₂ abundance ≥ for  cm^-3, i.e. % of the elemental silicon.

Conclusions.This similar jet width, regardless of the presence of a dense envelope, definitely rules out jet collimation by external pressure, and favors a common MHD self-collimation (and possibly acceleration) process at all stages of star formation. We propose that the more abundant SiO in Class 0 jets could mainly result from rapid (≤25 yrs) molecular synthesis at high jet densities.