PdBI sub-arcsecond study of the SiO microjet in HH212 - Origin and collimation of class 0 jets

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 (0. ′′34 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 Tk 50−500 K and an SiO/H2 abundance ≥4 × 10−8−6 × 10−5 for n(H2) = 107−105 cm−3, i.e. 0.05−90% 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.