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 H-2 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 T-k similar or equal to 50-500 K and an SiO/H-2 abundance >= 4 x 10(-8)- 6 x 10(-5) for n(H-2) = 10(7)-10(5) 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.