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

International audienceContext: 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 farcsec 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(H_2) = 10^7-105 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