A highly-collimated SiO jet in the HH212 protostellar outflow

Context. In young stars, jets are believed to play a role in removing angular momentum from the circumstellar disk, allowing accretion onto the central star. Recent results suggest that in earlier phases of star formation, SiO might trace the primary jet launched close to the protostar, but further observations are required in order to reveal the properties of this molecular component. Aims. We wish to exploit the combination of high angular and spectral resolution provided by millimetre interferometry to investigate the collimation and kinematics of molecular protostellar jets, and their angular momentum content. Methods. We mapped the inner 40 of the HH212 Class 0 outflow in SiO(2-1), SiO(5-4) and continuum using the Plateau de Bure interferometer in its extended configurations. The unprecedented angular resolution (down to 0.34) allows accurate comparison with a new, deep H-2 image obtained at the VLT. Results. The SiO emission is confined to a highly-collimated bipolar jet (width similar to 0.5 close to the protostar) along the outflow axis. The jet can be traced down to within 500 AU of the protostar, in a region that is heavily obscured in H2 images. Where both species are detected, SiO shows the same overall kinematics and structure as H-2, indicating that both molecules are tracing the same material. Transverse cuts reveal no velocity gradient compatible with jet rotation above 1 km s(-1), in contrast to previous claims based on H-2 spectra. The central continuum peak is unresolved and close to optically thick, suggesting an edge-on disk with diameter <= 117 AU. Conclusions. SiO proves to be a powerful tracer of molecular jets in Class 0 sources, in particular of their obscured innermost regions. The very small blue/red overlap in the SiO outflow lobes, despite the nearly edge-on view to HH212, further implies that the high-velocity SiO gas is not tracing a wide-angle wind but is already confined to a flow inside a narrow cone of half-opening angle < 6 degrees at <= 500 AU from the protostar. The broad SiO line widths and the transverse velocity gradients both appear significantly affected by internal bowshocks, and should thus be interpreted with caution.