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Previous SiO maps of the innermost regions of HH212 set strong constraints onthe structure and origin of this jet. They rule out a fast wide-angle wind, andtentatively favor a magneto-centrifugal disk wind launched out to 0.6 AU. Weaim to assess the SiO content at the base of the HH212 jet to set anindependent constraint on the location of the jet launch zone with respect tothe dust sublimation radius. We present the first sub-arcsecond (0"44x0"96) COmap of the HH212 jet base, obtained with the IRAM Plateau de BureInterferometer. Combining this with previous SiO(5-4) data, we infer theCO(2-1) opacity and mass-flux in the high-velocity jet and arrive at a muchtighter lower limit to the SiO abundance than possible from the (opticallythick) SiO emission alone. Gas-phase SiO at high velocity contains at least 10%of the elemental silicon if the jet is dusty, and at least 40% if the jet isdust-free, if CO and SiO have similar excitation temperatures. Such a high SiOcontent is challenging for current chemical models of both dust-free winds anddusty interstellar shocks. Updated chemical models (equatorial dust-free winds,highly magnetized dusty shocks) and observations of higher J CO lines arerequired to elucidate the dust content and launch radius of the HH212high-velocity jet.

High SiO abundance in the HH212 protostellar jet