The jet and the disk of the HH 212 low-mass protostar imaged by ALMA: So and so emission

Abstract

Context. The investigation of the disk formation and jet launching mechanism in protostars is crucial to understanding the earliest stages of star and planet formation. Aims. We aim to constrain the physical and dynamical properties of the molecular jet and disk of the HH 212 protostellar system at unprecedented angular scales, exploiting the capabilities of the Atacama Large Millimeter Array (ALMA). Methods. The ALMA observations of HH 212 in emission lines from sulfur-bearing molecules, SO 98 SO 1010 SO7 are compared with simultaneous CO 3-2, SiO 8-7 data. The molecules column density and abundance are estimated using simple radiative transfer models. Results. SO 98and SO7show broad velocity profiles. At systemic velocity, they probe the circumstellar gas and the cavity walls. Going from low to high blue- and red-shifted velocities the emission traces the wide-angle outflow and the fast (∼100-200 km s-1), collimated (∼90 AU) molecular jet revealing the inner knots with timescales ≤50 yr. The jet transports a mass-loss rate ≥ 0.2-2 × 10-6 Myr-1, implying high ejection efficiency (≥ 0.03-0.3). The SO and SOabundances in the jet are ∼ 10-7-10-6. SO 1010emission is compact and shows small-scale velocity gradients, indicating that it originates partly from the rotating disk previously seen in HCO+ and C17O, and partly from the base of the jet. The disk mass is ≥ 0.002-0.013 Mand the SO abundance in the disk is ∼ 10-8-10-7. Conclusions. SO and SOare effective tracers of the molecular jet in the inner few hundreds AU from the protostar. Their abundances indicate that 1-40% of sulfur is in SO and SOdue to shocks in the jet/outflow and/or to ambipolar diffusion at the wind base. The SO abundance in the disk is 3-4 orders of magnitude larger than in evolved protoplanetary disks. This may be due to an SO enhancement in the accretion shock at the envelope-disk interface or in spiral shocks if the disk is partly gravitationally unstable.