ALMA observations of the extremely high velocity, massive and compact molecular outflow G331.512-0.103

Cycle 0 ALMA observations are presented for one of the most energetic and luminous molecular outflows known in the Galaxy, G331.512-0.103. High angular resolution observations with Band 7 (350 GHz; 0.86 mm) show that the bipolar molecular outflow is very compact and extremely young, with velocities relative to the ambient cloud of 70 km/s. The SiO (8-7) emission is confined in a region less than 5’’ in size, corresponding to 0.18 pc at the source distance 7.5 kpc, and it reveals the presence of a ring-type structure toward the systemic velocity of the source. This feature is also observed in the H13CO+ emission line, and the cavity is coincident with a strong and compact radio continuum source detected at 4.8 and 8.6 GHz, using ATCA. We interpret these observations as a young stellar object producing a compact H II region, with an expansion shock propagating into the medium and possible dense material still infalling around this shell. The expansion velocity of the shocked shell is ~ 24 km/s, suggesting a crossing time of about 2000 yr. The spectral index of the radio continuum source is consistent with an ionized jet, which is likely to drive the associated energetic molecular outflow. The H13CO+ emission also shows the presence of a feature in the position-velocity maps, associated with the presence of a bullet of dense material. The complete Band 7 spectra show a rich set of complex emission lines, possibly associated with hot core chemistry, with gas rotational temperature ~ 100 K. The source is one of the youngest examples of a massive molecular outflow found associated with a high-mass star.