Oral Presentation

An Sio Toroid and Wide-angle Outflow associated with the Massive Protostar W75N(B)-VLA2

Author(s): J.F. Gómez, J.M. Torrelles, J.M. Girart, G. Surcis, J-S. Kim, J. Cantó, G. Anglada, S. Curiel, W.T. Vlemmings, C. Carrasco-González, A. Rodríguez-Kamenetzky, S-W. Kim, C. Goddi, H.J. van Langevelde, Á. Sánchez-Monge

Presenter: José María Torrelles (Institut de Ciències de l'Espai (ICE-CSIC/IEEC))

(2023, Astrophysical Journal Letters, in press)
Over the past 20 years we have observed in real time the evolution of the massive protostar W75N(B)-VLA2. We have seen that the expanding maser shell and the radio continuum emission in this source have both evolved from an isotropic outflow into a jet. This phenomenon has never been seen before in any protostar. We interpret this unique behavior in terms of an episodic, short-lived (tens of years), originally isotropic ionized wind whose shape changes as it moves within a toroidal density stratification. These results are surprising because, according to the monolitic core-accretion model, collimated outflows are already expected at the very early phases of star formation, rather than outflows expanding without any preferential direction. Therefore, it is an open question whether the mass ejection and collimation mechanisms in the early life of massive protostars differ from the case of low-mass protostars.
Now with ALMA observations at 1.3 mm, we have found SiO emission highly concentrated around VLA2, indicating very strong shocks generated near this massive protostar. The SiO emission is resolved into an elongated structure of 0.6" x 0.3" (780 au x 390 au) perpendicular to the major axis of the wind-driven maser shell. The structure and kinematics of the SiO are consistent with a toroid and a wide-angle outflow surrounding VLA2, as predicted by our previous work. We have also detected, with H2CO emission, a dense gas structure that could be responsible for the previously observed halting of the maser shell expansion.