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We have carried out ALMA observations of the massive star-forming regionW75N(B), which contains the massive protostars VLA1, VLA2, and VLA3.Particularly, VLA2 is an enigmatic protostar associated with a wind-drivenH$_2$O maser shell, which has evolved from an almost isotropic outflow to acollimated one in just 20 years. The shell expansion seemed to be halted by anobstacle located to the northeast of VLA2. Here we present our findings fromobserving the 1.3 mm continuum and H$_2$CO and SiO emission lines. Within aregion of $\sim 30"$ ($\sim 39,000$ au) diameter, we have detected 40 compactmm-continuum sources, three of them coinciding with VLA1, VLA2, and VLA3. Whilethe H$_2$CO emission is mainly distributed in a fragmented structure around thethree massive protostars, but without any of the main H$_2$CO clumps spatiallycoinciding with them, the SiO is highly concentrated on VLA2, indicating thepresence of very strong shocks generated near this protostar. The SiO emissionis clearly resolved into an elongated structure ($\sim 0.6"\times0.3"$; $\sim780$ au$\times$390 au) perpendicular to the major axis of the wind-driven masershell. The structure and kinematics of the SiO emission are consistent with atoroid and a wide-angle outflow surrounding a central mass of $\sim 10$M$_{\odot}$, thus supporting previous theoretical predictions regarding theevolution of the outflow. Additionally, we have identified the expectedlocation and estimated the gas density of the obstacle that is hindering theexpansion of the maser shell.

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