We have carried out ALMA observations of the massive star-forming region
W75N(B), which contains the massive protostars VLA1, VLA2, and VLA3.
Particularly, VLA2 is an enigmatic protostar associated with a wind-driven
H2​O maser shell, which has evolved from an almost isotropic outflow to a
collimated one in just 20 years. The shell expansion seemed to be halted by an
obstacle located to the northeast of VLA2. Here we present our findings from
observing the 1.3 mm continuum and H2​CO and SiO emission lines. Within a
region of ∼30" (∼39,000 au) diameter, we have detected 40 compact
mm-continuum sources, three of them coinciding with VLA1, VLA2, and VLA3. While
the H2​CO emission is mainly distributed in a fragmented structure around the
three massive protostars, but without any of the main H2​CO clumps spatially
coinciding with them, the SiO is highly concentrated on VLA2, indicating the
presence of very strong shocks generated near this protostar. The SiO emission
is clearly resolved into an elongated structure (∼0.6"×0.3"; ∼780 au×390 au) perpendicular to the major axis of the wind-driven maser
shell. The structure and kinematics of the SiO emission are consistent with a
toroid and a wide-angle outflow surrounding a central mass of ∼10
M⊙​, thus supporting previous theoretical predictions regarding the
evolution of the outflow. Additionally, we have identified the expected
location and estimated the gas density of the obstacle that is hindering the
expansion of the maser shell.Comment: To be published in The Astrophysical Journal Letters. Sixteen pages,
seven figures. Updated metadat