The rotating gas toroid surrounding the K3-50A ionized bipolar outflow

We present high spatial resolution, aperture synthesis images of K3-50A as part of an ongoing study of high-mass star formation regions. Maps in the HCO⁺ (J = 1→0), H¹³CO⁺ (J = 1→0), and SiO (v = 0, J = 2 → 1) emission lines at ∼2″.5 resolution reveal both a flattened cloud of dense molecular gas ∼1.1 pc in extent and a newly discovered inner torus ∼0.5 pc in extent. The axis of the inner torus is tilted ∼20° with respect to the outer cloud. The inner torus surrounds a bright continuum source, associated with the origin of a bipolar ionized gas outflow, and has HCO⁺(J = 1 → 0) optical depth greater than 14. Chemical abundances in the inner torus are azimuthally asymmetric, possibly due to differential impact of the ionized outflow on the inner region of the torus. Comparison with a kinematic radiative transfer model confirms that the torus is rotating and the rotation axis of the inner torus is aligned with the ionized outflow. While the cloud is not in solid body rotation, the rotational velocity of material in the cloud increases with radius. We estimate molecular abundance ratios by comparison with high-resolution dust extinction maps previously obtained at the same spatial resolution. The abundance ratio [HCO⁺]/[H₂] = 3.9 × 10^-8 is enhanced and falls in the upper range of typically measured values for star formation regions. SiO is likewise enhanced relative to both H₂ and to HCO⁺. The ratio [SiO]/[H₂] = 3.9 × 10^-10 is within the range observed toward shock-excited regions. The cloud mass is estimated to be ≥2600 M_⊙.