Massive star formation and feedback in W49A: The source of our Galaxy's most luminous water maser outflow

We present high spatial resolution mid-infrared (mid-IR) images of the ring of ultracompact H ii regions in W49A obtained at Gemini North, allowing us to identify the driving source of its powerful H₂O maser outflow. These data also confirm our previous report that several radio sources in the ring are undetected in the mid-IR because they are embedded deep inside the cloud core. We locate the source of the water maser outflow at the position of the compact mid-IR peak of source G (source G:IRS1) to within 0.07 arcsec. This IR source is not coincident with any identified compact radio continuum source, but is coincident with a hot molecular core, so we propose that G:IRS1 is a hot core driving an outflow analogous to the wide-angle bipolar outflow in OMC-1. G:IRS1 is at the origin of a larger bipolar cavity and CO outflow. The water maser outflow is orthogonal to the bipolar CO cavity, so the masers probably reside near its waist in the thin cavity walls. Models of the IR emission require a massive protostar with M_* ≃ 45 M_⊙, L _* ≃ 3 × 10⁵ L_⊙ and an effective envelope accretion rate of ∼10^-3 M_⊙ yr^-1. Feedback from the central star could potentially drive the small-scale H₂O maser outflow, but it has insufficient radiative momentum to have driven the large-scale bipolar CO outflow, requiring that this massive star had an active accretion disc over the past 10⁴ yr. Combined with the spatially resolved morphology in IR images, G:IRS1 in W49 provides compelling evidence for a massive protostar that formed by accreting from a disc, accompanied by a bipolar outflow.