G11.92-0.61 MM 1: a fragmented Keplerian disk surrounding a proto-O star

We present high-resolution (~300 au) Atacama Large Millimeter/submillimeter Array observations of the massive young stellar object G11.92–0.61 MM 1. We resolve the immediate circumstellar environment of MM 1 in 1.3 mm continuum emission and CH₃CN emission for the first time. The object divides into two main sources—MM 1a, which is the source of a bipolar molecular outflow, and MM 1b, located 0."57 (1920 au) to the southeast. The main component of MM 1a is an elongated continuum structure, perpendicular to the bipolar outflow, with a size of 0."141 × 0."050 (480 × 170 au). The gas kinematics toward MM 1a probed via CH₃CN trace a variety of scales. The lower energy J = 12–11 K = 3 line traces extended, rotating gas within the outflow cavity, while the v8 = 1 line shows a clearly resolved Keplerian rotation signature. Analysis of the gas kinematics and dust emission shows that the total enclosed mass in MM 1a is 40 ± 5 M _⊙ (where between 2.2 and 5.8 M _⊙ is attributed to the disk), while MM 1b is <0.6 M _⊙. The extreme mass ratio and orbital properties of MM 1a and MM 1b suggest that MM 1b is one of the first observed examples of the formation of a binary star via disk fragmentation around a massive young (proto)star.