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We present high resolution ($\sim$300 au) Atacama LargeMillimeter/submillimeter Array (ALMA) observations of the massive young stellarobject G11.92-0.61 MM 1. We resolve the immediate circumstellar environment ofMM 1 in 1.3 mm continuum emission and CH$_{3}$CN emission for the first time.The object divides into two main sources - MM 1a, which is the source of abipolar molecular outflow, and MM 1b, located 0.57'' (1920 au) to theSouth-East. The main component of MM 1a is an elongated continuum structure,perpendicular to the bipolar outflow, with a size of $0.141'' \times 0.050''$($480\times170$ au). The gas kinematics toward MM 1a probed via CH$_{3}$CNtrace a variety of scales. The lower energy $J=12-11$ $K=3$ line tracesextended, rotating gas within the outflow cavity, while the $v$8=1 line shows aclearly-resolved Keplerian rotation signature. Analysis of the gas kinematicsand dust emission shows that the total enclosed mass in MM 1a is $40\pm5$M$_{\odot}$ (where between 2.2-5.8 M$_{\odot}$ is attributed to the disk),while MM 1b is $<0.6$ M$_{\odot}$. The extreme mass ratio and orbitalproperties of MM 1a and MM 1b suggest that MM 1b is one of the first observedexamples of the formation of a binary star via disk fragmentation around amassive young (proto)star.

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