We present high resolution ( \sim 300 au ) Atacama Large Millimeter/submillimeter Array ( ALMA ) 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 CH _ { 3 } CN 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 \farcs 57 ( 1920 au ) to the South-East .
The main component of MM 1a is an elongated continuum structure , perpendicular to the bipolar outflow , with a size of 0 \farcs 141 \times 0 \farcs 050 ( 480 \times 170 au ) .
The gas kinematics toward MM 1a probed via CH _ { 3 } CN trace a variety of scales .
The lower energy J = 12 –11 K = 3 line traces extended , rotating gas within the outflow cavity , while the v 8=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 \pm 5 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 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 .