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

Abstract

We present high resolution (300 au) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the massive young stellar object G11.92-0.61 MM1. We resolve the immediate circumstellar environment of MM1 in 1.3mm continuum emission and CH3CN emission for the first time. The object divides into two main sources - MM1a, which is the source of a bipolar molecular outflow, and MM1b, located 0:0057 (1920 au) to the South-East. The main component of MM1a is an elongated continuum structure, perpendicular to the bipolar outflow, with a size of 0.00141arcsec X 0.00050arcsec (480 X 170 au). The gas kinematics toward MM1a 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 MM1a is 40 +/- 5M_solar (where between 2.2-5.8M_solar is attributed to the disk), while MM1b is < 0.6M_solar. The extreme mass ratio and orbital properties of MM1a and MM1b suggest that MM1b is one of the first observed examples of the formation of a binary star via disk fragmentation around a massive young (proto)star.