Discovery of a 500 au Protobinary in the Massive Prestellar Core
  G11.92-0.61 MM2
  We present high-resolution (< ~160 au) Atacama Large Millimeter/submillimeter
Array (ALMA) 1.3 mm observations of the high-mass prestellar core candidate
G11.92-0.61 MM2, which reveal that this source is in fact a protobinary system
with a projected separation of 505 au. The binary components, MM2E and MM2W,
are compact (radii < 140 au) sources within the partially optically thick dust
emission with alpha$_{0.9 cm-1.3 mm}$=2.47-2.94. The 1.3 mm brightness
temperatures, T$_b$=68.4/64.6 K for MM2E/MM2W, imply internal heating and
minimum luminosities L$_*$ > 24.7 Lsun for MM2E and L$_*$ > 12.6 Lsun for MM2W.
The compact sources are connected by a "bridge" of lower-surface-brightness
dust emission and lie within more extended emission that may correspond to a
circumbinary disk. The circumprotostellar gas mass, estimated from ~0.2"
resolution VLA 0.9 cm observations assuming optically thin emission, is
6.8+/-0.9 Msun. No line emission is detected towards MM2E and MM2W in our
high-resolution 1.3 mm ALMA observations. The only line detected is 13CO J=2-1,
in absorption against the 1.3 mm continuum, which likely traces a layer of
cooler molecular material surrounding the protostars. We also report the
discovery of a highly asymmetric bipolar molecular outflow that appears to be
driven by MM2E and/or MM2W in new deep, ~0.5" resolution (1680 au) ALMA 0.82 mm
observations. This outflow, traced by low-excitation CH3OH emission, indicates
ongoing accretion onto the protobinary system. Overall, the super-Alfvenic
models of Mignon-Risse et al. (2021) agree well with the observed properties of
the MM2E/MM2W protobinary, suggesting that this system may be forming in an
environment with a weak magnetic field.
