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 α_{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 L⊙ for MM2E and L_* > 12.6 L⊙ 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 M⊙. 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.