2017A&A...605A..61B

Context. High-mass stars have a high degree of multiplicity and most likely form via disk accretion processes. The detailed physics of the binary and disk formation are still poorly constrained.
Aims. We seek to resolve the central substructures of the prototypical high-mass star-forming region NGC 7538IRS1 at the highest possible spatial resolution in line and continuum emission to investigate the protostellar environment and kinematics.
Methods. Using the Karl G. Jansky Very Large Array (VLA) in its most extended configuration at ∼24GHz has allowed us to study the NH₃ and thermal CH₃OH emission and absorption as well as the cm continuum emission at an unprecedented spatial resolution of 0.06''x0.05'', corresponding to a linear resolution of ∼150AU at a distance of 2.7kpc.
Results. A comparison of these new cm continuum data with previous VLA observations from 23yr ago reveals no recognizable proper motions. If the emission were caused by a protostellar jet, proper motion signatures should have been easily identified. In combination with the high spectral indices S∝ν^α (α between 1 and 2), this allows us to conclude that the continuum emission is from two hypercompact HII regions separated in projection by about 430AU. The NH₃ spectral line data reveal a common rotating envelope indicating a bound high-mass binary system. In addition to this, the thermal CH₃OH data show two separate velocity gradients across the two hypercompact HII regions. This indicates two disk-like structures within the same rotating circumbinary envelope. Disk and envelope structures are inclined by ∼33°, which can be explained by initially varying angular momentum distributions within the natal, turbulent cloud.
Conclusions. Studying high-mass star formation at sub-0.1'' resolution allows us to isolate multiple sources as well as to separate circumbinary from disk-like rotating structures. These data show also the limitations in molecular line studies in investigating the disk kinematics when the central source is already ionizing a hypercompact HII region. Recombination line studies will be required for sources such as NGC 7538IRS1 to investigate the gas kinematics even closer to the protostars.