Multiplicity and disks within the high-mass core NGC7538IRS1: Resolving
  cm line and continuum emission at ~0.06"x0.05" resolution
  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. Methods: Using the VLA in its most
extended configuration at ~24GHz toward the prototypical high-mass star-forming
region NGC7538IRS1 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 23yrs 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~nu^{alpha} (alpha 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 ~33deg, 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.
