We have observed a high-mass protobinary system in the hot core W3 ( H _ { 2 } O ) with the BIMA Array .
Our continuum maps at wavelengths of 1.4 mm and 2.8 mm both achieve sub-arcsecond angular resolutions and show a double-peaked morphology .
The angular separation of the two sources is 1 \farcs 19 corresponding to 2.43 \times 10 ^ { 3 } AU at the source distance of 2.04 kpc .
The flux densities of the two sources at 1.4 mm and 2.8 mm have a spectral index of 3 , translating to an opacity law of \kappa _ { \nu } \propto \nu .
The small spectral indices suggest that grain growth has begun in the hot core .
We have also observed 5 K components of the methyl cyanide ( \mathrm { CH _ { 3 } CN } ) J = 12 \rightarrow 11 transitions .
A radial velocity difference of 2.81 \pm 0.10 \mathrm { km s ^ { -1 } } is found towards the two continuum peaks .
Interpreting these two sources as binary components in orbit about one another , we find a minimum mass of 22 M _ { \odot } for the system .
Radiative transfer models are constructed to explain both the continuum and methyl cyanide line observations of each source .
Power-law distributions of both density and temperature are derived .
Density distributions close to the free-fall value , r ^ { -1.5 } , are found for both components , suggesting continuing accretion .
The derived luminosities suggest the two sources have equivalent zero-age main sequence ( ZAMS ) spectral type B0.5 - B0 .
The nebular masses derived from the continuum observations are about 5 M _ { \odot } for source A and 4 M _ { \odot } for source C. A velocity gradient previously detected may be explained by unresolved binary rotation with a small velocity difference .