Abstract
We used the BIMA array to observe the hot molecular core W3(H2O). Our continuum maps at wavelengths of 1.4mm and 2.8mm both achieve sub-arcsecond angular resolutions and show a double-peaked morphology. The angular separation of the two sources is 1.19 arcsec corresponding to 2.43 × 103 AU at the source distance of 2.04~kpc. The flux densities of the two sources at λλ 1.4mm and 2.8mm have a spectral index of 3, translating to an opacity law of κν ∝ ν. The small spectral indices suggest that grain growth has begun in the hot core. We have also observed 5 of the K-components of the methyl cyanide (CH3CN) J=12-11 transition. A radial velocity difference of 2.81 km s-1 is found for the gas in the two continuum peaks. Interpreting these two sources as binary components in orbit about one another, we find a minimum mass of 22 M⊙ for the system. Power-law 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. A velocity gradient previously detected may be explained by unresolved binary rotation with a small velocity difference.