A High‐Mass Protobinary System in the Hot Core W3(H2O)

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 and 2.8 mm both achieve subarcsecond angular resolutions and show a double-peaked morphology. The angular separation of the two sources is 1 19, corresponding to 2.43 × 10 3 AU at the source distance of 2.04 kpc. The flux densities of the two sources at 1.4 and 2.8 mm have a spectral index of 3, translating to an opacity law of κ ν ∝ ν. The small spectral indices suggest that grain growth has begun in the hot core. We have also observed five K components of the methyl cyanide (CH 3 CN) J = 12 → 11 transitions. A radial velocity difference of 2.81 ± 0.10 km s -1 is found toward the two continuum peaks. Interpreting these two sources as binary components in orbit about each other, we find a minimum mass of 22 M ☉ 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 that 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 ☉ for source A and 4 M ☉ for source C. A velocity gradient previously detected may be explained by unresolved binary rotation with a small velocity difference.