Sub-Arcsecond Imaging of W3(OH) at 87.7 GHz

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Abstract
We report continuum observations of the W3(OH) region with the Berkeley-Illinois-Maryland array at an effective frequency of 87.7 GHz with sub-arcsecond resolution (07 × 05). The bright ultracompact H II region shows a shell-like morphology, consistent with an extrapolation from lower frequencies of optically thin free-free emission with no significant contribution from dust. A weak unresolved continuum source is detected 6'' to the east of the ultracompact H II region, coincident with the elongated centimeter-wave synchrotron source associated with the Turner-Welch (TW) object. The source brightness temperature is approximately 20 K, and the spectral index between 87.7 GHz and 111 GHz is characteristic of optically thin thermal dust emission. Assuming standard dust properties, we estimate that the underlying source luminosity is 103-104 L☉, and we derive a mass of 10-20 M☉ for the emission region. We speculate that this represents circumstellar material about to be incorporated into a newly forming star, making the TW object the high-mass equivalent of a "class 0" low-mass protostar.