SUB-ARCSECOND IMAGING OF W3(OH) AT 87.7 GHZ

We report continuum observations of the W3(OH) region with the Berkele y-Illinois-Maryland array at an effective frequency of 87.7 GHz with s ub-arcsecond resolution (0 ''.7 x 0 ''.5). The bright ultracompact H I I region shows a shell-like morphology, consistent with an extrapolati on from lower frequencies of optically thin free-free emission with no significant contribution from dust. A weak unresolved continuum sourc e is detected 6 '' to the east of the ultracompact H II region, coinci dent with the elongated centimeter-wave synchrotron source associated with the Turner-Welch (TW) object. The source brightness temperature i s approximately 20 K, and the spectral index between 87.7 GHz and 111 GHz is characteristic of optically thin thermal dust emission. Assumin g standard dust properties, we estimate that the underlying source lum inosity is 10(3)-10(4) L., and we derive a mass of 10-20 M. for the em ission region. We speculate that this represents circumstellar materia l about to be incorporated into a newly forming star, making the TW ob ject the high-mass equivalent of a ''class 0'' low-mass protostar.