High‐Resolution Continuum Imaging at 1.3 and 0.7 Centimeters of the W3 IRS 5 Region

Abstract

High-resolution images of the hypercompact H II (HC H II) regions in W3 IRS 5 taken with the Very Large Array (VLA) at 1.3 and 0.7 cm are presented. Four HC H II regions were detected with sufficient signal-to-noise ratios to allow the determination of relevant parameters, such as source position, size, and flux density. The sources are slightly extended in our ~02 beams; the deconvolved radii are less than 240 AU. A comparison of our data with VLA images taken at epoch 1989.1 shows proper motions for sources IRS 5a and IRS 5f. Between 1989.1 and 2002.5, we find a proper motion of 210 mas at a position angle of 12° for IRS 5f and a proper motion of 190 mas at a position angle of 50° for IRS 5a. At the assumed distance to W3 IRS 5, 1.83 ± 0.14 kpc, these offsets translate to proper motions of ~135 and ~122 km s-1, respectively. These sources are either shock-ionized gas in an outflow or ionized gas ejected from high-mass stars. We find no change in the positions of IRS 5d1/d2 and IRS 5b, and we show through a comparison with archival NICMOS 2.2 μm images that these two radio sources coincide with the infrared double constituting W3 IRS 5. These sources contain B or perhaps O stars. The flux densities of the four sources have changed compared to the epoch 1989.1 results. In our epoch 2002.5 data, none of the spectral indices obtained from flux densities at 1.3 and 0.7 cm are consistent with optically thin free-free emission; IRS 5d1/d2 shows the largest increase in flux density from 1.3 to 0.7 cm. This may be an indication of free-free optical depth within an ionized wind, a photoevaporating disk, or an accretion flow. It is less likely that this increase is caused by dust emission at 0.7 cm.