The compact, time-variable radio source projected inside W3(OH): Evidence for a Photoevaporated Disk?

Abstract

We present new Karl G. Jansky Very Large Array observations of the compact (~ 0.05"), time-variable radio source projected near the center of the ultracompact HII region W3(OH). The analysis of our new data as well as of VLA archival observations confirms the variability of the source on timescales of years and for a given epoch indicates a spectral index of \alpha = 1.3 +- 0.3 (S_\nu \propto \nu^\alpha). This spectral index and the brightness temperature of the source (~6,500 K) suggest that we are most probably detecting partially optically thick free-free radiation. The radio source is probably associated with the ionizing star of W3(OH), but an interpretation in terms of an ionized stellar wind fails because the detected flux densities are orders of magnitude larger than expected. We discuss several scenarios and tentatively propose that the radio emission could arise in a static ionized atmosphere around a fossil photoevaporated disk.