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

TL;DR

This study reports on a variable, compact radio source near W3(OH), likely originating from a photoevaporated disk around a star, based on new and archival VLA observations.

Contribution

It provides evidence supporting the hypothesis that the radio emission stems from a photoevaporated disk, challenging previous interpretations of stellar wind origins.

Findings

Radio source shows variability over years.

Spectral index indicates partially optically thick free-free emission.

Flux densities exceed expectations for a stellar wind.

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.