Australia Telescope Compact Array Radio Imaging of the Proplyd-like Objects in the Giant H II Region NGC 3603

Three cometary-shaped objects in the giant H II region NGC 3603, originally found and identified as proto-planetary disks (proplyds) by Brandner and coworkers using the Hubble Space Telescope and the Very Large Telescope in the optical and near-infrared, have been detected with the Australia Telescope Compact Array in the radio continuum at 3 and 6 cm. All three proplyd-like objects are clearly resolved with an extent of a few arcseconds. The integrated 6 cm fluxes are up to 1.3 times higher than the 3 cm fluxes with spectral indices averaged over the whole clump between α = -0.1 and -0.5 (S_ν ∝ ν^α), indicating the likely presence of nonthermal emission in at least some of the sources. We present spectral-index maps and show that the sites of negative radio spectral indices are predominantly concentrated in the direction of the tails in at least two of the three proplyd-like nebulae, while positive spectral indices are found in the region facing the ionizing star cluster. We propose that thermal bremsstrahlung and nonthermal synchrotron radiation are at work in all three proplyd-like sources. In at least one of the three objects, optically thin nonthermal synchrotron emission appears to dominate when averaged over its whole spatial extent, while the spectrum of a second source shows a marginal indication of a nonthermal spectrum. The average spectrum of the third source is in agreement with thermal bremsstrahlung. All measured fluxes are at least one order of magnitude higher than those predicted by Brandner and coworkers. Upper limits for mass-loss rates due to photoevaporation are calculated to be ~10^-5 M_☉ yr^-1 and for electron densities to be ~10⁴ cm^-3. Due to the unexpectedly large radio luminosities of the proplyd-like features and because the radio emission is extended, a (proto-) stellar origin of the nonthermal emission from a dust enshrouded star appears unlikely. Instead, we propose that magnetized regions within the envelope of the proplyd-like nebulae exist.