Very compact radio emission from high-mass protostars. II. Dust disks and ionized accretion flows

This paper reports 43 GHz imaging of the high-mass protostars W 33A, AFGL 2591 and NGC 7538 IRS9 at 0.04'' and 0.6'' resolution. In each case, weak ( mJy-level), compact (Ø 100 AU) emission is detected, which has an elongated shape (axis ratio 3). However, for AFGL 2591 and NGC 7538 IRS9, the emission is single-peaked, while for the highest-luminosity source, W 33A, a “mini-cluster” of three sources is detected. The derived sizes, flux densities, and broad-band radio spectra of the sources support recent models where the initial expansion of H ii regions around very young O-type stars is prevented by stellar gravity. In these models, accretion flows onto high-mass stars originate in large-scale molecular envelopes and become ionized close to the star. These models reproduce our observations of ionized gas as well as the structure of the molecular envelopes of these sources on 10^3{-}10⁴ AU scales derived previously from single-dish sub-millimeter continuum and line mapping. For AFGL 2591, the 43 GHz flux density is also consistent with dust emission from a disk recently seen in near-infrared “speckle” images. However, the alignment of the 43 GHz emission with the large-scale molecular outflow argues against an origin in a disk for AFGL 2591 and NGC 7538 IRS9. In contrast, the outflow from W 33A does not appear to be collimated. Together with previously presented case studies of W 3 IRS5 and AFGL 2136, our results indicate that the formation of stars and stellar clusters with luminosities up to 10⁵ L_⊙ proceeds through accretion and produces collimated outflows as in the solar-type case, with the “additional feature” that the accretion flow becomes ionized close to the star. Above 10⁵ L_⊙, clusters of H ii regions appear, and outflows are no longer collimated, possibly as the result of mergers of protostars or pre-stellar cores.