Hypercompact H II Regions: Resolved Images of G34.26+0.15 A and B

Abstract

We report high angular resolution observations at λ = 7 mm of the hypercompact (HC) H II regions G34.26 + 0.15 A and B. The images and the intensity profiles obtained give possible evidence of limb brightening, which may indicate the existence of inner "holes" in the ionized gas. These "holes" were previously inferred from a study of the spectra and angular sizes at different frequencies from lower angular resolution observations. Using a spherical ionized model bounded by an inner and an outer radii with an electron density gradient ne ∝ r –α, we can reproduce the 7 mm intensity profile, spectral energy distribution, and observed angular sizes of both sources. These models indicate inner radii R 1 ~ 400 AU, outer radii R 2 ~ 1000 AU, and shallow density gradients with slopes α ~ 0.3-1.0. These profiles are similar in physical size and shape to those found recently for the HC H II regions G24.78+0.08 A1 and G28.20 – 0.04N. Therefore, at least four HC H II regions seem to have thick shells of ionized gas with fractional widths, (R 2 – R 1)/R 2 ~ 0.1-0.6, in contrast with the thin ionized shells expected in the case of shells swept up by stellar winds. More observations are needed to determine the origin of these thick ionized shells and their frequency of occurrence in the HC H II region stage.