Spectra and Sizes of Hypercompact HiiRegions

Abstract

We present the analysis of the spectra and sizes of the hypercompact H II regions G34.26+0.15 A and B. We compare their radio continuum spectra and angular sizes with simple models of spherical ionized regions bounded by an inner and an outer radius, with a power-law electron density profile, ne(r) ∝ r-α. The radio continuum spectra and sizes can be reasonably reproduced by both uniform sphere models and shell models with large inner radii of the order of 500 AU. High spatial resolution observations at ν > 15 GHz, where the regions are optically thin, could distinguish between the two models. The optically thin radio emission implies, after accounting for dust absorption of ionizing photons, that the ionizing stars of sources A and B have spectral types earlier than main-sequence B1 type.