Density gradients in Galactic ultra-compact H ii regions

Abstract

We have used recent radiative transfer solutions for cavity-centred shells to investigate the prevalence of density gradients in Galactic ultra-compact H ii regions. We find that an analysis of 5 and 1.4 GHz data, taken from the recent compilation of Giveon et al., implies that ∼76 per cent of sources may have appreciable density gradients. It would also seem that the central cavities of these sources must be relatively small, with radii no greater than ∼20 per cent of those of the outer shells. The remainder of these sources are presumably homogenous, have much larger cavities, or possess reverse density gradients (densities which increase with increasing radius). A good fraction of the H ii regions also appear to have high brightness temperatures, implying mean electron temperatures 〈Te〉 of the order of ≈1.3 × 104 K. This value is higher than has been determined for other such sources.