The hyperyoung H ii region in G24.78+0.08 A1

Abstract

Context. G24.78+0.08 A1 is a 20 $M_\odot$ star surrounded by a hypercompact (HC) Hii region, driving a CO bipolar outflow, and located at the center of a massive rotating toroid undergoing infall towards the HC Hii region. Recent water maser observations suggest that the HC Hii region is expanding and accretion onto the star is halted. Aims. To confirm the expansion scenario proposed for the HC Hii region on the basis of recent H 2 O maser observations. Methods. We carried out continuum VLA observations at 1.3 cm and 7 mm with the A array plus Pie Town configuration to map the HC Hii region towards G24.78+0.08 A1. Results. The emission of the HC Hii region has been resolved and shows a ring-shape structure. The profiles of the emission obtained by taking slices at different angles passing through the barycenter of the HC Hii region confirm the shell structure of the emission. The ratio between the inner and the outer radius of the shell, $R_{\rm i}/R_{\rm o}$, derived by fitting the normalized brightness temperature profile passing through the peak of the 7 mm emission, is 0.9, which indicates that the shell is thin. The deconvolved outer radius estimated from the fit is 590 AU. These results imply that the HC Hii region in G24 A1 cannot be described in terms of a classical, homogeneous Hii region but is instead an ionized shell. This gives support to the model of an expanding wind-driven, ionized shell suggested by the kinematics and distribution of the H 2 O masers associated with the HC Hii region. According to this model, the HC Hii region is expanding on very short times scales, 21–66 yr.