The hyperyoung HII region in G24.78+0.08 A1

TL;DR

This study confirms that the hypercompact HII region in G24.78+0.08 A1 is an expanding, thin ionized shell driven by wind, with recent observations supporting a short timescale expansion model of 21-66 years.

Contribution

The paper provides high-resolution VLA observations revealing the shell structure of the HC region, supporting the wind-driven expansion model and refining the understanding of its morphology and dynamics.

Findings

HC region is a thin ionized shell with a radius of 590 AU.

The shell is expanding on a timescale of 21-66 years.

Observations support the wind-driven expansion model.

Abstract

Context. G24.78+0.08 A1 is a 20 Msun 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 region. Recent water maser observations suggest that the HC region is expanding and accretion onto the star is halted. Aims. This study aims to confirm the expansion scenario proposed for the HC region on the basis of recent H2O maser observations. Methods. We carried out continuum VLA observations at 1.3cm and 7mm with the A array plus Pie Town configuration to map the HC region towards G24 A1. Results. The emission of the HC 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 region confirm the shell structure of the emission. The ratio between the inner and the outer radius of the shell, Ri/Ro, derived fitting the normalized brightness temperature profile passing through the peak of the 7mm 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 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 H2O masers associated with the HC region. According to this model, the HC region is expanding on very short times scales, 21-66 yr.