2008A&A...492..111C

A near-infrared study of the main ionizing star of the ultracompact HII region G23.96+0.15 (IRAS 18317-0757) is presented, along with a re-evaluation of the distance to this source, and a re-assessment of H- and K-band classification diagnostics for O dwarfs We have obtained near-IR VLT/ISAAC imaging and spectroscopy of G23.96+0.15, plus archival imaging from UKIRT/UFTI. A spectroscopic analysis was carried out using a non-LTE model atmosphere code. A quantitative H- and K-band classification scheme for O dwarfs is provided, from which we establish an O7.5V spectral subtype for the central star of G23.96+0.15. We estimate an effective temperature of T_eff∼38kK from a spectral analysis. A spectroscopic distance of 2.5kpc is obtained for G23.96+0.15, substantially lower than the kinematic distance of 4.7kpc, in common with recent studies of other Milky way HII regions. Such discrepancies would be alleviated if sources are unresolved binaries or clusters.