Research Note

On the central ionizing star of G23.96+0.15 and near-IR spectral classification of O stars

Department of Physics & Astronomy, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH, UK e-mail: Paul.Crowther@sheffield.ac.uk

Received: 9 September 2008
Accepted: 14 October 2008

Abstract

Aims. 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

Methods. 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.

Results. 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  kK from a spectral analysis.

Conclusions. A spectroscopic distance of 2.5 kpc is obtained for G23.96+0.15, substantially lower than the kinematic distance of 4.7 kpc, in common with recent studies of other Milky way H ii regions. Such discrepancies would be alleviated if sources are unresolved binaries or clusters.