EDP Sciences
Free access
Volume 492, Number 1, December II 2008
Page(s) 111 - 115
Section Galactic structure, stellar clusters, and populations
DOI http://dx.doi.org/10.1051/0004-6361:200810942
Published online 27 October 2008

A&A 492, 111-115 (2008)
DOI: 10.1051/0004-6361:200810942

Research Note

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

P. A. Crowther and J. P. Furness

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

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 $T_{\rm eff}\sim 38$ 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.

Key words: ISM: HII regions -- ISM: dust, extinction -- stars: early-type -- stars: fundamental parameters

© ESO 2008