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 Teff ~
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 HII
regions. Such discrepancies would be alleviated if sources are unresolved
binaries or clusters.Comment: 5 pages, 3 figures, accepted for Astronomy & Astrophysic