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

TL;DR

This study uses near-infrared observations and spectral analysis to classify the ionizing star of G23.96+0.15 as an O7.5V type, re-evaluate its distance, and improve classification diagnostics for O stars.

Contribution

It provides a new near-infrared classification scheme for O dwarfs and determines the spectral type and distance of the G23.96+0.15 star using advanced spectral modeling.

Findings

Classified the star as O7.5V with Teff ~ 38 kK

Estimated a spectroscopic distance of 2.5 kpc

Revealed discrepancies with kinematic distance estimates

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 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.