2002A&A...381.1007R

We present a spectral analysis of the sdO central star K648 based on medium-resolution optical and high-resolution UV spectra. The photospheric parameters are determined by means of state-of-the-art NLTE model atmosphere techniques. We found T_eff=39±2kK and logg=3.9±0.2. The helium (n_He/n_H=0.08) and oxygen (n_O/n_H=0.001) abundances are about solar while carbon is enriched by a factor of 2.5 (n_C/n_H=0.001). Nitrogen (n_N/n_H=1x10^–6, [N/H]=-2.0) appears at a sub-solar value. However, these metal abundances are much higher than the cluster's metallicity (M 15: [Fe/H]=-2.25). The surface composition appears to be a mixture of the original hydrogen-rich material and products of helium burning (3α process) which have been mixed up to the surface. The abundances of He, C, and N are consistent with the nebular abundance, while O is considerably more abundant in the photosphere than in the nebula. From a comparison of its position in the logT_eff-logg plane with evolutionary calculations a mass of 0.57^+0.02_–0.01M_☉ and a luminosity of 3810±1200L_☉ are deduced. Our spectroscopic distance d=11.1^+2.4_–2.9kpc is in agreement with the distance of M 15 as determined by Alves et al. (2000AJ....120.2044A). From the GHRS spectra we measure a radial velocity of v_rad=-130km/s.