Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)

Abstract

We present a spectral analysis of the sdO central star K 648 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 Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08) and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-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 (3 alpha 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 log Teff - log g plane with evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity of 3810 +/- 1200 Lsun are deduced. Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the distance of M 15 as determined by Alves et al. (2000). From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.Comment: 8 pages, 13 figure