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Institut für Astronomie und Astrophysik

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Preprint 04/01


Spectral analysis of the sdO K648, the exciting star of the planetary nebula Ps1 in the globular cluster M15 (NGC 7078}

T. Rauch (1), U. Heber (2), and K. Werner (1)

(1) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Sand 1, D-72076 Tübingen, Germany
(2) Dr. Remeis-Sternwarte, Universität Erlangen-Nürnberg, D-96049 Bamberg, Germany

To appear in: Astronomy & Astrophysics

Abstract.

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 Teff=39+/-2 kK and log g=3.9+/-0.2. The helium (nHe/nH=0.08) and oxygen (nO/nH=0.001) abundances are about solar while carbon is enriched by a factor of 2.5 (nC/nH=0.001). Nitrogen (nN/nH=1*10-6, [N/H]=-2.0) appears at a sub-solar value. However, these metal abundances are much higher than the cluster's metallicity (M15: [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 logTeff - logg plane with evolutionary calculations a mass of 0.57+0.02-0.01 Msol and a luminosity of 3810+/-1200 Lsol are deduced. Our spectroscopic distance d=11.1+2.4-2.9 kpc is in agreement with the distance of M15 as determined by Alves et al. (2000). From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.

Key words: Galaxy: globular clusters: individual: M15 - ISM: planetary nebulae: individual: Ps1 - Stars: abundances - Stars: AGB and post-AGB - Stars: evolution - Stars: individual: K648}

Paper (340k gzip'ed Postscript including figures)


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