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 \mbox { $T _ { \mathrm { eff } } $ } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 39 \mbox { % \raisebox { 2.0 pt } { { \tiny \mbox { $ \pm$ } } } } 2 \mathrm { kK } and \log g \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 3.9 \mbox { \raisebox { 2.0 pt } { { \tiny% \mbox { $ \pm$ } } } } 0.2 .
The helium ( n _ { He } / n _ { H } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 0.08 ) and oxygen ( n _ { O } / n _ { H } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 0.001 ) abundances are about solar while carbon is enriched by a factor of 2.5 ( n _ { C } / n _ { H } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 0.001 ) .
Nitrogen ( n _ { N } / n _ { H } \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 1 \cdot 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 \mbox { $T _ { \mathrm { eff } } $ } – \log g plane with evolutionary calculations a mass of 0.57 ^ { +0.02 } _ { -0.01 } \mathrm { M } _ { \odot } and a luminosity of 3810 \mbox { \raisebox { 2.0 pt } { { \tiny \mbox { $ \pm$ } } } } 1200 \mathrm { L } _ { \odot } are deduced .
Our spectroscopic distance d = 11.1 ^ { +2.4 } _ { -2.9 } \mathrm { 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 v _ { \mathrm { rad } } = -130 \mathrm { km / sec } .