Quantitative analysis of the O-subdwarf HD 49789.

HD 49798, the brightest O-subdwarf known, is analyzed by evaluation of a blue plate (6.8 A/mm), 25 image- tube spectra, scans and broad- and narrow-band colors. The spectrum shows helium- and nitrogen-lines enhanced relative to the hydrogen lines that stffl contribute most to the blends with the Pickering lines. We derive the atmospheric parameters by fitting the outer wings of typical lines to proffles computed with LTE-models. First flux-constant unblanketed models are used for computing line-widths (Tables 2 and 3). A better reproduction of the line-centers is achieved by an approximately line-blanketed model and by an empirical model. The atmospheric parameters obtained in the three approximations differ slightly. We adopt as final parameters Te = 45000 K, log g = 5.0, log CR = 11.8, log CR = 9.7 (one line ouly) (relative to log = 12.0). From the interstellar lines Ca ii K and Na D an absolute visual magnitude MR - and a mass of about 2 e are estimated. Considering also the close relation between O-subdwarfs and static helium-star models as far as the mass-luminosity-ratio and the position in the HR-diagram are concerned, we propose an evolutionary track resembling that of a massive star losing a considerable fraction of its envelope while hydrogen is burn in a shell source. Key words: O-subdwarfs - stellar atmosphere - fine analysis - abundances - evolution