Spectral analysis of HD 49798, a bright, hydrogen-deficient sdO-type donor star in an accreting X-ray binary

Context. HD 49798 is a bright (mV = 8.287), hot (effective temperature Teff = 45 000 K) subdwarf star of the spectral type O (sdO). It is the only confirmed sdO-type mass-donor star of an X-ray binary that has a high-mass (1.28 M☉) white-dwarf or neutron-star primary with a spin period of only 13.2 s. Aims. Since a high-quality spectrum of HD 49798, obtained with the Tübingen Ultraviolet Echelle Spectrometer (TUES), that has never been analyzed before is available in the database of the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS), we performed a spectral analysis based on observations from the far ultraviolet (FUV) to the optical wavelength range. Methods. We used advanced non-local thermodynamic equilibrium (NLTE) model atmospheres of the Tübingen Model-Atmosphere Package (TMAP) to determine the effective temperature, the surface gravity (log g), and the abundances of those elements that exhibit lines in the available observed spectra. Results. We determined Teff = 45 000 ± 1000 K, log g = 4.46 ± 0.10, and re-analyzed the previously determined photospheric abundances of H, He, N, O, Mg, Al, Si, Fe, and Ni. For the first time, we measured the abundances of C, Ne, P, S, Cr, and Mn. Conclusions. Our panchromatic spectral analysis of HD 49798 – from the FUV to the optical – allowed us to reduce the error limits of the photospheric parameters and to precisely measure the metal abundances. HD 49798 is a stripped, intermediate-mass (zero-age main sequence mass of MZAMS ≈ 7.15 M☉) He star with a mass of 1.14−0.24+0.30 M⊙. At its surface, it exhibits abundances that are the result of CNO-cycle and 3 α burning nucleosynthesis as well as enhanced Cr, Mn, Fe, and Ni abundances.