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

Abstract: Context: HD 49798 is a bright ($m_{\rm V} = 8.287$), hot (effective temperature $T_{\rm eff} = 45000$ 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 $M_\odot$) 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 T\"ubingen 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 Tuebingen 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 $T_{\rm eff} = 45000 \pm 1000$ K, log (g / cm/s$^2$) = $4.46 \pm 0.10$, 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. Conclusons: Our panchromatic spectral analysis of HD 49798 -- from the FUV to the optical -- allowed 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 about 7.15 $M_\odot$) He star with a mass of 1.14(+0.30/-0.24) $M_\odot$. At its surface it exhibits abundances that are the result of CNO-cycle and 3 alpha burning nucleosynthesis as well as enhanced Cr, Mn, Fe, and Ni abundances.