Extreme helium stars ( EHe stars ) are hydrogen-deficient supergiants of spectral type A and B .
They are believed to result from mergers in double degenerate systems .
In this paper we present a detailed quantitative non-LTE spectral analysis for BD+10 ^ { \circ } 2179 , a prototype of this rare class of stars , using UVES and FEROS spectra covering the range from \sim 3100 to 10 000 Å .
Atmosphere model computations were improved in two ways .
First , since the UV metal line blanketing has a strong impact on the temperature-density stratification , we used the Atlas12 code .
Additionally , We tested Atlas12 against the benchmark code Sterne3 , and found only small differences in the temperature and density stratifications , and good agreement with the spectral energy distributions .
Second , 12 chemical species were treated in non-LTE .
Pronounced non-LTE effects occur in individual spectral lines but , for the majority , the effects are moderate to small .
The spectroscopic parameters give T _ { \mathrm { eff } } =17 300 \pm 300 K and \log g = 2.80 \pm 0.10 , and an evolutionary mass of 0.55 \pm 0.05 M _ { \odot } .
The star is thus slightly hotter , more compact and less massive than found in previous studies .
The kinematic properties imply a thick-disk membership , which is consistent with the metallicity [ Fe/H ] \approx -1 and \alpha -enhancement .
The refined light-element abundances are consistent with the white dwarf merger scenario .
We further discuss the observed helium spectrum in an appendix , detecting dipole-allowed transitions from about 150 multiplets plus the most comprehensive set of known/predicted isolated forbidden components to date .
Moreover , a so far unreported series of pronounced forbidden He i components is detected in the optical-UV .