Non-Local Thermodynamic Equilibrium abundance analyses of the extreme helium stars: V652 Her and HD 144941

Optical high-resolution spectra of V652 Her and HD 144941, the two extreme helium stars with exceptionally low C/He ratios, have been subjected to a non-LTE abundance analysis using the tools TLUSTY and SYNSPEC. Defining atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of spectroscopic indicators including He I and He II line profiles, ionization equilibrium of ion pairs such as C II/C III and N II/N III. The various indicators provide a consistent set of atmospheric parameters: $T_{\rm eff}$=25000$\pm$300K, $\log g$ = 3.10$\pm$0.12(cgs), and $\xi=13\pm2 {\rm km\,s^{-1}}$ are provided for V652 Her, and $T_{\rm eff}$=22000$\pm$600K, $\log g$ = 3.45$\pm$0.15 (cgs), and $\xi=10 {\rm km\,s^{-1}}$ are provided for HD 144941. In contrast to the non-LTE analyses, the LTE analyses - LTE atmospheres and a LTE line analysis - with the available indicators do not provide a consistent set of atmospheric parameters. The principal non-LTE effect on the elemental abundances is on the neon abundance. It is generally considered that these extreme helium stars with their very low C/He ratio result from the merger of two helium white dwarfs. Indeed, the derived composition of V652 Her is in excellent agreement with predictions by Zhang & Jeffery (2012) who model the slow merger of helium white dwarfs; a slow merger results in the merged star having the composition of the accreted white dwarf. In the case of HD 144941 which appears to have evolved from metal-poor stars a slow merger is incompatible with the observed composition but variations of the merger rate may account for the observed composition. More detailed theoretical studies of the merger of a pair of helium white dwarfs are to be encouraged.