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 _ { eff } =25000 \pm 300K , \log g = 3.10 \pm 0.12 ( cgs ) , and \xi = 13 \pm 2 { km s ^ { -1 } } are provided for V652 Her , and T _ { eff } =22000 \pm 600K , \log g = 3.45 \pm 0.15 ( cgs ) , and \xi = 10 { 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 .