Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652Her and HD144941

Abstract

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. De fi ning atmospheric parameters were obtained from a grid of non-LTE atmospheres and a variety of spectroscopic indicators including He I and He II line pro fi les, and the 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  =  25,000  ±  300 K, log g  =  3.10  ±  0.12 ( cgs ) , and ξ  =  13  ±  2kms − 1 are provided for V652 Her, and T eff  =  22,000  ±  600 K, log g  =  3.45  ±  0.15 ( cgs ) , and ξ  =  10 km s − 1 are provided for HD 144941. In contrast to the non-LTE analyses, the LTE analyses — LTE atmospheres and an 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, 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.