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Title: | Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652Her and HD144941 |
Authors: | Pandey, G Lambert, D. L |
Keywords: | stars: abundances stars: atmospheres stars: chemically peculiar stars: evolution stars: fundamental parameters |
Issue Date: | Oct-2017 |
Publisher: | IOP Publishing |
Citation: | The Astrophysical Journal, Vol. 847, No. 2, 127 |
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. |
Description: | Restricted Access © The American Astronomical Society https://doi.org/10.3847/1538-4357/aa88bb |
URI: | http://prints.iiap.res.in/handle/2248/6985 |
ISSN: | 0004-637X |
Appears in Collections: | IIAP Publications |
Files in This Item:
File | Description | Size | Format | |
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Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652Her and HD144941.pdf | Restricted Access | 1.23 MB | Adobe PDF | ![]() View/Open |
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