GALEX J184559.8−413827: a new extreme helium star identified using SALT★

Jeffery, C. S.

doi:10.1093/mnras/stx1442

Cited by 20 publications

(9 citation statements)

References 54 publications

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“…The star GALEX J184559.8-413827 according to the LTE abundance analysis by Jeffery (2017) has a subsolar (−0.4) metallicity with C, N and O abundances similar to that of V652 Her, i.e., the atmosphere of the new discovery is rich in CNO-cycled material. Our abundance analyses of V652 Her show that a non-LTE reanalysis of J184559.8-413827 will not alter this conclusion.…”

Section: Discussionmentioning

confidence: 99%

“…In this paper, we have used TLUSTY and SYNSPEC for calculating LTE and non-LTE model atmospheres and line profiles (Hubeny 1988;Hubeny et al 1994;Hubeny & Lanz 1995, 2017. Stellar atmospheric parameters provided in Table 1 are determined from the spectra of V652 Her and HD 144941 on the assumption of both non-LTE and LTE using the lines given in Tables 2 and 3.…”

Section: Quantitative Fine Analysesmentioning

confidence: 99%

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Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652 Her and HD 144941

Pandey

Lambert

2017

ApJ

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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±300K, log g = 3.10±0.12(cgs), and ξ = 13 ± 2km s −1 are provided for V652 Her, and T eff =22000±600K, log g = 3.45±0.15 (cgs), and ξ = 10km 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.

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Section: Discussionmentioning

confidence: 99%

Section: Quantitative Fine Analysesmentioning

confidence: 99%

Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652 Her and HD 144941

Pandey

Lambert

2017

ApJ

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“…There are only 22 extreme helium (EHe) stars known. Jeffery (2017) recently reported the identification of the first new eHe star in 40 years. There are only 5 low-gravity He-sdO stars and 4 O(He) stars known (Jeffery 2008a).…”

Section: Lifetimes and The Expected Number Of Rcb Starsmentioning

confidence: 99%

Evolving R Coronae Borealis stars with mesa

Lauer

Chatzopoulos

Clayton

et al. 2019

Monthly Notices of the Royal Astronomical Society

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The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich supergiants. They undergo extreme, irregular declines in brightness of many magnitudes due to the formation of thick clouds of carbon dust. It is thought that RCB stars result from the mergers of CO/He white dwarf (WD) binaries. We constructed postmerger spherically-symmetric models computed with the MESA code, and then followed the evolution into the region of the HR diagram where the RCB stars are located. We also investigated nucleosynthesis in the dynamically accreting material of CO/He WD mergers which may provide a suitable environment for significant production of 18 O and the very low 16 O/ 18 O values observed. We have also discovered that the N abundance depends sensitively on the peak temperature in the He-burning shell. Our MESA modeling consists of engineering the star by adding He-WD material to an initial CO-WD model, and then following the post-merger evolution using a nuclearreaction network to match the observed RCB abundances as it expands and cools to become an RCB star. These new models are more physical because they include rotation, mixing, mass-loss, and nucleosynthesis within MESA. We follow the later evolution beyond the RCB phase to determine the stars' likely lifetimes. The relative numbers of known RCB and Extreme Helium (EHe) stars correspond well to the lifetimes predicted from the MESA models. In addition, most of computed abundances agree very well with the observed range of abundances for the RCB class.

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“…In an attempt to address the shortage of known helium stars on the post-merger track, one of us commenced a spectroscopic survey of chemically-peculiar subdwarfs, targeting all southern hot subdwarfs classified as helium-rich (Jeffery et al 2017), (Jeffery et al, in prep.). An early discovery was GALEX J184559.8 − 413827, (Jeffery 2017), which is almost a twin of V652 Her; it is also nitrogen-rich but not yet detected to pulsate. Analyses of several others are currently in progress.…”

Section: Observational Tests: He+he Modelsmentioning

confidence: 99%

Merged white dwarfs and nucleosynthesis

Jeffery,

Zhang

2020

Preprint

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Orbital decay mechanisms argue that double white dwarf mergers are inevitable, but extremely rare. Whilst some mergers result in explosions, the survivors re-ignite helium and burn brightly for tens of thousands or millions of years. Candidate survivors include extreme helium stars, R CrB variables and various classes of helium-rich subluminous star. Nuclear waste on the survivors' surfaces provides evidence of the stars' nuclear history prior to and their nucleosynthesis during the merger. Extensive and deep spectroscopic surveys offer rich prospects for future discoveries.

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GALEX J184559.8−413827: a new extreme helium star identified using SALT★

Cited by 20 publications

References 54 publications

Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652 Her and HD 144941

Non-local Thermodynamic Equilibrium Abundance Analyses of the Extreme Helium Stars V652 Her and HD 144941

Evolving R Coronae Borealis stars with mesa

Merged white dwarfs and nucleosynthesis

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