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

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. 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.

HD144941: The most extreme helium-strong star

“…Moreover, a detection of weak Hα emission is reported for the first time, suggesting the existence of a centrifugal magnetosphere (Petit et al 2013). Using the rigidly rotating magnetosphere model of Townsend & Owocki (2005), they propose this as strong evidence of the star having a mass around 1 M , which they derive from adopting the radius determined from the Gaia EDR3 parallax and the low surface gravity value of Pandey & Lambert (2017) that we ruled out here. Interestingly, Shultz et al (2021) note that their model could be reconciled with the massive star nature of HD144941 in important aspects if v sin i were lower than what they derived (11.2±0.7 km s −1 ), requiring about 7 km s −1 (c.f.…”

Detection of an extremely strong magnetic field in the double-degenerate binary merger product HD 144941

“…The line list was obtained from the Vienna Atomic Line Database (VALD3) (Piskunov et al 1995;Ryabchikova et al 1997;Kupka et al 1999Kupka et al , 2000Ryabchikova et al 2015) using an 'extract stellar' request for a star with solar metallicity and atmospheric parameters corresponding to those of HD 144941 (Pandey & Lambert 2017). Solar metallicity was used rather than the star's much lower, measured abundances so as to ensure all possible lines are available for analysis.…”

“…The conclusion that HD 144941 is in fact a merger product rests upon its identification as an EHe star. The chemical peculiarity of HD 144941 as compared to other EHe stars has been much remarked upon (Pandey & Lambert 2017). The possibility that it might be a He-s mCP star was previously rejected on the basis of its Gaia Data Release 1 (DR1) parallax (Jeffery & Ramsay 2018), which yielded a luminosity below the MS.…”

“…Surface gravity measurements vary widely: log g = 3.45 ± 0.15 (Pandey & Lambert 2017), 3.9 ± 0.2 (Harrison & Jeffery 1997), and 4.15 ± 0.1 (Przybilla et al 2005). Using the radius determined with the Gaia EDR3 parallax, the spectroscopic mass is then 1.2 ± 0.5 M⊙ using the lowest log g, and 6 ± 2 M⊙ using the highest value.…”

K2 spots rotation in the helium star HD144941

“…HD144941 was reported helium-rich by MacConnell et al (1970). The spectrum was shown to be metal-poor with a helium/hydrogen ratio ≈ 10 (Hunger & Kaufmann 1973;Beauchamp et al 1997;Przybilla et al 2005;Pandey & Lambert 2017). Its effective temperature and surface gravity are appropriate for a main-sequence B star, but it is 6 magnitudes too faint to be associated with the Sco-Cen OB2 association within which it lies (MacConnell et al 1970).…”