Heavy-metal enrichment of intermediate He-sdOB stars: the pulsators Feige 46 and LS IV -14 116 revisited

Abstract

Hot subdwarf stars of types O and B represent a poorly understood phase in the evolution of low-mass stars. Many subdwarfs show rich oscillations and are among the most chemically peculiar stars known. Two intermediate He-rich hot subdwarf stars, LS IV-14${}^{\circ }$116 and Feige 46, are particularly interesting, because they show extreme enrichments of heavy elements such as Ge, Sr, Y, and Zr, strikingly similar in both stars. Also their light oscillations are similar, but occur at periods incompatible with standard pulsation theory. We investigate the metal abundances in both stars and validate the pulsations in Feige 46 using its recent TESS light curve. High-resolution spectroscopy is combined with non-LTE model atmospheres calculated with Tlusty and Synspec to determine metal abundances consistently. Many lines are identified with transitions originating from Ga III, Ge III-IV, Se III, Kr III, Sr II-III, Y III, Zr III-IV, and Sn IV, most of which have not been observed so far in any star. The abundances of 19 metals in both stars are almost identical, light metals being slightly more abundant in Feige 46 while Zr, Sn, and Pb are slightly less enhanced compared to LS IV$-$14${}^{\circ }$116. Both abundance patterns are distinctively different from those of He-poor subdwarfs of similar temperature. The enrichment in heavy metals of more than 4 dex compared to the Sun is likely the result of strong atmospheric diffusion processes while the stars' similar patterns of C, N, O, and Ne abundances might provide clues to their as yet unclear evolutionary history. Finally, we find that the periods of the pulsation modes in Feige 46 are stable to better than $\dot{P}\lesssim 10^{-8}$ s/s. This is not compatible with $\dot{P}$ predicted for pulsations driven by the $\epsilon$-mechanism and excited by helium-shell flashes in a star which is evolving, for example, onto the extended horizontal branch.

31 pages, 13 figures, accepted for publication in A&A

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