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

Abstract

Hot subdwarf stars of spectral types O and B represent a poorly understood phase in the evolution of low-mass stars, in particular of close compact binaries. A variety of phenomena are observed, which make them important tools for several astronomical disciplines. For instance, the richness of oscillations of many subdwarfs are important for asteroseismology. Furthermore, hot subdwarfs are among the most chemically peculiar stars known. Two intermediate He-rich hot subdwarf stars, LS IV–14°116 and Feige 46, are particularly interesting, because they show extreme enrichments of heavy elements such as Ge, Sr, Y, and Zr, which are strikingly similar in both stars. In addition, both stars show light oscillations at periods incompatible with standard pulsation theory and form the class of V366 Aqr variables. We investigated whether the similar chemical compositions extend to more complete abundance patterns in both stars and validate the pulsations in Feige 46 using its recent TESS light curve. High-resolution optical and near-ultraviolet spectroscopy are combined with non-local thermodynamical-equilibrium model atmospheres and synthetic spectra calculated with TLUSTY and SYNSPEC to consistently determine detailed metal abundance patterns in both stars. Many previously unidentified lines were identified for the first time 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 yet been observed in any star. The abundance patterns of 19 metals in both stars are almost identical, light metals being only slightly more abundant in Feige 46, while Zr, Sn, and Pb are slightly less enhanced compared to LS IV–14°116. Both abundance patterns are distinctively different from those of normal He-poor hot subdwarfs of a similar temperature. The extreme enrichment in heavy metals of more than 4 dex compared to the Sun is likely the result of strong atmospheric diffusion processes that operate similarly in both stars while their 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 Ṗ ≲ 10−8 s s−1. This is not compatible with Ṗ predicted for pulsations driven by the ɛ-mechanism and excited by helium-shell flashes in a star that is evolving, for example, onto the extended horizontal branch.

The SALT survey of helium-rich hot subdwarfs: methods, classification, and coarse analysis

“…It was included in the RSS sample as a control. The survey parameters are consistent with other recent measurements (Randall et al 2015;Dorsch et al 2020).…”

Electron-Impact Excitation of Ge iii and Photoionisation of Ge ii

“…These represented the first detection of these ions in the optical spectrum of any star, and yielded measured abundances between 3 and 4.6 dex above the solar value and > 10 ppm in absolute terms. These detections and overabundances were recently confirmed in LS IV−14 • 116, and also in the hot subdwarfs Feige 46 and PHL 417 (Dorsch et al 2020;Østensen et al 2020). Such abundances raise the question of their contribution to the opacity and structure of the stellar atmosphere, for which accurate atomic data are essential.…”

“…Bottom Right: relative difference between models with and without the additional contribution of photo-ionization from Ge + and photo-excitation transitions from Ge 2 + (from GRASP). Dorsch et al 2020). Zinc, gallium, germanium, krypton, strontium, yttrium, zirconium, tin and lead are overabundant compared with the Sun by factors from 300 (zinc) to 40 000 (strontium, yttrium and zirconium).…”

“…For the measurement of abundances using individual spectral lines, departures from LTE can be accommodated in the calculation of the emergent spectrum, as done for LS IV−14 • 116 by Dorsch et al (2020). Since we are not concerned with measuring abundances in this paper, the LTE approximation implemented here is sufficient to evaluate the impact on the overall opacity of improved structure calculations for individual ions.…”

Predicted spatial and velocity distributions of ejected companion stars of helium accretion-induced thermonuclear supernovae

“…The distinction between HVS and fast halo stars is, besides kinematics, as discussed above, made even more complicated by the fact that hot subdwarfs are affected by diffusion and mixing processes in their atmospheres (Schindewolf et al 2018;Dorsch et al 2020), which change their primordial abundances and make chemical tagging as a population tracer not feasible. The fraction of bound runaways and halo stars high above the Galactic plane is therefore hard to estimate.…”