Spectroscopic analysis of BPS CS 22940−0009: connecting evolved helium stars

Abstract

BPS CS 22940−0009 is a helium-rich B-star that shares characteristics with both helium-rich B subdwarfs and extreme helium stars. The optical spectrum of BPS CS 22940−0009 has been analysed from SALT observations. The atmospheric parameters were found to be Teff = 34 970 ± 370 K, log g/cm s−2 = 4.79 ± 0.17, nH/nHe ≃ 0.007, nC/nHe ≃ 0.007, nN/nHe ≃ 0.002, although further improvement to the helium line fits would be desirable. This places the star as a link between the He-sdB and EHe populations in g-T space. The abundance profile shows enrichment of N from CNO-processing, and C from 3α burning. Depletion of Al, Si, S and a low upper limit for Fe show the star to be intrinsically metal-poor. The results are consistent with BPS CS 22940−0009 having formed from the merger of two helium white dwarfs and currently evolving toward the helium main sequence.

The SALT survey of chemically-peculiar hot subdwarfs

“…Some have been introduced by Jeffery et al (2017b). Analysis of the low-gravity extreme-helium subdwarf BPS CS 22940-0009 (Snowdon et al, 2022) shows it to be an intrinsically metal-poor N-rich star. Analysis of the extreme-helium subdwarf EC 20187-4939 (Scott et al, in preparation) shows it to be neon-rich as well as a member of a wide binary.…”