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

Abstract

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 $T_{\rm eff} = 34\, 970 \pm 370$ K, $\log g/{\rm cm\, s^{-2}} = 4.79 \pm 0.17$, 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 towards the helium main sequence.