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

TL;DR

This study analyzes the optical spectrum of the helium-rich star BPS CS 22940-0009, revealing its properties and suggesting it formed from a white dwarf merger, connecting helium subdwarfs and extreme helium stars.

Contribution

The paper provides detailed spectroscopic analysis of BPS CS 22940-0009, establishing it as a link between helium-rich subdwarfs and extreme helium stars, and supports its formation via white dwarf merger.

Findings

Star's atmospheric parameters determined: Teff ~35000 K, log g ~4.8.

Abundance profile shows N and C enrichment, metal-poor composition.

Star likely formed from helium white dwarf merger, evolving toward helium main sequence.

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 $T_{\rm eff} = 34970 \pm 370$ K, $\log g/{\rm cm \, s^{-2}} = 4.79 \pm 0.17$, $n_{\rm H}/n_{\rm He} \simeq 0.007$, $n_{\rm C}/n_{\rm He} \simeq 0.007$, $n_{\rm N}/n_{\rm He} \simeq 0.002$, 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 $3\alpha$ 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.