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

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 $T_{\mathrm{e}\mathrm{f}\mathrm{f}}=34970\pm 370$ K, $\log g\mathrm{/}\mathrm{c}\mathrm{m}{\mathrm{s}}^{-2}=4.79\pm 0.17$, $n_{\mathrm{H}}\mathrm{/}{n}_{\mathrm{H}\mathrm{e}}\simeq 0.007$, $n_{\mathrm{C}}\mathrm{/}{n}_{\mathrm{H}\mathrm{e}}\simeq 0.007$, $n_{\mathrm{N}}\mathrm{/}{n}_{\mathrm{H}\mathrm{e}}\simeq 0.002$, 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.