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The chemical composition of β Cephei and hybrid pulsators in the solar neighbourhood

Abstract β Cephei variables are the most prominent pulsators among the massive stars on the upper main sequence, extending into the class of the rare hybrid β Cephei-SPB pulsators in the overlap region with the instability strip of the Slowly Pulsating B-stars. While the κ-mechanism has been identified as the driver of the pulsations, a comprehensive explanation of the excitation of the observed p- and g-modes is still lacking. In particular, the instability regions for these main-sequence B-type pulsators are still not fully consistent with any current opacity calculations. We have determined tight observational constraints on the chemical composition of a sample of β Cephei and hybrid pulsators in the solar neighbourhood, covering all elements with abundances log (X/H) + 12 > 7.3. The star sample turns out to be chemically homogeneous, with a metallicity Z ≈ 0.014 and a non-solar abundance mix. The availability of accurate and precise abundances eliminates one of the two variables in the opacity calculations for asteroseismic applications, allowing to focus on (missing) atomic data.