Abstract
Here we report the detection of polarization variations due to nonradial modes in the β Cep star β Crucis. In so doing we confirm 40-year-old predictions of pulsation-induced polarization variability and its utility in asteroseismology for mode identification. In an approach suited to other β Cep stars, we combine polarimetry with space-based photometry and archival spectroscopy to identify the dominant nonradial mode in polarimetry, f2, as `=3, m = −3 (in them-convention of Dziembowski) and determine the stellar axis position angle as 25 (or 205) ±8◦. The rotation axis inclination to the line of sight was derived as ∼ 46◦ from combined polarimetry and spectroscopy, facilitating identification of additional modes and allowing for asteroseismic modelling. This reveals a star of 14.5 ± 0.5M and a convective core containing ∼ 28% of its mass – making β Crucis the most massive star with an asteroseismic age.