Here we report the detection of polarization variations due to non-radial modes in the β Cephei 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 Cephei stars, we combine polarimetry with space-based photometry and archival spectroscopy to identify the dominant non-radial mode in polarimetry, f2, as mode degree ℓ = 3, azimuthal order m = −3 (in the m-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 ± 0.5 M⊙ and a convective core containing ~28% of its mass—making β Crucis the most massive star with an asteroseismic age.
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The new data that support the plots within this paper and other findings of this study will be available from the VizieR service upon print publication. All other data analysed in this work come from public repositories; where this is the case, the origin of the data is indicated in the text.
Our polarimetric modelling code is based on the publicly available ATLAS9, SYNSPEC and VLIDORT codes. Our modified version of SYNSPEC is available on request. The spectroscopic mode identification was performed with the software package FAMIAS available from https://fys.kuleuven.be/ster/Software/famias/famias, applied to the time-series spectroscopy available from https://fys.kuleuven.be/ster/Software/helas/helas. The neural network is available from https://github.com/l-hendriks/asteroseismology-dnn. The joint frequency analysis was conducted using a custom MATLAB package that is available on request.
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This research has made use of the SIMBAD database and VizieR catalogue access tool, operated at CDS (Strasbourg, France). This research has made use of NASA’s Astrophysics Data System. We thank the Director of Siding Spring Observatory, C. Lidman, for his support of the HIPPI-2 project on the AAT. We thank M. Filipovic for providing access to the Penrith Observatory. D.V.C. would also like to thank M. Filipovic and B. Carter for their support of his initially unfunded research on this project in the form of adjunct positions at WSU and USQ. We thank N. Cohen, G. Santucci and D. Maybour for assisting with the observations. We thank Wm. B. Weaver for useful comments on the manuscript. Funding for the construction of HIPPI-2 was provided by UNSW through the Science Faculty Research Grants Program (J.B.). Part of the research leading to these results has received funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme by means of a European Research Council AdG to C.A. (grant agreement No. 670519: MAMSIE). This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958 (M.G.P.). D.L.B. acknowledges support from the TESS Guest Investigator Program through award NNH17ZDA001N-TESS.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Dietrich Baade, Swetlana Hubrig and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Cotton, D.V., Buzasi, D.L., Aerts, C. et al. Polarimetric detection of non-radial oscillation modes in the β Cephei star β Crucis. Nat Astron 6, 154–164 (2022). https://doi.org/10.1038/s41550-021-01531-9
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