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Polarimetric detection of non-radial oscillation modes in the β Cephei star β Crucis

Nature Astronomy volume 6, pages 154–164 (2022)Cite this article

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Abstract

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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Fig. 1: β Cru time-series examples.
Fig. 2: Selected amplitude spectra for β Cru.
Fig. 3: Photometric and polarimetric data phase-folded to f2 for β Cru.
Fig. 4: Polarimetric mode determination diagrams for the f2 mode of β Cru.
Fig. 5: Line-profile analysis for β Cru.
Fig. 6: Distributions of β Cru’s rotation axis inclination angle.

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Data availability

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.

Code availability

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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Acknowledgements

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.

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Authors and Affiliations

  1. Monterey Institute for Research in Astronomy, Marina, CA, USA

    Daniel V. Cotton

  2. Anglo Australian Telescope, Australian National University, Coonabarabran, New South Wales, Australia

    Daniel V. Cotton

  3. Western Sydney University, Penrith-South, New South Wales, Australia

    Daniel V. Cotton, Jeremy Bailey & Ain De Horta

  4. Centre for Astrophysics, University of Southern Queensland, Toowoomba, Queensland, Australia

    Daniel V. Cotton, Lucyna Kedziora-Chudczer & Duncan J. Wright

  5. Department of Chemistry & Physics, Florida Gulf Coast University, Fort Myers, FL, USA

    Derek L. Buzasi

  6. Institute of Astronomy, KU Leuven, Leuven, Belgium

    Conny Aerts, Siemen Burssens & May G. Pedersen

  7. Department of Astrophysics, IMAPP, Radboud University Nijmegen, Nijmegen, the Netherlands

    Conny Aerts

  8. Max Planck Institute for Astronomy, Heidelberg, Germany

    Conny Aerts

  9. School of Physics, UNSW Sydney, Sydney, New South Wales, Australia

    Jeremy Bailey, Dennis Stello, Fiona Lewis & Sai Prathyusha Malla

  10. Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA, USA

    May G. Pedersen

  11. Royal Observatory of Belgium, Brussels, Belgium

    Peter De Cat

  12. Department of Earth and Planetary Science, University of California, Riverside, CA, USA

    Kimberly Bott

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Contributions

All authors contributed to the discussion and drafting of the final manuscript. D.V.C., D.L.B., C.A., J.B., D.S., M.G.P., P.D.C., L.K.-C. and A.D.H. contributed to observing proposals and/or scheduling. D.V.C., J.B., D.L.B., A.D.H., L.K.-C., F.L. and S.P.M. carried out polarimetric observations. In addition, the following authors made specific contributions to the work: D.V.C. initiated the work, contributed the polarimetric data processing and analysis, calculations of and comparisons with the analytical model, investigated binary effects, the interstellar polarization and co-ordinated the observations and analysis. D.L.B. carried out the frequency analysis and contributed analysis of asteroseismic data. C.A. analysed the spectroscopic data and carried out the associated mode identification, as well as the asteroseismic modelling. J.B. contributed the polarized radiative transfer modelling, investigated binary effects and aided with interpretation of the analytical model. S.B. computed theoretical stellar models and pulsation modes for the asteroseismic modelling and ran the neural network. M.G.P. computed bolometric corrections and the luminosity of β Cru, based on its spectroscopic properties. D.S. helped facilitate the initial collaboration and provided valuable context for the work.

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Correspondence to Daniel V. Cotton or Derek L. Buzasi.

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Supplementary discussion, Figs. 1–5, Tables 1–7 and references.

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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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