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# Polarization due to rotational distortion in the bright star Regulus

## Abstract

Polarization in stars was first predicted by Chandrasekhar1, who calculated a substantial linear polarization at the stellar limb for a pure electron-scattering atmosphere. This polarization will average to zero when integrated over a spherical star but could be detected if the symmetry was broken, for example, by the eclipse of a binary companion. Nearly 50 years ago, Harrington and Collins2 modelled another way of breaking the symmetry and producing net polarization—the distortion of a rapidly rotating hot star. Here we report the first detection of this effect. Observations of the linear polarization of Regulus, with two different high-precision polarimeters, range from +42 ppm at a wavelength of 741 nm to –22 ppm at 395 nm. The reversal from red to blue is a distinctive feature of rotation-induced polarization. Using a new set of models for the polarization of rapidly rotating stars, we find that Regulus is rotating at $$96.{5}_{-0.8}^{+0.6} \%$$ of its critical angular velocity for break-up, and has an inclination greater than 76.5°. The rotation axis of the star is at a position angle of 79.5 ± 0.7°. The conclusions are independent of, but in good agreement with, the results of previously published interferometric observations of Regulus3. The accurate measurement of rotation in early-type stars is important for understanding their stellar environments4 and the course of their evolution5.

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## Acknowledgements

The work was supported by the Australian Research Council through Discovery Project grants DP140100121 and DP160103231. We thank D. Opitz, J. Sturges and the staff at the AAT for their assistance in making the HIPPI observations. We thank R. Spurr of RT Solutions for providing the VLIDORT software.

## Author information

Authors

### Contributions

J.B., D.V.C., L.K.-C., K.B., P.W.L. and J.H.H. drafted the initial proposals to observe Regulus with HIPPI, following an extended discussion of the PlanetPol results dating back to 2010 that included I.D.H., J.H.H., J.B. and P.W.L. All authors contributed to the discussion and drafting of the final manuscript. The HIPPI observations were carried out and directed by J.B., D.V.C., L.K.-C. and K.B. In addition, the following authors made specific contributions to the work: D.V.C. contributed the initial data analysis, telescope polarization subtraction, stellar atmosphere modelling, interstellar subtraction, model comparison to data, and other calculations including the position angle calculation and the calculations related to Regulus’ companions. J.B. contributed the polarized radiative transfer modelling and verification, gravity-darkening calculations and PlanetPol bandpass model calculations and code. I.D.H. contributed rotational velocity calculations, the knowledge and calculations needed to constrain parameter space, and other miscellaneous calculations. K.B. contributed HIPPI bandpass model calculations and code, and research on Regulus’ companions. P.W.L. contributed details of the PlanetPol observations not otherwise available.

### Corresponding author

Correspondence to Daniel V. Cotton.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

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## Electronic supplementary material

### Supplementary Information

Supplementary Text, Supplementary References, Supplementary Figures 1–6 and Supplementary Tables 1–8

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Reprints and Permissions

Cotton, D.V., Bailey, J., Howarth, I.D. et al. Polarization due to rotational distortion in the bright star Regulus. Nat Astron 1, 690–696 (2017). https://doi.org/10.1038/s41550-017-0238-6

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• DOI: https://doi.org/10.1038/s41550-017-0238-6

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