Polarized reflected light from the Spica binary system


Close binary systems often show linear polarization varying over the binary period, usually attributed to light scattered from electrons in circumstellar clouds1,2,3. One of the brightest close binary systems is Spica (alpha Virginis) consisting of two B-type stars orbiting with a period of just over four days. Past observations of Spica have shown low polarization with no evidence for variability4,5,6. Here we report new high-precision polarization observations of Spica that show variation with an amplitude of about 200 parts per million. By including polarized radiative transfer in a binary star model, we show that the phase-dependent polarization is mainly due to light reflected from the primary component of the binary system off the secondary component and vice versa. The stars reflect only a few per cent of the incident light, but the reflected light is very highly polarized. The polarization results show that the binary orbit is clockwise and the position angle of the line of nodes is 130.4° ± 6.8°, in agreement with intensity interferometer results7. We suggest that reflected light polarization may be much more important in binary systems than has previously been recognized and may be a way of detecting previously unrecognized close binaries.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Polarization observations and modelling of Spica.
Fig. 2: Reflected light and polarization for stellar atmosphere grid models.
Fig. 3: Polarization modelling of Spica at phase 0.222 (80°).

Data availability

All processed data generated during this study are included in this published article (the reduced observational data points are in Supplementary Table 4 and plotted in Fig. 1), and the raw data files are available upon reasonable request. All other data analysed in this work comes from public repositories: where this is the case, the origin of the data is indicated in the text.

Code availability

Our modelling code is based on the publicly available ATLAS9, SYNSPEC and VLIDORT codes. Our modified version of SYNSPEC and our binary modelling code is available on request.


  1. 1.

    Brown, J. C., McLean, I. S. & Emslie, A. G. Polarisation by Thomson scattering in optically thin stellar envelopes. Astron. Astrophys. 68, 415–427 (1978).

    ADS  Google Scholar 

  2. 2.

    Rudy, R. J. & Kemp, J. C. A polarimetric determination of binary inclinations: results for five systems. Astrophys. J. 221, 200–210 (1978).

    ADS  Article  Google Scholar 

  3. 3.

    Berdyugin, A., Piirola, V., Sakanoi, T., Kagitani, M. & Yoneda, M. High-precision broad-band linear polarimetry of early-type binaries. II. Variable, phase-locked polarization in triple Algol-type system λ Tauri. Astron. Astrophys. 611, A69 (2018).

    ADS  Article  Google Scholar 

  4. 4.

    Pfeiffer, R. J. & Koch, R. H. On the linear polarization of close binaries. Publ. Astron. Soc. Pacif. 89, 147–154 (1977).

    ADS  Article  Google Scholar 

  5. 5.

    Tinbergen, J. Interstellar polarization in the immediate solar neighbourhood. Astron. Astrophys. 105, 53–64 (1982).

    ADS  Google Scholar 

  6. 6.

    Elias, N. M., Koch, R. H. & Pfeiffer, R. J. Polarimetric measures of selected variable stars. Astron. Astrophys. 489, 911–921 (2008).

    ADS  Article  Google Scholar 

  7. 7.

    Herbison-Evans, D., Hanbury-Brown, R., Davis, J. & Allen, L. R. A study of alpha Virginis with an intensity interferometer. Mon. Not. R. Astron. Soc. 151, 161–176 (1971).

    ADS  Article  Google Scholar 

  8. 8.

    Bailey, J. et al. A high-sensitivity polarimeter using a ferro-electric liquid crystal modulator. Mon. Not. R. Astron. Soc. 449, 3064–3073 (2015).

    ADS  Article  Google Scholar 

  9. 9.

    Bailey, J., Cotton, D. V. & Kedziora-Chudczer, L. A high-precision polarimeter for small telescopes. Mon. Not. R. Astron. Soc. 465, 1601–1607 (2017).

    ADS  Article  Google Scholar 

  10. 10.

    Cotton, D. V. et al. The linear polarization of southern bright stars measured at the parts-per-million level. Mon. Not. R. Astron. Soc. 455, 1607–1628 (2016).

    ADS  Article  Google Scholar 

  11. 11.

    Rudy, R. J. & Kemp, J. C. Phase-locked polarization in u Herculis: evidence for the reflection mechanism. Astrophys. J. 216, 767–775 (1977).

    ADS  Article  Google Scholar 

  12. 12.

    Berdyugin, A. V. & Harries, T. J. Discovery of the variable phase-locked polarization in LZ Cephei. Astron. Astrophys. 352, 177–181 (1999).

    ADS  Google Scholar 

  13. 13.

    Cotton, D. V. et al. Polarization due to rotational distortion in the bright star Regulus. Nat. Astron. 1, 690–696 (2017).

    ADS  Article  Google Scholar 

  14. 14.

    Eddington, A. S. The reflection effect in eclipsing variables. Mon. Not. R. Astron. Soc. 86, 320–327 (1926).

    ADS  Article  Google Scholar 

  15. 15.

    Russell, H. N. Idealized models and rectified light-curves for eclipsing variables. Astrophys. J. 108, 388–412 (1948).

    ADS  Article  Google Scholar 

  16. 16.

    Castelli, F. & Kurucz, R. New grids of ATLAS9 model atmospheres. Preprint at http://arXiv.org/abs/astroph/0405087 (2004).

  17. 17.

    Hubeny, I., Stefl, S. & Harmanec, P. How strong is the evidence of superionization and large mass outflows in B/Be stars? Bull. Astron. Inst. Czech. 36, 214–230 (1985).

    ADS  Google Scholar 

  18. 18.

    Spurr, R. J. D. VLIDORT: a linearized pseudo-spherical vector discrete ordinate radiative transfer code for forward model and retrieval studies in multilayer scattering media. J. Quant. Spectrosc. Radiat. Transf. 102, 316–342 (2006).

    ADS  Article  Google Scholar 

  19. 19.

    Wilson, R. E. & Devinney, E. J. Realization of accurate close-binary light curves: application to MR Cygni. Astrophys. J. 166, 605–619 (1971).

    ADS  Article  Google Scholar 

  20. 20.

    Wilson, R. E. Eccentric orbit generalization and simultaneous solution of binary star light and velocity curves. Astrophys. J. 234, 1054–1066 (1979).

    ADS  Article  Google Scholar 

  21. 21.

    Harrington, D. et al. Alpha Virginis: line-profile variations and orbital elements. Astron. Astrophys. J. 590, A54 (2016).

    Article  Google Scholar 

  22. 22.

    Tkachenko, A. et al. Stellar modelling of Spica, a high-mass spectroscopic binary with a β Cep variable primary component. Mon. Not. R. Astron. Soc. 458, 1964–1976 (2016).

    ADS  Article  Google Scholar 

  23. 23.

    Palate, M., Koenigsberger, G., Rauw, G., Harrington, D. & Moreno, E. Spectral modeling of the α Virginis (Spica) binary system. Astron. Astrophys. 556, A49 (2013).

    ADS  Article  Google Scholar 

  24. 24.

    Van Leeuwen, F. Validation of the new Hipparcos reduction. Astron. Astrophys. 474, 653–664 (2007).

    ADS  Article  Google Scholar 

  25. 25.

    Marshall, J. P. et al. Polarization measurements of hot dust stars and the local interstellar medium. Astrophys. J. 825, 124 (2016).

    ADS  Article  Google Scholar 

  26. 26.

    Cotton, D. V. et al. The intrinsic and interstellar broad-band linear polarization of nearby FGK dwarfs. Mon. Not. R. Astron. Soc. 467, 873–897 (2017).

    ADS  Google Scholar 

  27. 27.

    Bailey, J. The Dawes Review 3: the atmospheres of extrasolar planets and brown dwarfs. Publ. Astron. Soc. Aust. 31, e043 (2014).

    ADS  Article  Google Scholar 

  28. 28.

    Duchene, G. & Kraus, A. Stellar multiplicity. Ann. Rev. Astron. Astrophys. 51, 269–310 (2013).

    ADS  Article  Google Scholar 

  29. 29.

    Chapellier, E. et al. Has alpha Vir ever stopped pulsating? Astron. Astrophys. 143, 466–468 (1985).

    ADS  Google Scholar 

  30. 30.

    Balona, L. A. On the amplitude decrease of the beta Cephei stars Spica and 16 Lacertae. Mon. Not. R. Astron. Soc. 217, 17–21 (1985).

    ADS  Article  Google Scholar 

  31. 31.

    Odell, A. P. Possible polarization effects in the beta Cephei stars. Publ. Astron. Soc. Pacif. 91, 326–328 (1979).

    ADS  Article  Google Scholar 

  32. 32.

    Bailey, J. & Kedziora-Chudczer, L. Modelling the spectra of planets, brown dwarfs and stars using VSTAR. Mon. Not. R. Astron. Soc 419, 1913–1929 (2012).

    ADS  Article  Google Scholar 

  33. 33.

    Bailey, J., Kedziora-Chudczer, L. & Bott, K. Polarized radiative transfer in planetary atmospheres and the polarization of exoplanets. Mon. Not. R. Astron. Soc. 480, 1613–1625 (2018).

    ADS  Article  Google Scholar 

  34. 34.

    Espinosa Lara, F. & Rietord, M. Gravity darkening in binary stars. Astron. Astrophys. 547, A32 (2012).

    ADS  Article  Google Scholar 

Download references


The work was supported by the Australian Research Council through Discovery Project grants DP140100121 and DP160103231. We thank the staff at the AAT for their support of our HIPPI observing runs. We thank R. Spurr of RT Solutions for providing the VLIDORT software.

Author information




All authors participated in the observations of Spica and commented on the manuscript. D.V.C. developed the data reduction code, carried out the data reductions, ran the ATLAS9 models and investigated Spica’s interstellar polarization. J.B. developed the binary system and polarized radiative transfer codes, carried out the modelling, and prepared the manuscript.

Corresponding author

Correspondence to Jeremy Bailey.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Journal peer review information: Nature Astronomy thanks J. Patrick Harrington and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary references, Supplementary Figures 1–4, Supplementary Tables 1–4.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Bailey, J., Cotton, D.V., Kedziora-Chudczer, L. et al. Polarized reflected light from the Spica binary system. Nat Astron 3, 636–641 (2019). https://doi.org/10.1038/s41550-019-0738-7

Download citation

Further reading


Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing