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