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 clouds1–3. 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 variability4–6. 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. High-precision polarization observations of the binary star system Spica reveal that the amount of light from the primary component that is reflected off the secondary component (and vice versa) is a few per cent of the incident light. Such observations will be useful in identifying close binary systems.