Phase-locked polarization by photospheric reflection in the semidetached eclipsing binary $\mu^1$ Sco

TL;DR

This study detects phase-locked polarization in the semidetached eclipsing binary $$ Sco, attributing it to photospheric reflection rather than scattering from gas, enabling mass determination in non-eclipsing systems.

Contribution

It introduces a new interpretation of polarization in semidetached binaries as due to photospheric reflection, supported by detailed modeling.

Findings

Polarization amplitude is wavelength-dependent and large (~700 ppm).

Polarization can be explained by reflected light from each star's photosphere.

This method allows mass measurements in non-eclipsing binaries.

Abstract

We report the detection of phase-locked polarization in the bright ($m_V$=2.98-3.24) semidetached eclipsing binary $\mu^1$ Sco (HD 151890). The phenomenon was observed in multiple photometric bands using two different HIPPI-class (HIgh Precision Polarimetric Instrument)polarimeters with telescopes ranging in size from 35-cm to 3.9-m. The peak-to-trough amplitude of the polarization is wavelength dependent and large, $\sim$700 parts-per-million in green light, and is easily seen with even the smallest telescope. We fit the polarization phase curve with a SYNSPEC/VLIDORT polarized radiative transfer model and a Wilson-Devinney geometric formalism, which we describe in detail. Light from each star reflected by the photosphere of the other, together with a much smaller contribution from tidal distortion and eclipse effects, wholly accounts for the polarization amplitude. In the past polarization in semidetached binaries has been attributed mostly to scattering from extra-stellar gas. Our new interpretation facilitates determining masses of such stars in non-eclipsing systems.