On using the beaming effect to measure spin-orbit alignment in stellar binaries with Sun-like components

TL;DR

This paper proposes a novel photometric method to measure the spin-orbit alignment in binary star systems using the beaming effect during eclipses, which can be detected with precise space-based photometry like Kepler.

Contribution

It introduces a new technique to determine the spin-orbit angle in stellar binaries by analyzing the beaming effect during eclipses, including analytic expressions and simulations.

Findings

The beaming effect during eclipse encodes spin-orbit information.

The combined effects are detectable with Kepler data for bright systems.

The method provides a new way to study stellar binary dynamics.

Abstract

The beaming effect (aka Doppler boosting) induces a variation in the observed flux of a luminous object, following its observed radial velocity variation. We describe a photometric signal induced by the beaming effect during eclipse of binary systems, where the stellar components are late type Sun-like stars. The shape of this signal is sensitive to the angle between the eclipsed star's spin axis and the orbital angular momentum axis, thereby allowing its measurement. We show that during eclipse there are in fact two effects, superimposed on the known eclipse light curve. One effect is produced by the rotation of the eclipsed star, and is the photometric analog of the spectroscopic Rossiter-McLaughlin effect, thereby it contains information about the sky-projected spin-orbit angle. The other effect is produced by the varying weighted difference, during eclipse, between the beaming signals of the two stars. We give approximated analytic expressions for the amplitudes of the two effects, and present a numerical simulation where we show the light curves for the two effects for various orbital orientations, for a low mass ratio stellar eclipsing binary system. We show that although the overall signal is small, it can be detected in the primary eclipse when using Kepler Long Cadence data of bright systems accumulated over the mission lifetime.