General Model for Light curves of Chromospherically Active Binary Stars

TL;DR

This paper introduces a general model explaining light curve variations in chromospherically active binary stars, linking orbital motion, starspot distribution, ellipticity, and flip-flop events, with implications for understanding stellar magnetic fields.

Contribution

The paper presents a unified model for light curves of active binaries that accounts for orbital motion, starspot behavior, and ellipticity, extending applicability to single stars and proposing a magnetic field origin hypothesis.

Findings

Long-term light curves follow a model combining orbital and active longitude periods.

Interference of two constant period waves explains light curve changes.

Model applies to both binary and single stars with known active longitude periods.

Abstract

The starspots on the surface of many chromospherically active binary stars concentrate on long--lived active longitudes separated by 180 degrees. The activity shifts between these two longitudes, the "flip-flop" events, have been observed in single stars like FK Comae and binary stars like $\sigma$ Geminorum. Recently, interferometry has revealed that ellipticity may at least partly explain the "flip-flop" events in $\sigma$ Geminorum. This idea was supported by the double peaked shape of the long--term mean light curve of this star. Here, we show that the long--term mean light curves of fourteen chromospherically active binaries follow a general model which explains the connection betweenm orbital motion, starspot distribution changes, ellipticity and \flip ~events. Surface differential rotation is probably weak in these stars, because the interference of two constant period waves may explain the observed light curve changes. These two constant periods are the active longitude period $(P_{\mathrm{act}})$ and the orbital period $(P_{\mathrm{orb}})$. We also show how to apply the same model to single stars, where only the value of $P_{\mathrm{act}}$ is known. Finally, we present a tentative interference hypothesis about the origin of magnetic fields in all spectral types of stars.