Modeling the Variable Polarization of epsilon Aurigae In and Out of Eclipse

TL;DR

This paper models the variable polarization of epsilon Aurigae during and outside eclipse using optical polarimetry and color variations, revealing surface temperature and polarization behavior despite complex data.

Contribution

It introduces a surface temperature variation model with spherical harmonics to interpret polarization and color changes in epsilon Aurigae.

Findings

Captured the overall polarization behavior during eclipse and post-eclipse.

Revealed surface temperature variations influence polarization signals.

Provided a framework for interpreting polarization in similar binary systems.

Abstract

The eclipsing binary epsilon Aur is unique in being a very long-period binary involving an evolved, variable F star and a suspected B main-sequence star enshrouded in an opaque circumstellar disk. The geometrical arrangement is that the disk is viewed almost perfectly edge-on, with alignment leading to a partial eclipse of the F star. Despite a global observing campaign for the 2009-11 eclipse, there remain outstanding questions about the nature of the binary, its components, the disk, and the evolutionary state of the system. We analyze optical-band polarimetry in conjunction with broad-band color variations to interpret brightness variations across the surface of the F star. We model this both during and after the 1982-84 eclipse for which an extensive and dense data set exists. We develop a model in terms of surface temperature variations characterized by a small global variation overlaid with a temperature variation described with low-order spherical harmonics. While not providing a detailed fit to the dataset, our modeling captures the overall characterization of the color and polarimetric variability. In particular, we are able to recover the gross behavior of the polarimetric excursion in the Q-U plane as observed during eclipse of the F star when compared to post-eclipse behavior.