Interferometric Images of the Starspot Evolution of $\zeta$ Andromedae

TL;DR

This study used interferometric imaging to directly observe the evolving starspots on $$ Andromedae, revealing complex activity patterns and polar spot growth over multiple rotations, providing new insights into stellar surface dynamics.

Contribution

First direct interferometric images of $$ Andromedae's starspots over several epochs, showing complex evolution and polar spot growth, advancing understanding of stellar surface activity.

Findings

Starspots are more complex than previously thought.

Polar spots grow over time.

No secondary star detected, possibly a white dwarf.

Abstract

The evolution of starspots of the giant primaries of RS CVn systems is typically detected indirectly with photometric and spectroscopic monitoring. These observations suggest slowly-evolving stellar surfaces and can constrain differential rotation as starspots move with respect to one another. However, starspot latitudes are difficult to constrain without resolved images of the stellar surfaces from which the unambiguous locations of starspots are determined. We imaged the active RS CVn primary $\zeta$ And with the 330-m-baseline Center for High Angular Resolution Astronomy Array for three epochs over approximately six rotations of the star. The resultant images show a more complicated picture of stellar activity than expected from the contemporaneous photometry and earlier Doppler images. The spot structures change on the timescale of rotation, making differential rotation difficult to study. Our observations show changes in the polar spot, growing over time. We do not detect the secondary star in the interferometric data, though the observations are sensitive to the predicted 0.75 $M_\odot$ main-sequence star, and we suggest the companion may be a white dwarf.