Interferometric Images of the Starspot Evolution of $ζ$ Andromedae

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 $ζ$ 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.

Figures (15)

• Figure 1: Visibility curve of UT 2019 July 17 $\zeta$ And CHARA/MIRC-X observations with $1\sigma$ error bars. Inset is the $uv$ coverage for the night of observation. The ellipsoidal shape of the star (data shown are at phase 0.98), limb darkening, and starspots contribute to the spread in visibility at a particular spatial frequency.
• Figure 2: Squared visibilities of UT 2019 July 17 $\zeta$ And CHARA/MIRC-X observations with $1\sigma$ error bars. Each block represents a pair of telescopes in a temporal block of observations.
• Figure 3: Closure phases of UT 2019 July 17 $\zeta$ And CHARA/MIRC-X observations with $1\sigma$ error bars. Each block represents a set of three telescopes in a temporal block of observations.
• Figure 4: Triple amplitudes of UT 2019 July 17 $\zeta$ And CHARA/MIRC-X observations with $1\sigma$ error bars. Each block represents a set of three telescopes in a temporal block of observations.
• Figure 5: Timeline representation of the interferometric and photometric observations. The CHARA Array observations are described in Section \ref{['section:mircx']}. The APT photometry are described in Section \ref{['section:apt']}, the AAVSO photometry in Section \ref{['section:aavso']}, and the Konkoly Observatory photometry in Section \ref{['section:konkoly']}.