Ellipsoidal primary of the RS CVn binary zeta And: Investigation using high-resolution spectroscopy and optical interferometry

TL;DR

This study combines high-resolution spectroscopy, optical interferometry, and long-term photometry to analyze the physical characteristics, surface activity, and chromospheric features of the ellipsoidal primary star in the RS CVn binary zeta And.

Contribution

It provides new measurements of zeta And's stellar radius, temperature, and surface spot distribution using combined observational techniques, enhancing understanding of RS CVn star activity.

Findings

Stellar radius of 15.9 +/- 0.8 R_sun derived from interferometry and Hipparcos data.

Detection of a strong equatorial spot belt and possible polar cap from Doppler imaging.

Hints of a 12-year spot activity cycle observed in long-term photometry.

Abstract

We have obtained high-resolution spectroscopy, optical interferometry, and long-term broad band photometry of the ellipsoidal primary of the RS CVn-type binary system zeta And. Based on the optical interferometry the apparent limb darkened diameter of zeta And is 2.55 +/- 0.09 mas using a uniform disk fit. The Hipparcos distance and the limb-darkened diameter obtained with a uniform disk fit give stellar radius of 15.9 +/- 0.8 Rsolar, and combined with bolometric luminosity, it implies an effective temperature of 4665 +/- 140 K. The temperature maps obtained from high resolution spectra using Doppler imaging show a strong belt of equatorial spots and hints of a cool polar cap. The equatorial spots show a concentration around the phase 0.75. This spot configuration is reminiscent of the one seen in the earlier published temperature maps of zeta And. Investigation of the Halpha line reveals both prominences and cool clouds in the chromosphere. Long-term photometry spanning 12 years shows hints of a spot activity cycle, which is also implied by the Doppler images, but the cycle length cannot be reliably determined from the current data.