Doppler imaging of stellar surface structure^*

XXIII. The ellipsoidal K giant binary ζ Andromedae

¹ Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, Hungary e-mail: [kovari;olah]@konkoly.hu
² Astrophysical Institute Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [jbartus;kstrassmeier;mweber;wasi]@aip.de
³ Department of Physics, Brandon University, Brandon, Manitoba R7A 6A9, Canada e-mail: rice@BrandonU.ca

Received: 5 July 2006
Accepted: 8 November 2006

Abstract

Aims.We present the first Doppler images of the bright RS CVn-type binary ζ And. The star is a magnetically active K1 giant with its rotation synchronized to the 17.8-day orbital period. Our revised lithium abundance of places ζ And in the vicinity of Li-rich RGB stars but it is nevertheless a Li-normal chromospherically active binary star. The star seems to undergo its first standard dredge-up dilution.

Methods.Four consecutive Doppler images were obtained from a continuous 67-night observing run at NSO-McMath in 1996/97. An additional single image was obtained from a continuous 19-night run at KPNO in 1997/98. These unique data allow to compute a small time series of the evolution of the star's surface structure. All line-profile inversions are done with a modified TempMap version that takes into account the non-spherical shape of the star. Representative test reconstructions are performed and demonstrate the code's reliability and robustness.

Results.High and low-latitude spot activity was recovered together with an asymmetric polar cap-like feature. The latter dominated the first half of the two-month time series in 1996/97. The second half showed mostly medium-to-high latitude activity and only a fainter polar spot. The coolest areas were restored with a temperature contrast of about  K. Some weaker features at equatorial latitudes were also recovered but these could be partially spurious and appear blurred due to imperfect phase coverage. We use our line profiles to reconstruct an average non-sphericity of which would, if not taken into account, mimic a temperature difference pole-to-equator of ≈220 K, especially at the phases of quadrature. Finally, we apply two different methods for restoring surface differential rotation and found a weak solar-type rotation law with a shear °/day (), i.e. roughly a factor of four weaker at a rotation rate roughly 1.5 times faster than the Sun's.