Doppler imaging of stellar surface structure XXIII. the ellipsoidal K giant binary ζ Andromedae

Zs Kovári, J. Bartus, K. G. Strassmeier, K. Oláh, M. Weber, J. B. Rice, A. Washuettl

Research output: Contribution to journalArticle

18 Citations (Scopus)

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 log n - 1.2 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 1000 ± 200 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 Rpole/Rpoint = 0.96 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 ΔΩ ≈ 0.95°/day (α = ΔΩ/Ωeq = +0.049 ± 0.003), i.e. roughly a factor of four weaker at a rotation rate roughly 1.5 times faster than the Sun's.

Original languageEnglish
Pages (from-to)1071-1080
Number of pages10
JournalAstronomy and Astrophysics
Volume463
Issue number3
DOIs
Publication statusPublished - Mar 2007

Keywords

  • Stars: activity
  • Stars: imaging
  • Stars: individual: ζ Andromedae
  • Stars: late-type
  • Stars: starspots

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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