* Astronomy

Blobrana · L

RE: Zeta Andromedae

Blobrana · L

Title: Measuring differential rotation of the K-giant zeta And
Authors: Zs. K\Hovári, H. Korhonen, L. Kriskovics, K. Vida, J.-F. Donati, H. Le Coroller, J. D. Monnier E. Pedretti, P. Petit

We investigate the temporal spot evolution of the K-giant component in the RS CVn-type binary system zeta Andromedae to establish its surface differential rotation. Doppler imaging is used to study three slightly overlapping spectroscopic datasets, obtained independently at three different observing sites. Each dataset covers one full stellar rotation with good phase coverage, and in total, results in a continuous coverage of almost three stellar rotations (P_rot=17.8\,d). Therefore, these data are well suited for reconstructing surface temperature maps and studying temporal evolution in spot configurations. Surface differential rotation is measured by the means of cross-correlation of all the possible image pairs. The individual Doppler reconstructions well agree in the revealed spot pattern, recovering numerous low latitude spots with temperature contrasts of up to ~1000\,K with respect to the unspotted photosphere, and also an asymmetric polar cap which is diminishing with time. Our detailed cross-correlation study consistently indicate solar-type differential rotation with an average surface shear \alpha ~0.055, in agreement with former results.

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Blobrana · L

Title: Ellipsoidal primary of the RS CVn binary zeta And: Investigation using high-resolution spectroscopy and optical interferometry
Authors: H. Korhonen (ESO), M. Wittkowski (ESO), Zs. Kovari (Konkoly), Th. Granzer (AIP), T. Hackman (Helsinki), K.G. Strassmeier (AIP)

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.

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