Ellipsoidal primary of the RS CVn binary ζ Andromedae^*

Investigation using high-resolution spectroscopy and optical interferometry

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: [hkorhone;mwittkow]@eso.org
² Konkoly Observatory, 1525 Budapest, PO Box 67, Hungary e-mail: kovari@konkoly.hu
³ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Observatory, PO Box 14, 00014 University of Helsinki, Finland

Received: 24 November 2009
Accepted: 24 January 2010

Abstract

Aims. 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 ζ And. These observations are used to obtain fundamental stellar parameters and to study surface structures and their temporal evolution.

Methods. Temperature maps of the stellar surface were obtained from high-resolution spectra with Doppler imaging techniques. These spectra were also used to investigate the chromospheric activity using the Hα  line and to correlate it with the photospheric activity. The possible cyclicity in the spot activity was investigated from the long-term broad  band photometry. Optical interferometry was obtained during the same time period as the high-resolution spectra. These observations were used to derive the size and fundamental parameters of ζ And.

Results. Based on the optical interferometry the apparent limb darkened diameter of ζ And is 2.55 ± 0.09 mas using a uniform disk fit. The expected ~4% maximum difference between the long and short axes of the ellipsoidal stellar surface cannot be confirmed from the current data which have 4% errors. The Hipparcos distance and the limb-darkened diameter obtained with a uniform disk fit give stellar radius of 15.9 ± 0.8 , and combined with bolometric luminosity, it implies an effective temperature of 4665 ± 140 K. The temperature maps obtained from 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, i.e., 0.25 in phase from the secondary, and another concentration spans the phases 0.0–0.4. This spot configuration is reminiscent of the one seen in the earlier published temperature maps of ζ And. Investigation of the Hα line reveals both prominences and cool clouds in the chromosphere. These features do not seem to have a clearly preferred location in the binary reference frame, nor are they strongly associated with the cool photospheric spots. The investigation of the 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.