Ellipsoidal primary of the RS CVn binaryζ Andromedae

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 R , 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.

V405 Andromeda Revisited

“…These features are common in Doppler imaging of RS CVn binaries, as can be seen in recent reconstructions of ζ And (Korhonen et al 2010).…”

Spotted Surface Structure of the Active Giant PZ Mon

“…Strassmeier 2000;Weber & Strassmeier 1998;Korhonen et al 2010). A constant cool polar region with a tem-perature of ∼3500 K is present on the maps of many such stars (Oláh et al 2002;García-Alvarez et al 2003;Strassmeier 2000;Weber & Strassmeier 1998;Korhonen et al 2010). In these papers, however, it is noted that the polar region can be an artefact due to the limitations of the applied technique, because in the spectral line profile at zero relative radial velocity the line is identified with the core, which, first, may be described poorly by the model and, second, has a signal-to-noise ratio smaller than that averaged over the spectrum.…”

“…However, its radius is much smaller than that in our model and does not exceed 20 • . Comparison of the surface maps for such stars shows that cool spots are present not only in the polar regions but also on the remaining stellar surface, occupying a significant fraction of the visible hemisphere (Korhonen et al 2010). However, the distribution of these spots is more or less uniform, so that their relative area on the visible hemisphere remains almost constant; as a consequence, there no modulation with the stellar rotation.…”

Differential Rotation in Two RS CVn Systems: σ Gem and ζ And

“…Observations showed the existence of spots concentrating at quadrature positions in ζ And (e.g. Kővári et al 2007a, Korhonen et al 2010. Likewise, the deformation could also account for the disparate DR laws obtained for ζ And and for σ Gem.…”