Long-term magnetic activity in close binary systems. I. Patterns of color variations
Aims.This is the first of a series of papers in which we present theresults of a long-term photometric monitoring project carried out atCatania Astrophysical Observatory aimed at studying magnetic activity inlate-type components of close binary systems, its dependence on globalstellar parameters, and its evolution on different time scales from daysto years. In this first paper, we present the complete observationdataset and new results of an investigation into the origin ofbrightness and color variations observed in the well-known magneticallyactive close binary stars: AR Psc, VYAri, UX Ari, V711 Tau,EI Eri, V1149 Ori, DHLeo, HU Vir, RS CVn,V775 Her, AR Lac, SZPsc, II Peg and BY Dra Methods: About 38 000 high-precision photoelectric nightly observationsin the U, B and V filters are analysed. Correlation and regressionanalyses of the V magnitude vs. U-B and B-V color variations are carriedout and a comparison with model variations for a grid of active regiontemperature and filling factor values is also performed. Results: Wefind the existence of two different patterns of color variation. Eightstars in our sample: BY Dra, VYAri, V775 Her, II Peg,V1149 Ori, HU Vir, EIEri and DH Leo become redder when theybecome fainter, as is expected from the presence of active regionsconsisting of cool spots. The other six stars show the oppositebehaviour, i.e. they become bluer when they become fainter. ForV711 Tau this behaviour could be explained by theincreased relative U- and B-flux contribution by the earlier-typecomponent of the binary system when the cooler component becomesfainter. On the other hand, for AR Psc, UXAri, RS CVn, SZ Psc andAR Lac the existence of hot photospheric faculae mustbe invoked. We also found that in single-lined and double-lined binarystars in which the fainter component is inactive or much less active theV magnitude is correlated to B-V and U-B color variations in more than60% of observation seasons. The correlation is found in less than 40% ofobservation seasons when the fainter component has a non-negligiblelevel of activity and/or hot faculae are present but they are eitherspatially or temporally uncorrelated to spots.I dedicate this paper to the memory of the P.I. of this project, Prof.Marcello Rodonò, who suddenly passed away on October 23, 2005. Tohim my sincere estimation and deepest gratitude.Based onobservations collected at INAF-Catania Astrophysical Observatory, Italy.
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