Cyclical Changes in the Extraeclipse Brightness and the Period of W UMa-TYPE Close Binary System AM Leo
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ical Changes in the Extraeclipse Brightness and the Period of W UMa-TYPE Close Binary System AM Leo S. Yu. Gordaa, * aKourovka
Astronomical Observatory, Ural Federal University, Yekaterinburg, 620000 Russia *e-mail: [email protected] Received May 12, 2020; revised June 30, 2020; accepted June 30, 2020
Abstract—The results of 14-year photometric monitoring of the contact close binary system (CBS) W UMatype AM Leo are presented. Low-amplitude cyclic variations in brightness with a period of 7.6 ± 0.3 years, not associated with the phenomena of eclipses and tidal deformations of the components, as well as lowamplitude cyclic variations of the CBS period with practically the same value of the oscillation period, were found. It is concluded that the mechanism generating such oscillations may be the process of changing the magnetic field of the convective shells of the components. Based on the detected decrease in the depths of both minima of the light curves and the change in their relative values, it was concluded that the system is currently transiting from the W-subtype class to the A-subtype class. The values of seven new values of the moments of minima are given. DOI: 10.1134/S1063772920110037
1. INTRODUCTION The eclipsing variable star AM Leo is a low-mass contact close binary system (CBS) of the W UMa type. For more than forty years, starting with the pioneering work of Lewcy, Björn, Thomas, Shu et al. [1– 8], it is assumed that the components of a CBS of this type, having different masses, are surrounded by a common optically thick convective gas shell heated to a temperature close to the temperature of the more massive and hot component. This fact is indicated by the almost identical depths of both minima on the light curves of such stars. The gas envelope surrounding both components is sometimes interpreted as a common photosphere of the components. Observations show that the light curves of W UMa-type CBS vary to a certain extent from cycle to cycle. The changes are mainly associated with an increase or decrease by an order of a few hundredths of the magnitude of the brightness at the maxima or minima of the light curves [9]. Sometimes when a decrease or increase in brightness at the maxima and minima occurs (synchronously) simultaneously, the effect of a slight increase or decrease in the total brightness of the system is created. These effects are currently attributed to several reasons. The main and the first of those proposed back in 1975 by Mullan [10] is the presence of cold or hot spots on the surface of the primary or secondary components (see, for example, [9, 11, 12]). Moreover, chromospheric activity has been detected for some W UMa-type stars [13–15]. At the same time, it is
assumed that similar effects on the light curves can be produced by the process of nonuniform heating of local regions of the common convective shell in the region of the point L1 so-called “necks” during the matter flowing from one component on the other, occurring under a common shell [16] or its heating and expansio
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