On The Orbital Period Changes Of RS CVn-type Binaries AR Lac And RT Lac

The RS CVn-type groups are the binary systems showing strong magnetic activity.These eclipsing binaries systems send strong Hα, CaII, H and K emission lines. RSCVn-type binaries show all features of solar activities: spots, flare, mass loss due tostar wind, X-ray emission. The relation between the orbital period changes andmagnetic activities of RS CVn-type binaries interests many astronomic researchers.In the present thesis, firstly, we have introduced the basic methods of analyzingthe mechanisms of the orbital period variation of close binaries. Then, we havestudied the orbital period changes of two RS CVn-type binaries of AR Lac and RTLac, and obtained the following results:1. Firstly, by collecting the light minima of all times from the literature and thedatabase, we reach the O Cvalues by using the new ephemeris and by calculatingthe theoretical value of the light minimum of AR Lac. Secondly, we use the leastsquares to fit O Ccurve. Finally, we investigate the mechanisms that may causechanges in the period of AR Lac. Our results suggest that the orbital period of AR Lacshows a cyclic oscillation with a period of50.93yr and amplitude of0.0362d, whilesuperimposed on a long-term secular period decrease at a rate ofdP/d t=-(2.128±0.060)×10-9d/d. The orbital period cyclic variation can beexplained by the magnetic activity, and that the long-term secular period decreasemay be caused by enhanced magnetic wind braking in the cooler component.2. The orbital period of RS CVn type binaries RT Lac was analyzed, and wefound a period of278.04yr, which is superimposed on a long-term period decrease ata rate ofdP/dt=(4.81±7.33)×109days/day. It is found that the period change ofthe binary systems may be caused by the magnetic activity instead of the light timeeffect due to the third body. Since RT Lac is a typical detached binary, the long-termsecular period decrease may not be caused by the transfer of materials between thetwo components. The computed value we get is more than two orders of magnitudeabove the observed value, therefore the long-term secular period decrease may not becaused by magnetic braking of the completely convective component.