Orbital Period Changes for the Weak-Contact Binary TY Bootis

Abstract

We present a detailed period analysis of TY Bootis, based on all available light minimum times covering about 80 years. We discovered that there exists a cyclic variation overlaying a secular period decrease from the distorted (O - C) diagram. A long-term decrease with a sinusoidal oscillation (i.e., eq. [2]) or with a light-time effect (i.e., eq. [3]) may be reasonable due to the smaller difference between their sum of residuals. The sinusoidal change with a period of 59.7 yr for equation (2) can be attributed to either a light-time effect or cyclic magnetic activity of both components. For the assumed third body, the period and eccentricity of the light-time orbit are P3 = 58.6 yr and e' = 0.1744, respectively. If the existence of an additional unseen body is true, this binary may be a tertiary system. The long-term orbital period decrease suggests that TY Boo is undergoing a mass transfer from the primary component to the secondary component at a rate of dm/dt = -3.15 ? 10-8 M? yr-1 for equation (2) or dm/dt = -2.90 ? 10-8 M? yr-1 for equation (3). As its mass transfers, accompanied by angular momentum loss due to mass outflow L2, this binary will evolve into a deep contact configuration.