Photometric analysis on RZ Horologii: An evolved and active Algol with a δ Scuti component

Abstract

RZ Hor (TIC 31653503) is a long-period (P = 6.68 d) EA-type eclipsing binary that exhibits both chromospheric activity and oscillation. The physical properties of RZ Hor are determined with binary modelling for the first time. It is found that it has a semi-detached configuration with a low mass ratio of q = 0.1606(13), where the secondary component is filled with its Roche lobe. By analyzing all available eclipse times, it is shown that the orbital period of RZ Hor is decreasing at a rate of dP/dt = −5.48 × 10−6 d yr−1. In a typical Algol, a rapid mass transfer from the secondary to the primary should cause the orbital period to be increasing and then the secondary will be temporarily detached from the critical Roche lobe. The present configuration reveals that RZ Hor has undergone a rapid mass transfer and the period decrease can be explained by magnetic stellar wind. We estimate the rate of mass loss initially to be $10^{-8}\, M_{\odot }\, \mathrm{yr}^{-1} < \vert \dot{M}\vert < 10^{-6}\, M_{\odot }$ yr−1. The variations and the asymmetries of the light curve are interpreted by adding a migrating cool spot on the surface of the late-type secondary. After subtracting the eclipsing changes from the light curve data, we analyzed the pulsation in the light residuals. Twenty-five frequencies of signal-to-noise amplitude ratio (S/N) larger than 5.2, including four multiples of tidally split frequencies, were detected. A total of 17 independent frequencies containing 2 radial modes, and 10 non-radial p modes were identified. All the investigations suggest that RZ Hor is an evolved Algol-type binary with a δ Sct-type primary and an active secondary.