Evidence for a close-in tertiary orbiting around the Algol-type system HZ Dra with tidal splitting and spots activities

TL;DR

This study presents evidence for a close-in third body orbiting the eclipsing binary HZ Dra, analyzes its pulsation modes, and investigates spot activities and tidal effects influencing the system's behavior.

Contribution

It provides the first detection of a tertiary companion in HZ Dra and explores the star's pulsation modes and spot activities in detail.

Findings

Detection of a third body with a 1.05-year orbit affecting the system's O-C diagram.

Identification of multiple pulsation modes, including radial and non-radial p- and g-modes.

Observation of tidally split pulsation modes caused by the close binary's equilibrium tides.

Abstract

We reported a cyclic variation of $O-C$ diagram with a semi-amplitude of 0.0033 days and a period of 1.05 years for the pulsating eclipsing binary HZ Dra. The cyclic variation can be explained by the light travel-time effect via the presence of a close-in third body orbiting around HZ Dra in an elliptical orbit with a maximum semi-major axis of 0.92 au. Based on the W-D code, the contribution of the third light to the total system is determined to be 29 $\%$, which is in agreement with the estimated value. Our light curve modelling indicates an evolving hot and cool spot on the surface of the primary and secondary components, respectively. Their positions are roughly symmetrical to the inner Lagrangian point L1, which could be used to explain the variation in the O$^{'}$Connell effect. Our frequency analysis detects 1 radial p-mode, 7 non-radial p-modes and 1 non-radial g-mode. In addition, a total of 6 multiplets are identified, spaced by the orbital frequency, which can be explained as a tidally split mode caused by the equilibrium tides of the close binary system with a circular orbit. These pulsating features suggest that the primary of HZ Dra is a $\delta$ Scuti star, pulsating in both p- and g-mode and influenced by tidal forces.