CZ Aqr: an oscillating eclipsing Algol-type system composed of a $\delta$ Sct primary star and a subgiant star in a quadruple system

TL;DR

This study characterizes CZ Aqr, an eclipsing Algol-type system with a δ Sct primary, revealing its orbital dynamics, potential third bodies, and pulsation frequencies using TESS data, contributing to understanding stellar evolution in complex systems.

Contribution

First detailed analysis of CZ Aqr combining TESS photometry and O-C analysis, identifying its quadruple system nature and pulsation modes.

Findings

Orbital period decreasing at a rate of -3.09×10^{-8} days per year.

Presence of a third body with minimum mass 0.312 solar masses affecting timing variations.

Detection of 26 pulsation frequencies, including two radial modes.

Abstract

Eclipsing Algol-type systems containing a $\delta$ Scuti (hereafter $\delta$ Sct) star enable precise determination of physical parameters and the investigation of stellar internal structure and evolution. We present the absolute parameters of CZ Aquarius (hereafter CZ Aqr) based on TESS data. CZ Aqr has an orbital period of 0.86275209 d, a mass ratio of 0.489 (6), and the secondary component nearly fills its Roche lobe. $O-C$ analysis reveals a downward parabolic trend and a cyclical variation with a period of 88.2 yr. The downward parabola suggests a long-term decrease in the orbital period with $\dot{P}$ = -3.09$\times$$10^{-8}$ d $\textrm{yr}^{-1}$. The mass loss rate is estimated to be 4.54$\times$$10^{-9}$ M$_{\odot}$ $\textrm{yr}^{-1}$, which possibly due to magnetic stellar wind or hot spot. The cyclical variation might be caused by the light travel time effect via the presence of a third body with a minimum mass of $M_{3min}$ = 0.312 (21) M$_{\odot}$. Additionally, there are two possible celestial bodies in a 2:7 resonance orbit around CZ Aqr. The asymmetric light curve is explained by adding a hot spot on the surface of the primary star. After removing the binary model, 26 frequencies were extracted from TESS data. Two radial modes were newly identified among three possible independent frequencies. Our results show that the eclipsing Algol-type system is composed of a $\delta$ Sct primary star and a subgiant star in a quadruple system.