Tidal perturbations and eclipse mapping in the pulsations in the hierarchical triple system U~Gru

TL;DR

This study investigates the pulsations of the hierarchical triple system U Gru, demonstrating tidal influence on at least one mode and proposing eclipse mapping as a mechanism for observed frequency patterns.

Contribution

The paper provides the first detailed analysis of tidal effects on pulsations in U Gru, identifying a tidally perturbed mode and introducing eclipse mapping as an explanation for frequency spacings.

Findings

U Gru is likely a hierarchical triple system.

A single pulsation mode shows amplitude and phase variations due to tides.

Eclipse mapping can reproduce frequency series separated by the orbital frequency.

Abstract

Context. Unambiguous examples of the influence of tides on self-excited, free stellar pulsations have recently been observationally detected in space-based photometric data. Aims. We aim to investigate U Gru and contextualise it within the growing class of tidally influenced pulsators. Initial analysis of U Gru revealed frequencies spaced by the orbital frequency that are difficult to explain by currently proposed tidal mechanisms. Methods. We re-investigate the TESS photometry of U Gru alongside new uves spectroscopy. We analyse the uves spectroscopy with least-squares deconvolution and spectral disentangling techniques, and perform an atmospheric analysis. We remove the binary signature from the light curve using an effective model in order to investigate the pulsation signal in the residuals. We track the amplitudes and phases of the residual pulsations as a function of the orbital period to reveal their tidal influence. Results. We establish that U Gru is likely a hierarchical triple system. We identify a single p mode oscillation to exhibit amplitude and phase variation over the binary orbit. We propose a toy model to demonstrate that the series of frequencies separated by the orbital frequency can be reproduced by eclipse mapping. We find no evidence of modulation to the other independent oscillation modes. Conclusions. We demonstrate that U Gru hosts at least one tidally perturbed pulsation. Additionally we argue that eclipse mapping of the dominant, tidally perturbed mode can produce the series of frequencies separated by the observed orbital frequency. Our simulations show that the effects of eclipse mapping are mode dependent, and are not expected to produce an observable signature for all pulsation modes in an eclipse binary.