Asteroseismology of Close Binary Stars: Tides and Mass Transfer

Abstract

The study of stellar oscillations allows us to infer the properties of stellar interiors. Meanwhile, fundamental parameters such as mass and radius can be obtained by studying stars in binary systems. The synergy between binarity and asteroseismology can constrain the parameter space of stellar properties and facilitate the asteroseismic inference. On the other hand, binarity also introduces additional complexities such tides and mass transfer. From an observational perspective, we briefly review the recent advances in the study of tidal effects on stellar oscillations, focusing on upper main sequence stars (F-, A-, or OB- type). The effect can be roughly divided into two categories. The first one concerns the tidally excited oscillations (TEOs) in eccentric binaries where TEOs are mostly due to resonances between dynamical tides and gravity modes of the star. TEOs appear as orbital-harmonic oscillations on top of the eccentric ellipsoidal light curve variations (the “heartbeat” feature). The second category is regarding the self-excited oscillations perturbed by static tides in circularized and synchronized close binaries. It includes the tidal deformation of the propagation cavity and its effect on eigenfrequencies, eigenfunctions, and the pulsation alignment. We list binary systems that show these two types of tidal effect and summarize the orbital and pulsation observables. We also discuss the theoretical approaches used to model these tidal oscillations and relevant complications such as non-linear mode coupling and resonance locking. Further information can be extracted from the observations of these oscillations which will improve our understanding of tides. We also discuss the effect of mass transfer, the extreme result of tides, on stellar oscillations. We bring to the readers' attention: (1) oscillating stars undergoing mass accretion (A-, F-, and OB type pulsators and white dwarfs), for which the pulsation properties may be changed significantly by accretion; (2) post-mass transfer pulsators, which have undergone a stable or unstable Roche-Lobe overflow. These pulsators have great potential in probing detailed physical processes in stellar interiors and mass transfer, as well as in studying the binary star populations.

Asteroseismology across the HR diagram

“…See Section D in the appendix for a discussion of these techniques and their impact. Guo (2021) has given an extensive review of asteroseismology in close binary stars with particular emphasis on tides and the effects of mass transfer on pulsations and stellar structure. He examined both tidally excited and tidally perturbed, self-excited oscillations.…”

“…In the highest eccentricity systems the periastron passage is within a few stellar radii, with stunning tidal distortions that then relax as the stars separate in the outer parts of the orbit. See Guo (2021) and Cheng et al ( 2020) for example light curves, FTs, and detailed discussion for some of these stars. Two results of the discovery of the heartbeat stars are that the previously untested theory of Kumar et al (1995) provides excellent fits to the wide variety of the orbital light variations that result primarily from different viewing angles and eccentricity, and that strong astrophysical inference about tidally excited oscillations can be made in the more than 20% of these systems that show pulsations.…”

A New Window to Tidal Asteroseismology: Non-linearly Excited Stellar Eigenmodes and the Period Spacing Pattern in KOI-54

“…Significant advances have been made in tidal seismology after the discovery of the prototype heartbeat binary KOI-54 (HD 187091, KIC 8112039) by (Welsh et al 2011)(hereafter W11). Tidally Excited Oscillations (TEOs) have been observed in tens of heartbeat binary systems by the Kepler telescope (Thompson et al 2012;Kirk et al 2016;Cheng et al 2020;Guo 2021), and the TESS mission is unfolding more massive heartbeat binaries (Jayasinghe et al 2019;Kołaczek-Szymański et al 2021). Recently, nearly one thousand heartbeat stars in the OGLE survey have been reported by Wrona et al (2021).…”

Tidally excited oscillations in MACHO 80.7443.1718: Changing amplitudes and frequencies, high-frequency tidally excited mode, and a decrease in the orbital period

“…A possibility of transferring energy from pulsations to orbit which may increase non-synchronicity of components or eccentricity of the system, so called inverse tides, was also found (Li et al 2020;Fuller 2021). A review of both the theory and observations of TEOs in massive binaries has been recently presented by Guo (2021).…”