Tidally Trapped Pulsations in a close binary star system discovered by TESS

Abstract: It has long been suspected that tidal forces in close binary stars could modify the orientation of the pulsation axis of the constituent stars. Such stars have been searched for, but until now never detected. Here we report the discovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 in TESS space photometry data. The system contains a Delta Scuti pulsator in a 1.6-d orbit, whose pulsation mode amplitude is strongly modulated at the orbital frequency, which can be explained if the pulsations have a much larger amplitude in one hemisphere of the star. We interpret this as an obliquely pulsating distorted dipole oscillation with a pulsation axis aligned with the tidal axis. This is the first time that oblique pulsation along a tidal axis has been recognized. It is unclear whether the pulsations are trapped in the hemisphere directed towards the companion or in the side facing away from it, but future spectral measurements can provide the solution. In the meantime, the single-sided pulsator HD 74423 stands out as the prototype of a new class of obliquely pulsating stars in which the interactions of stellar pulsations and tidal distortion can be studied.

• The paper identifies the first instance of tidally trapped pulsations in a δ Scuti star within a close binary system.
• It employs detailed TESS light curve analysis to demonstrate that orbital forces modulate pulsation amplitudes with a dominant hemisphere effect.
• The findings challenge traditional pulsation models and encourage further research into tidal influences on stellar oscillations.

Tidally Trapped Pulsations in a Close Binary Star System: A Summary of TESS Observations of HD 74423

The paper presents a notable astrophysical discovery concerning tidally trapped pulsations within a close binary star system, specifically identified in the star HD 74423 through data collected by the Transiting Exoplanet Survey Satellite (TESS). This discovery provides a novel perspective on the interaction between tidal forces and stellar pulsations, particularly in the context of binary star systems.

Discovery and Observations

HD 74423 is classified as a $\delta$ Scuti pulsator within a close binary system, characterized by a 1.6-day orbital period. The discovery reveals a distinctive modulation in the pulsation amplitude, synchronized with the star's orbital frequency. Notably, these pulsations exhibit a disproportionately large amplitude on one side of the star, which presents a unique form of nonradial oscillation. This is the first recognized instance of oblique pulsation aligned with a tidal axis, potentially indicating a new class of stars characterized by such phenomena.

Analysis and Modeling

A thorough analysis of the TESS light curves of HD 74423 exposes clear ellipsoidal light variations, alongside high-frequency pulsations. The pulsation frequency centered at 8.756917 days$^{-1}$ manifests alongside its orbital frequency sidelobes, suggesting an oblique distortion of the pulsation axis. Such variations are argued to fit the framework of an oblique pulsator model where the pulsation axis corresponds to the tidal axis, similar to magnetic axis alignment observed in roAp stars.

The paper also discusses amplitude and phase variability over orbital cycles, indicating that the pulsation primarily occurs in one of the hemispheres. Detailed modeling highlights that despite the asymmetry, the pulsations are largely restricted to a dominant hemisphere, inconsistent with symmetrical axisymmetric eigenfunction expectations.

Implications and Future Research

The identification of tidally trapped pulsations contributes to the broader understanding of stellar dynamics in binary systems, offering insights relevant to both theoretical modeling and observational strategies. It challenges existing paradigms by demonstrating that tidal forces can significantly impact the excitation and modulation of pulsations beyond classical predictions.

Future research is anticipated to focus on exploratory searches for similar stars with pulsation axes influenced by tidal distortions, leveraging advances in space-based photometry. Additionally, investigations aimed at determining whether the hemisphere facing or away from the binary companion predominantly exhibits pulsations will be crucial in refining the theoretical understanding.

Conclusion

The findings surrounding HD 74423 open new domains of astrophysical inquiry, particularly concerning the interactions of gravitational forces and stellar oscillations. Such research not only innovates the asteroseismic assessment of binary systems but also enhances the capability to classify and interpret stellar variations facilitated by modern observational technology, such as TESS. As researchers continue to explore these complexities, the potential for discovering analogous tidal phenomena promises to advance the understanding of stellar physics and dynamics within binary systems.