Tidally Trapped Two-pole Pulsations Discovered in a Close Binary with a Massive $β$ Cephei Star

TL;DR

The study addresses the presence of tidally trapped two-pole pulsations in a close binary hosting a massive beta Cephei star, extending the TTP phenomenon beyond previously known delta Scuti and sdB hosts. Using high-precision, two-minute cadence photometry from TESS, it identifies pulsation amplitude modulation at the orbital frequency in HD 329379, consistent with tidal confinement near the two apsides. The authors interpret the detected mode as a tidally distorted quadrupole pulsation with its axis aligned to the tidal axis, marking the first such identification in a beta Cephei star and establishing a prototype for a new TTP subclass. This discovery broadens the TTP family to massive stars and provides a novel avenue to probe interior structure and pulsation–tidal interactions in close binaries.

Abstract

Tidally tilted pulsators (TTPs), whose pulsation axis aligns with the binary's semi-major axis, represent a newly established class of oscillators in binary systems. While all previously known TTPs are either $δ$ Scuti or subdwarf B-type stars, their existence has remained unidentified in more massive $β$ Cephei variables. Here, we report the discovery of tidally trapped pulsations in the massive ellipsoidal variable HD~329379, based on photometry from the Transiting Exoplanet Survey Satellite (TESS). Our analysis reveals a $β$ Cephei pulsator in a 2.25-day orbit whose pulsation mode amplitude is strongly modulated with the orbital frequency. Based on our analysis, we concluded that this modulation can be explained by pulsations with significantly larger amplitude near the star's two tidal poles (apsides). We interpret this as a tidally distorted quadrupole pulsation chariacteristiced by trapped two-pole pulsations, with a pulsation axis aligned with the tidal axis. This represents the first identification of such a pulsation mode in a $β$ Cephei star, which differs from single-sided pulsations observed in previous works, marking a rare and important discovery. Our work extends the family of TTPs beyond $δ$ Scuti and subdwarf B-type stars to include more massive $β$ Cephei variables. In particular, the two-pole pulsator HD~329379 stands out as the prototype of a new class of TTPs in massive stars. These results not only provide a new insight to probe the interior structure and evolutionary state for massive stars but also offer a unique opportunity to study the interaction between pulsations and strong tidal distortions.

Figures (2)

• Figure 1: TESS light curves of HD 329379.(a), A segment of the light curve displaying both ellipsoidal variations and high-frequency pulsations. Similar features are present in the other observed sector (not shown). (b), The same segment after removal of the orbital variations and low-frequency artifacts, revealing the modulation of the pulsation amplitude with the orbital period. Time is given in TESS Julian Date (TJD = BJD $-$ 2,457,000), where BJD is the Barycentric Julian Date.
• Figure 2: Fourier amplitude spectra of the TESS light curve for HD 329379.(a), Initial amplitude spectrum. The highest peak corresponds to twice the orbital frequency ($2\nu_{\mathrm{orb}}$; $\nu_{\mathrm{orb}} = 0.445665 \pm 0.000009\,\mathrm{d}^{-1}$), characteristic of ellipsoidal variation. (b), Amplitude spectrum of the residuals after pre-whitening with a sixteen-term harmonic series at the orbital frequency. The pulsation multiplet centered on $\nu_1 = 6.309599 \pm 0.000009\,\mathrm{d}^{-1}$ (see Table \ref{['tab:frequency_multiplets']}) is visible, along with low-frequency instrumental artifacts. (c), Same as panel (b) after applying a high-pass filter to remove the low-frequency artifacts, clearly revealing the multiplet structure. (d), Residual amplitude spectrum after subtracting the multiplet mode detailed in Table \ref{['tab:frequency_multiplets']}. Note the change in scale.