The planet-hosting subdwarf B star V391 Pegasi is a hybrid pulsator

A noticeable fraction of subdwarf B stars shows either short-period (p-mode) or long-period (g-mode) luminosity variations, with two objects so far known to exhibit hybrid behaviour, i.e. showing both types of modes at the same time. The pulsating subdwarf B star V391 Pegasi (or HS2201+2610), which is close to the two known hybrid pulsators in the log g - Teff plane, has recently been discovered to host a planetary companion. In order to learn more about the planetary companion and its possible influence on the evolution of its host star (subdwarf B star formation is still not well understood), an accurate characterisation of the host star is required. As part of an ongoing effort to significantly improve the asteroseismic characterisation of the host star, we investigate the low-frequency behaviour of HS2201+2610. We obtained rapid high signal-to-noise photometric CCD (B-filter) and PMT (clear-filter) data at 2m-class telescopes and carried out a careful frequency analysis of the light curves. In addition to the previously known short-period luminosity variations in the range 342s-367s, we find a long-period variation with a period of 54min and an amplitude of 0.15 per cent. This can most plausibly be identified with a g-mode pulsation, so that HS2201+2610 is a new addition to the short list of hybrid sdB pulsators. Along with the previously known pulsating subdwarf B stars HS0702+6043 and Balloon090100001 showing hybrid behaviour, the new hybrid HS2201+2610 is the third member of this class. This important property of HS2201+2610 can lead to a better characterisation of this planet-hosting star, helping the characterisation of its planetary companion as well. Current pulsation models cannot yet reproduce hybrid sdBV stars particularly well and improved pulsation models for this object have to include the hybrid behaviour.

💡 Research Summary

The paper presents a detailed asteroseismic investigation of the subdwarf B (sdB) star V391 Pegasi (HS 2201+2610), which is already known to host a planetary companion. The authors aim to improve the stellar characterisation because the planet’s inferred properties and the evolutionary history of the host depend critically on the star’s mass, radius, and internal structure. To this end, they obtained high‑signal‑to‑noise, rapid‑cadence photometry using both a B‑filter CCD and a clear‑filter photomultiplier tube (PMT) on 2‑meter class telescopes. The data were subjected to a rigorous frequency analysis employing Fourier transforms and multi‑sinusoid fitting.

The analysis reproduces the previously identified short‑period pressure‑mode (p‑mode) oscillations with periods ranging from 342 s to 367 s. More importantly, the authors detect a low‑frequency signal with a period of approximately 54 minutes (≈3240 s) and an amplitude of 0.15 % of the stellar flux. The period, amplitude, and phase stability of this signal are consistent with a gravity‑mode (g‑mode) pulsation, which had not been reported for this object before. Consequently, V391 Peg is classified as a hybrid sdB pulsator, joining only two other known hybrids: HS 0702+6043 and Balloon 090100001.

Hybrid pulsators are of particular interest because they simultaneously excite p‑ and g‑modes, implying that the star’s interior contains regions where both the acoustic and buoyancy restoring forces are effective. This dual excitation challenges current non‑adiabatic pulsation models, which struggle to reproduce the observed mode spectra for hybrid sdB stars. The detection of a g‑mode in V391 Peg therefore provides a crucial benchmark for refining theoretical models, especially regarding the role of metallicity, envelope thickness, and the precise location in the log g–Teff diagram.

Beyond stellar physics, the hybrid nature of V391 Peg has direct implications for the planetary companion. The planet’s mass and orbital parameters have been derived from timing variations (the O‑C method) that assume a stable pulsation clock. The presence of a g‑mode introduces an additional, independent clock that can be used to cross‑validate the timing analysis, potentially reducing systematic uncertainties. Moreover, the interaction between the planet and the host star—through angular momentum exchange, tidal forces, or enhanced mass‑loss during the red‑giant phase—might leave subtle imprints on the g‑mode frequencies or amplitudes. Detecting such signatures would offer a rare observational probe of star‑planet interaction during the extreme evolutionary phase that produces sdB stars.

The authors propose future work that includes long‑baseline, multi‑site photometric campaigns to resolve mode multiplets, high‑resolution spectroscopy to measure line‑profile variations, and detailed asteroseismic modelling that incorporates both p‑ and g‑mode constraints. Such efforts will enable a more precise determination of the star’s mass, radius, and internal rotation profile, which in turn will tighten the constraints on the planet’s true mass (as opposed to the minimum mass derived from timing alone).

In summary, this study establishes V391 Peg as the third confirmed hybrid sdB pulsator, demonstrates the feasibility of detecting low‑amplitude g‑modes in a planet‑hosting sdB star, and highlights the dual scientific payoff: advancing the physics of hybrid pulsations and improving the characterization of the planetary companion. The results underscore the need for improved pulsation models that can simultaneously account for p‑ and g‑mode excitation, and they open a promising avenue for probing the interplay between stellar evolution and planetary survival in the post‑red‑giant phase.