Wildly oscillating stars - Unexplained dense ridge-like frequency agglomerations in A- and F-type pulsators

@article{antoci2026wildly,
  title = {{Wildly oscillating stars - Unexplained dense ridge-like frequency agglomerations in A- and F-type pulsators}},
  author = {V. Antoci and J. Labadie-Bartz and M. Święch and O. Dürfeldt-Pedros and S. Murphy and D. Kurtz and T. Bedding and G. Handler and J. Fuller and R.-M. Ouazzani and H. Kjeldsen and L. Fellay and M. Gade Pedersen and E. Niemczura and D. Bowman and M. Deal and P. Mani},
  journal = {Astronomy \& Astrophysics - A\&A},
  year = {2026},
  month = aug,
  doi = {10.1051/0004-6361/202660809},
  eprint = {2606.17207},
  archivePrefix = {arXiv},
  url = {https://hal.science/hal-05760761v1},
  note = {CC BY 4.0},
}

TY  - JOUR
TI  - Wildly oscillating stars - Unexplained dense ridge-like frequency agglomerations in A- and F-type pulsators
AU  - V. Antoci
AU  - J. Labadie-Bartz
AU  - M. Święch
AU  - O. Dürfeldt-Pedros
AU  - S. Murphy
AU  - D. Kurtz
AU  - T. Bedding
AU  - G. Handler
AU  - J. Fuller
AU  - R.-M. Ouazzani
AU  - H. Kjeldsen
AU  - L. Fellay
AU  - M. Gade Pedersen
AU  - E. Niemczura
AU  - D. Bowman
AU  - M. Deal
AU  - P. Mani
PY  - 2026
DA  - 2026/08/24
JO  - Astronomy & Astrophysics - A&A
DO  - 10.1051/0004-6361/202660809
UR  - https://hal.science/hal-05760761v1
AB  - We investigate the origin of the dense, ridge-like frequency clusters observed in a subset of A- and F-type pulsating stars, which we refer to as ‘wildly oscillating’ stars (WOSs). These agglomerated frequency regions occupy a confined part of the frequency spectrum, typically below the fundamental radial mode, and they cannot be explained on the basis of classical pulsation theory. Methods. We analysed high-precision space photometry from Kepler and TESS, constructed échelle diagrams, and performed systematic searches for combination frequencies. We determined the expected fundamental radial mode using pulsation constants, period–luminosity relations, and stellar models to place the agglomerated regions in a seismic context. The rotational modulation was examined through phase-folded light curves and an amplitude–phase analysis, followed by tests of the binarity and geometric modulation scenarios. In addition, we computed non-adiabatic stability models for representative stellar parameters to assess whether standard excitation mechanisms reproduce the observed structures. Results. The WOS phenomenon is confined to a narrow region of the Hertzsprung-Russell diagram (HRD) near the overlap of the δ Sct and γ Dor instability strips. The observed ridge morphology and mode density cannot be reproduced by simple asymptotic g -mode behaviour, standard low-order p modes, binarity, or typical rotational splitting. In at least two stars (KIC 5443410 and KIC 9347095), a significant fraction of peaks in the agglomerated region can be explained as non-linear combination frequencies involving high-order g modes. However, these combinations require parent modes to be located within the agglomerated frequency band itself, indicating that intrinsic pulsation modes must be present there. Non-adiabatic stability calculations successfully reproduce the classical instability domains; however, they do not predict unstable modes with the observed density or organised ridge structure in the agglomerated region. Conclusions. The WOSs appear to represent a pulsational regime that is not captured by current models of mode excitation or rotational modulation. The agglomerated frequency phenomenon requires a mechanism that selects or excites a confined intermediate-frequency band and produces organised ridge structures within a narrow region of the stellar parameter space.
SN  - 0004-6361
SN  - 1432-0746
ER  -