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Wildly Oscillating Stars -- Unexplained dense ridge-like frequency agglomerations in A and F type pulsators

Lookup NU author(s): Dr Dominic BowmanORCiD

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Abstract

Aims. 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 sys- tematic searches for combination frequencies. We determined the expected fundamental radial mode using pulsation constants, pe- riod–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 mod- ulation 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.

Publication metadata

Author(s): Antoci V, Labadie-Bartz J, Swiech M, Deurfeldt-Pedros O, Murphy SJ, Kurtz DW, Bedding TR, Handler G, Fuller J, Ouazzani RM, Kjeldsen H, Fellay L, Pedersen MG, Niemczura E, Bowman DM, Deal M, Mani P

Publication type: Article

Publication status: Published

Journal: A&A

Year: 2026

Volume: 712

Pages: 29

Online publication date: 24/08/2026

Acceptance date: 15/06/2026

Date deposited: 31/07/2026

ISSN (print): 0004-6361

ISSN (electronic): 1432-0746

Publisher: EDP Sciences

URL: https://doi.org/10.1051/0004-6361/202660809

DOI: 10.1051/0004-6361/202660809

Data Access Statement: Data contained within the paper itself

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