Wildly Oscillating Stars -- Unexplained dense ridge-like frequency agglomerations in A and F type pulsators
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 (WOS). These agglomerated frequency regions occupy a confined part of the frequency spectrum, typically below the fundamental radial mode, and are not explained by pulsation theory. We analyse Kepler and TESS data, construct echelle diagrams, and perform searches for combination frequencies. We determine the fundamental radial mode in order to place the agglomerated regions in a seismic context. Rotational modulation is examined through phase-folded light curves and amplitude-phase analysis, and binarity and geometric modulation scenarios are tested. The WOS phenomenon is confined to a narrow region near the overlap of the delta Sct and gamma Dor instability strips. The observed ridge morphology and mode density cannot be reproduced by simple asymptotic g-mode behaviour, low-order p modes, binarity, or typical rotational splitting. In at least two stars, a significant fraction of peaks in the agglomerated region can be explained as nonlinear combination frequencies involving high-order g modes. However, these combinations require parent modes located within the agglomerated frequency band itself, indicating that intrinsic pulsation modes must be present there. Non-adiabatic stability calculations reproduce the classical instability domains but do not predict unstable modes with the observed density or organised ridge structure in the agglomerated region. The WOS appear to represent a pulsational regime 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 stellar parameter space (abridged for arXiv).