Harmonics as a Hidden Window into the Turbulent Convective Envelope of non-Blazhko RRab Stars

Abstract

Harmonics in pulsating stars have traditionally been regarded as mere Fourier artifacts, fully determined by their parent mode. Recent observations of high-amplitude $δ$ Scuti stars, however, have revealed the existence of disharmonized harmonics, which exhibit amplitude and frequency variations uncorrelated with their parent pulsation mode. Here we test the universality of this phenomenon by examining a class of stars long considered pulsationally stable: non-Blazhko RRab stars. Using short-cadence {\it Kepler} photometric data, we not only identify a distinct hump structure formed by harmonics of the primary pulsation mode in their frequency spectra, but also find significant amplitude and frequency variations associated with the harmonics around the onset and in the tail of the hump structure. These findings are consistent with the interpretation that the hump structure originates from the turbulent convective envelope of the star. Interestingly, several important phenomena can be understood within this framework as a working hypothesis. Thus, the hump structure of harmonics and their intrinsic variability could constitute a hidden window into the stellar convective envelope, potentially tracing energy injection at the convection-pulsation interaction and stochastic perturbations from turbulent convection -- although detailed modeling is required to confirm this interpretation.