Granulation signatures as seen by Kepler short-cadence data. I. A decoupling between granulation and oscillation timescales for dwarfs

Granulation is the observable surface signature of convection in the envelopes of low-mass stars, forming the background in stellar power spectra. While well-studied in evolved giants, granulation on the main sequence has received less attention. We aim to study and characterise granulation signatures of main-sequence and subgiant stars, extending previous studies of giants to provide a continuous physical picture across evolutionary stages. We analysed ępler short-cadence stars using a Bayesian nested-sampling framework to evaluate three background descriptions and compare model preferences. This yields full posterior distributions for all parameters, enabling robust comparisons across a diverse stellar sample. No universal preference between background models is found, and thus an a priori choice is not justified. Assuming a Gaussian oscillation envelope, νmax estimates become sensitive to model misspecification, with the resulting systematics being comparable to or exceeding the formal uncertainties. The envelope width scales with νmax across models and shows a dependence on effective temperature. Total granulation amplitudes in dwarfs broadly follow giant-based scalings; however, a decoupling appears between the timescale of the primary granulation and the oscillations for main-sequence stars cooler than the Sun. The prolonged granulation timescale was reproduced by 3D hydrodynamical simulations of a K dwarf, driven by reduced convective velocities resulting from more efficient convective energy transport in denser envelopes. Our study represents the most extensive Bayesian background modelling of ępler short-cadence stars to date and reveals a decoupling between granulation and oscillation timescales in K dwarfs. The prolonged granulation timescale increases the frequency separation to the oscillation excess, potentially aiding seismic detectability, while the reduced convective velocities may influence the excitation of stellar oscillations and relate to the low amplitudes observed in cool dwarfs. Finally, we contribute a dataset linking granulation, oscillations, and stellar parameters, establishing a foundation for future investigations into their interdependence across the Hertzsprung–Russell diagram.