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

TL;DR

This study extends granulation analyses from evolved stars to the main sequence and subgiants by applying a robust Bayesian background-inference framework to Kepler short-cadence data for a large, diverse sample. By comparing three background descriptions and implementing correlated-inference between granulation and the oscillation envelope, the authors show there is no universal background model and reveal a notable decoupling between granulation and oscillation timescales for MS stars cooler than the Sun. They demonstrate that the primary granulation timescale plateaus at high $ u_{ ext{max}}$, lengthening the separation from the oscillation excess, a result that is supported by 1D and 3D convection simulations of K-dwarfs. The work also introduces peakbogging as an alternative modelling approach, documents its limitations, and provides a rich, publicly available dataset linking granulation, oscillations, and stellar parameters to guide future asteroseismic and convective studies across the HR diagram.

Abstract

Granulation is the observable signature of convection in envelopes of low-mass stars, forming the background in stellar power spectra. While well-studied in evolved giants, granulation on the MS has received less attention. We here study and characterise granulation signatures of MS and SGB stars, extending previous studies of giants to provide a continuous physical picture across evolutionary stages. We analyse 753 Kepler 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. Assuming a Gaussian oscillation envelope, $ν_\mathrm{max}$ estimates are sensitive to model misspecification, with the resulting systematics exceeding the formal uncertainties. The envelope width scales with $ν_\mathrm{max}$ 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 MS stars cooler than the Sun. The prolonged granulation timescale is reproduced by 3D simulations of a K-dwarf, driven by reduced convective velocities due to more efficient convective energy transport in denser envelopes. 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, providing a foundation for future investigations into their interdependence across the HR diagram.

Figures (14)

• Figure 1: Kiel diagram of the 733 stars with available effective temperatures and surface gravities, taken from the sample after the sorting in Sect. \ref{['sec:Sample']}. The $T_{\textup{eff}}$ values are retrieved from Table 3 of Sayeed25, while the seismic $\log g$ was calculated using the asteroseismic scaling relations with this $T_{\textup{eff}}$ and the $\Delta\nu$ estimate from Sayeed25. Simple stellar evolution tracks of solar metallicity and a range of masses are overplotted to guide the eye. The solar location is indicated by the yellow star symbol.
• Figure 2: Power density spectra with overlaid results of the background model inference using model H (see Table \ref{['tab:Models']}) when binning to $0.5$$\mu \mathrm{Hz}$ resolution for three stars: KIC6679371 (top), KIC8866102 (middle) and KIC8006161 (bottom). The unbinned PDS is shown in grey with the binned version overplotted in black. The model is plotted in red using the median of the obtained posteriors for each fit parameter. Additionally, 50 randomly drawn samples from the posteriors are used to replot the model to indicate the scatter. The individual granulation components are plotted as dashed green profiles. The fitted value of $\nu_{\textup{max}}$ is given in each panel and indicated by the vertical dashed black line, while the noise is shown by the horizontal dashed orange line. The activity component is the dash-dotted green line. The model without the influence of the Gaussian oscillation excess is plotted as the dashed blue profile, visible underneath the oscillation excess.
• Figure 3: Kiel diagram with colouring according to normalised evidence ratios, with model preferences as indicated by the legend. When models are comparable in their evidences the colour is blended between the two competing models. The Sun is overplotted as the enlarged star symbol at the solar location and significantly prefers model T.
• Figure 4: Comparison of the $\nu_{\textup{max}}$ determination across the different models. The $\nu_{\textup{max}}$ fractional residuals of models J and T to those obtained by model H are plotted in yellow and blue, respectively. The horizontal dashed line indicates perfect agreement in $\nu_{\textup{max}}$ determinations, while the dot-dashed show the $2\%$ bounds. The RMS scatter was calculated for both cases and is provided in the inserted box in the top right. The insert shows a split violin plot of the $\nu_{\textup{max}}$ residual distributions for model J (left) and model T (right) versus model H, with medians and 16th/84th percentiles overplotted as full and dashed horizontal lines, respectively.
• Figure 5: The FWHM of the Gaussian oscillation excess as a function of the determined $\nu_{\textup{max}}$, coloured by the temperature of the star. The dashed and dot-dashed lines indicate the predictions by Stello09 and Mosser12, respectively.