NASA's Kepler mission has observed almost 200,000 stars during its nominal operation, including more than 20,000 red giants with solar-like oscillations.

We fitted the power spectra of light curves of thousands of those red giants, carried out with the new Apollinaire code, which describes the granulation part with two Harvey-law components.

We perform a regression of the granulation parameters, amplitude and timescale, against the atmospheric parameters, effective temperature, surface gravity, and metallicity. The latter are obtained from the APOKASC collaboration (between the APOGEE spectroscopic survey and the Kepler Asteroseismic Scientific Consortium). In the regression we also account for the dilution of the granulation signal by the number of granules on the stellar disk, which amounts to a scaling by radius. For that purpose, we use radii from GAIA DR2 as well as seismic radii.

Investigating red clump and red giant branch stars separately, we find curious differences in the behavior of granulation, which is not expected from our current understanding of convection as supported by 3-D simulations. We are still exploring the reasons behind these differences. Ideas and suggestions are most welcome.