GRANULATION IN RED GIANTS: OBSERVATIONS BY THE<i>KEPLER</i>MISSION AND THREE-DIMENSIONAL CONVECTION SIMULATIONS

Abstract

The granulation pattern that we observe on the surface of the Sun is due to\nhot plasma from the interior rising to the photosphere where it cools down, and\ndescends back into the interior at the edges of granules. This is the visible\nmanifestation of convection taking place in the outer part of the solar\nconvection zone. Because red giants have deeper convection zones and more\nextended atmospheres than the Sun, we cannot a priori assume that granulation\nin red giants is a scaled version of solar granulation. Until now, neither\nobservations nor 1D analytical convection models could put constraints on\ngranulation in red giants. However, thanks to asteroseismology, this study can\nnow be performed. The resulting parameters yield physical information about the\ngranulation. We analyze \\sim1000 red giants that have been observed by Kepler\nduring 13 months. We fit the power spectra with Harvey-like profiles to\nretrieve the characteristics of the granulation (time scale tau_gran and power\nP_gran). We also introduce a new time scale, tau_eff, which takes into account\nthat different slopes are used in the Harvey functions. We search for a\ncorrelation between these parameters and the global acoustic-mode parameter\n(the position of maximum power, nu_max) as well as with stellar parameters\n(mass, radius, surface gravity (log g) and effective temperature (T_eff)). We\nshow that tau_eff nu_max^{-0.89} and P_gran nu_max^{-1.90}, which is consistent\nwith the theoretical predictions. We find that the granulation time scales of\nstars that belong to the red clump have similar values while the time scales of\nstars in the red-giant branch are spread in a wider range. Finally, we show\nthat realistic 3D simulations of the surface convection in stars, spanning the\n(T_eff, log g)-range of our sample of red giants, match the Kepler observations\nwell in terms of trends.\n