1) Speaker: M.L. Khodachenko (Space Research Institute, Austrian Academy of Sciences, Graz, Austria)

Title:  “New insights in stellar physics with Kepler high precision photometry: Probing of deep mixing, stellar activity, and starspot diagnostics”

Abstract: The current knowledge on plasma convective mixing in stellar interiors is based on theoretical modelling, which calibrates the empirical relations between the mixing and observable stellar parameters. A model-independent approach to study the mixing process is still a challenge. Application of an originally developed analysis algorithms to the light curves of 1361 main sequence stars from the Kepler mission enabled a possibility to study complex internal motions, so far inaccessible to astero-seismology methods, by analyzing the dynamics of stellar surface activity pattern. The applied new spectral-correlation method shows that deep mixing is manifested in stellar photometric light curves. The timescales tau_1 and tau_2 of the stochastic change in the spectral power of stellar rotational variability were measured for the first and second rotational harmonics of hundreds of Kepler stars. Both, the amplitudes and variability timescales of the rotational harmonics’ spectral power demonstrate features, typical for the Kolmogorov’s turbulence. It has been shown that the timescale of deep mixing, known as the turnover time in the standard mixing length theory can be obtained by extrapolation of the measured timescales of the first and second harmonics of the turbulent cascade to the scale of laminar convection. This connection opens the way to determine the model-independent turnover time. Besides of that, a new branch on the omega_cyc – Ro^{-1} diagram which connects the branch of inactive stars with the area populated by super-active objects was discovered. The formation of the discovered branch has been shown to be due to the alpha-quenching effect, which saturates the magnetic dynamo and decreases the cycle periods with the increase of Ro^{-1}. The lifetimes of spots in the photospheres of cold and fast-rotating stars have been measured with the proposed analysis algorithm. The realistic values of the diffusivity of the magnetic elements in the photosphere were obtained for the broad interval of stellar effective temperatures from 3300 to 6600K.

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