Manifestations of deep convection in stellar photometry

Convection is the main factor of uncertainty in the modeling of stars and one of the drivers of stellar variability. Unfortunately, the theories of turbulent convection are far from perfect. For example, there is considerable variation in assumed values of key parameters such as the length of the mixing and turnover time. In particular, measurements of the turnover time are usually calibrated using calculations for the sun with the mixing length theory (MLT). The, MLT is a standard, but vaguely justified element in most modern models of the structure and evolution of stars. It represents an extreme simplification of the actual physical process of convection. The wide spectrum of sizes of turbulent eddies is replaced by only large eddies of the same size. The certification of such giant convective cells could be an explanation of the MLT success in stellar modeling. In any case, a new approach is needed for the measuring of MLT-independent turnover time. Elaboration of such an approach is a subject of the project. Space observatories Kepler, CoRoT, MOST have given a huge number of stellar photometric measurements that need processing and analysis. Extraction of the hidden information on stellar convection will facilitate the understanding of the physics of stars, their interiors, and activity. This project is aimed at searching and analysis of the effects of convection in deep layers of stars using the integral photometry of main-sequence stars. For this purpose, the light curves of stars from archives of orbital observatories (MOST, CoRoT, Kepler) will be processed. "Fingerprints" of deep convection recently were found in a large-scale pattern and dynamics of the surface activity of the Sun (e. g., Arkhypov et al., Sol. Phys., 2011, 270, 1; 2012, 278, 285). That is why the Sun will be used to adjust our spectral-autocorrelation method for extracting the information on deep convection from the data of stellar broadband integral photometry. After processing of a large sample of pre-selected stellar light curves, we plan to obtain: experimental evidence of large scale turbulence in stars; detection of super-giant convection cells in red dwarfs; mass measurements of turnover times for different scales independently on MLT, etc. The results obtained can be used in various fields of astrophysics and space research: from modeling of stars and stellar activity up to space weather prediction.

This project was aimed at the search and analysis of effects of plasma mixing (convection) in the stellar deep interiors. This mixing controls stellar activity and radiation as well as radius and surface temperature of a star, influencing therefore the habitability and evolution of the nearby planets. Unfortunately, the deep stellar convection still is inaccessible for direct instrumental probing (even with asteroseismology methods). At the same time, during this project it has been justified that the pattern of stellar surface activity may be used as a kind of display for the internal stellar plasma motions. In view of the fact that the stellar activity is a result of magnetic tube emersion which is influenced by plasma flows in the stellar convection zone, the deep convection is manifested in the surface pattern of starspots. The starspots, rotating with the star, modulate the radiation flux, which carries therefore an imprint of the longitudinal distribution of stellar activity, and hence, the deep convective flows. Following this logic, the project was focused on the analysis of rotational variations of light curves of 1998 main-sequence stars, observed by the Kepler space observatory. A number of unique algorithms have been developed and used to extract the hidden information about the structure and dynamics of the surface activity pattern in these stars. As a main result, the fingerprints of giant turbulence were discovered in the surface distribution of the stellar activity pattern. Since the signatures of analogous global turbulence were not directly observed at the solar surface, it should be an attribute of deep convection.Another fundamental result of the project consists in the finding of the non-turbulent (laminar) component of the stellar deep convection. This type of mixing is realized in the form of a statistically identical convective cells, like in the standard mixing length theory (MLT) widely used in modern astrophysics. The general applicability of MLT to the turbulent stellar convection was however reasonably criticized, in spite of the fact that it gives acceptable results in modeling of stars. The project study sheds light on this paradox. It has been shown that the turnover time of a standard cell in MLT indeed is a proxy of the measured time-scale, corresponding to the laminar convection.Among other valuable astrophysical findings of the project are the estimation of lifetime of activity complexes; detection of the short cycles of stellar activity (periods < 1000 days) revealing unknown behavior in relation to stellar parameters; investigation of the stellar variability timescales and introducing new activity indexes, etc. Besides astrophysics and astrobiology, the project results are of interest and potential use in education, popular science, planetary science, space weather, and planning of future space observatories.