Granulation in Red Giants: Observations by the Kepler Mission and Three-dimensional Convection Simulations

Mathur, S.; Hekker, S.; Trampedach, R.; Ballot, J.; Kallinger, T.; Buzasi, D.; García, R. A.; Huber, D.; Jiménez, A.; Mosser, B.; Bedding, T. R.; Elsworth, Y.; Régulo, C.; Stello, D.; Chaplin, W. J.; De Ridder, J.; Hale, S. J.; Kinemuchi, K.; Kjeldsen, H.; Mullally, F.; Thompson, S. E.
Bibliographical reference

The Astrophysical Journal, Volume 741, Issue 2, article id. 119 (2011).

Advertised on:
11
2011
Number of authors
21
IAC number of authors
2
Citations
147
Refereed citations
118
Description
The granulation pattern that we observe on the surface of the Sun is due to hot plasma rising to the photosphere where it cools down and descends back into the interior at the edges of granules. This is the visible manifestation of convection taking place in the outer part of the solar convection zone. Because red giants have deeper convection zones than the Sun, we cannot a priori assume that their granulation is a scaled version of solar granulation. Until now, neither observations nor one-dimensional analytical convection models could put constraints on granulation in red giants. With asteroseismology, this study can now be performed. We analyze ~1000 red giants that have been observed by Kepler during 13 months. We fit the power spectra with Harvey-like profiles to retrieve the characteristics of the granulation (timescale τgran and power P gran). We search for a correlation between these parameters and the global acoustic-mode parameter (the position of maximum power, νmax) as well as with stellar parameters (mass, radius, surface gravity (log g), and effective temperature (T eff)). We show that τeffvpropν-0.89 max and P granvpropν-1.90 max, which is consistent with the theoretical predictions. We find that the granulation timescales of stars that belong to the red clump have similar values while the timescales of stars in the red giant branch are spread in a wider range. Finally, we show that realistic three-dimensional simulations of the surface convection in stars, spanning the (T eff, log g) range of our sample of red giants, match the Kepler observations well in terms of trends.
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The principal objectives of this project are: 1) to study the structure and dynamics of the solar interior, 2) to extend this study to other stars, 3) to search for extrasolar planets using photometric methods (primarily by transits of their host stars) and their characterization (using radial velocity information) and 4) the study of the planetary
Savita
Mathur