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P. Demarque, F. J. Robinson, S. Sofia (Yale University), Y.-C. Kim (Yonsei University), K. L. Chan (Hong Kong University of Science & Technology), D. B. Guenther (Saint Mary's University, Canada)
Results of a 3D radiative hydrodynamical simulation of atmospheric convection, applicable to a red giant star of one solar mass with log g = 2.6, are presented, with special emphasis on the scale of the convective cells. Since the work of Schwarzschild (1975), it has been expected, by analogy with the Sun, that convective cell size scales with the atmospheric scale height Hp and the width of the highly superadiabatic transition layer (SAL). Based on his estimate of the SAL, Schwarzschild (1975) predicted a few hundred cells on a giant and fewer (possibly even just a few) cells on a supergiant like Betelgeuse, in contrast with the two million cells observed on the solar surface. Freytag et al (1999)'s 10Hp scaling law, based on 2D numerical simulations, suggests a few hundred cells on a supergiant.
This work is an extension to a lower surface gravity of earlier 3D simulations for the Sun (Robinson et al. 2003a) and subgiant stars (Robinson et al. 2003b). Preliminary extrapolation from these simulations predicts that the surface of a giant with log g = 2.6 has approximately 40,000 cells. Betelgeuse, with log g = -0.5, would have approximately 300 cells. This number of cells is compatible with Gray's (2001) spectroscopic observations of Betelgeuse, although other observations of its surface brightness have been interpreted as due to fewer giant cells.
The convection scale size from the giant simulation is compared to previous extrapolations and to observation.
This work was supported in part by research grants from NASA (NAG5-8406), Yonsei University and NSERC of Canada.
The author(s) of this abstract have provided an email address for comments about the abstract: demarque@astro.yale.edu
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.