AAS 201st Meeting, January, 2003
Session 115. Cool Star Atmospheres and Envelopes
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[115.06] The Scale of Granulation in Red Giant Atmospheres

P. Demarque, F. J. Robinson (Yale U.), K. L. Chan (Hong Kong UST), Y. -C. Kim (Yonsei U.), D. B. Guenther (Saint Mary's U.), S. Sofia (Yale U.)

The scale of photospheric convection in red giants is generally associated, by analogy with the Sun, with the atmospheric pressure scale height and the thickness of the superadiabatic transition layer. As pointed out by Schwarzschild (1975), both of these quantities are relatively much larger in terms of the stellar radius in a red giant than in the Sun. On these grounds, Schwarzschild concluded that only a few cells must be present on a red giant surface, in contrast with two millions cells observed on the solar surface. Observations of brightness variations in the TiO band on Betelgeuse (Gaustad 1986) and direct imaging of Betelgeuse (Gilliland & Dupree 1996) with HST have yielded results that are compatible with giant cells, although other interpretations (e.g. pulsation) are possible. 2D numerical simulations (Freytag et al. 2001) also claim compatibility with the giant cell interpretation. On the other hand, Gray (2001) has pointed out that his extensive spectroscopic observations of Betelgeuse are more easily interpreted in terms of many convection cells per hemisphere. We present preliminary results of 3D radiative hydrodynamical simulations which corroborate Gray's interpretation.

This work is supported in part by NASA grant NAG5-8406

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