The Scale of Granulation in Red Giant Atmospheres

Details The Scale of Granulation in Red Giant Atmospheres The Scale of Granulation in Red Giant Atmospheres

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aas...20111506d&link_type=abstract

American Astronomical Society, 201st AAS Meeting, #115.06; Bulletin of the American Astronomical Society, Vol. 34, p.1291

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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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