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Astron. Astrophys. 328, 229-234 (1997)

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Is stellar granulation turbulence?

Å. Nordlund 1, 2, H.C. Spruit 3, H.-G. Ludwig 2, 3 and R. Trampedach 4

1 Theoretical Astrophysics Center, Juliane Mariesvej 30, DK-2100 Copenhagen O, Denmark
2 Astronomical Observatory, NBIfAFG, Juliane Mariesvej 30, DK-2100 Copenhagen O, Denmark
3 Max Planck institute for Astrophysics, Box 1523, D-85740 Garching, Germany
4 Theoretical Astrophysics Center, Ny Munkegade bldg. 520, DK-8000 Århus C, Denmark

Received 27 March 1997 / Accepted 10 July 1997


We show that power spectra of granulation images or velocity fields cannot be compared meaningfully with spectra from theoretical models based on turbulent cascades. The small scale power in these images is due almost entirely to the sharp edges between granules and intergranular lanes, not to turbulence in the usual sense. This is demonstrated with a number of experiments with result from numerical simulations, and with simpler synthetic data with power spectra similar to that of granulation. The reason for the seemingly laminar behavior of the granulation flow, in spite of the high Reynolds numbers involved, is the influence of stratification on the local ratio of turbulence to bulk flow. The rapid expansion of upflows, their deep origin and near-adiabatic stratification lead to low levels of turbulence in the overturning fluid at the surface. Higher levels of turbulence are expected in the converging flows near downdrafts, but mostly at scales that are below current observational resolution limits, and contributing relatively little to the total convective flux and to spectral line broadening.

Key words: convection – Sun: granulation – stars: atmospheres

Send offprint requests to: H.C. Spruit


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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998