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Astron. Astrophys. 359, 729-742 (2000)


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Line formation in solar granulation

I. Fe line shapes, shifts and asymmetries

M. Asplund 1, Å. Nordlund 2, R. Trampedach 3, C. Allende Prieto 4 and R.F. Stein 3

1 NORDITA, Blegdamsvej 17, 2100 Copenhagen O, Denmark
2 Astronomical Observatory, NBIfAFG, Juliane Maries Vej 30, 2100 Copenhagen O, Denmark
3 Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48823, USA
4 McDonald Observatory and Department of Astronomy, University of Texas, Austin, TX 78712-1083, USA

Received 24 January 2000 / Accepted 4 May 2000

Abstract

Realistic ab-initio 3D, radiative-hydrodynamical convection simulations of the solar granulation have been applied to Fe I and Fe II line formation. In contrast to classical analyses based on hydrostatic 1D model atmospheres the procedure contains no adjustable free parameters but the treatment of the numerical viscosity in the construction of the 3D, time-dependent, inhomogeneous model atmosphere and the elemental abundance in the 3D spectral synthesis. However, the numerical viscosity is introduced purely for numerical stability purposes and is determined from standard hydrodynamical test cases with no adjustments allowed to improve the agreement with the observational constraints from the solar granulation.

The non-thermal line broadening is mainly provided by the Doppler shifts arising from the convective flows in the solar photosphere and the solar oscillations. The almost perfect agreement between the predicted temporally and spatially averaged line profiles for weak Fe lines with the observed profiles and the absence of trends in derived abundances with line strengths, seem to imply that the micro- and macroturbulence concepts are obsolete in these 3D analyses. Furthermore, the theoretical line asymmetries and shifts show a very satisfactory agreement with observations with an accuracy of typically 50-100 m s-1 on an absolute velocity scale. The remaining minor discrepancies point to how the convection simulations can be refined further.

Key words: convection – hydrodynamics – line: formation – radiative transfer – Sun: granulation – Sun: photosphere

Present address: Uppsala Astronomical Observatory, Box 515, 751 20 Uppsala, Sweden

Send offprint requests to: M. Asplund (martin@astro.uu.se)

© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000

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