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Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2009

Impact of granulation effects on the use of Balmer lines as temperature indicators

Résumé

Context: Balmer lines serve as important indicators of stellar effective temperatures in late-type stellar spectra. One of their modelling uncertainties is the influence of convective flows on their shape.
Aims: We aim to characterize the influence of convection on the wings of Balmer lines.
Methods: We perform a differential comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical model atmospheres and 1D hydrostatic standard ones. The model parameters are appropriate for F, G, K dwarf and subgiant stars of metallicity ranging from solar to 10-3 solar.
Results: The shape of the Balmer lines predicted by 3D models can never be exactly reproduced by a 1D model, irrespective of its effective temperature. We introduce the concept of a 3D temperature correction, as the effective temperature difference between a 3D model and a 1D model which provides the closest match to the 3D profile. The temperature correction is different for the different members of the Balmer series and depends on the adopted mixing-length parameter alphaMLT in the 1D model. Among the investigated models, the 3D correction ranges from -300 K to +300 K. Horizontal temperature fluctuations tend to reduce the 3D correction.
Conclusions: Accurate effective temperatures cannot be derived from the wings of Balmer lines, unless the effects of convection are properly accounted for. The 3D models offer a physically well justified way of doing so. The use of 1D models treating convection with the mixing-length theory do not appear to be suitable for this purpose. In particular, there are indications that it is not possible to determine a single value of alphaMLT which will optimally reproduce the Balmer lines for any choice of atmospheric parameters. The investigation of a more extended grid and direct comparison with observed Balmer profiles will be carried out in the near future.

Dates et versions

hal-03786030 , version 1 (23-09-2022)

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Hans-Günter Ludwig, Natalie Behara, Matthias Steffen, Piercarlo Bonifacio. Impact of granulation effects on the use of Balmer lines as temperature indicators. Astronomy and Astrophysics - A&A, 2009, 502, pp.L1-L4. ⟨10.1051/0004-6361/200911997⟩. ⟨hal-03786030⟩
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