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

Abstract

<i>Context. <i/>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.<i>Aims. <i/>We aim to characterize the influence of convection on the wings of Balmer lines.<i>Methods. <i/>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<sup>-3<sup/> solar.<i>Results. <i/>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 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.<i>Conclusions. <i/>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 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.