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

TL;DR

This study investigates how convection affects Balmer line profiles in stellar spectra, revealing that 3D models provide more accurate temperature diagnostics than traditional 1D models with mixing-length theory.

Contribution

It introduces the concept of 3D temperature correction and demonstrates the limitations of 1D models in reproducing Balmer line profiles for stellar temperature estimation.

Findings

3D models cannot be exactly matched by 1D models regardless of temperature.

3D temperature correction ranges from -300K to +300K depending on the Balmer line.

Convection effects significantly impact the interpretation of Balmer lines in stellar spectra.

Abstract

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. We aim to characterize the influence of convection on the wings of Balmer lines. 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 1/1000 solar. 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 -300K to +300K. Horizontal temperature fluctuations tend to reduce the 3D correction. 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 the mixing-length parameter which will optimally reproduce the Balmer lines for any choice of atmospheric parameters.