Impact of the convective mixing length parameter alpha on stellar metallicity

Context. Mixing-length theory is used to treat stellar convection. As a simulation in one-dimensional stellar atmospheres models, the mixing-length parameter alpha is calibrated from the Sun and then applied to other stars. However, there is no strong evidence to suggest that alpha should be the same for stars of different evolutionary stages.Aims. We evaluate the impact of the alpha value on the metallicity of different types of stars and investigate the correlation between the metallicity discrepancy (Delta [Fe/H]) and stellar parameters (T-eff, log g).Methods. We selected ten well-studied field stars and one open cluster of nine members for which high-resolution and high signal-to-noise spectra are available. The model atmospheres were calculated with the code MAFAGS-OS. We derived iron abundances from FeI and FeII lines both under local thermodynamic equilibrium and non-LTE conditions using a spectrum synthesis method. After deriving [Fe/H] for each line, we calculated Delta [Fe/H] with two different alpha values, fixed solar-calibrated alpha, and alpha obtained for each star individually. Finally, we investigated the correlation between Delta [Fe/H] caused by revised alpha with stellar parameters.Results. For FGK dwarf stars, the Delta [Fe/H] caused by the alpha correction is less than 0.02 dex, while for turn-off and giant stars, the Delta [Fe/H] values are no more than 0.03 dex, which are lower than typical uncertainties in metallicity. For main-sequence stars, Delta [Fe/H] versus T-eff and Delta [Fe/H] versus log g are well fit by linear relations.