Balmer lines in cool dwarf stars. I. Basic influence of atmospheric models.

Based on horizontally homogeneous model atmospheres the analysis of the first members of the hydrogen Balmer line series leads to a revision of the convective efficiency near the surfaces of cool dwarf stars. The synthesis of the Balmer lines includes Stark and resonance broadening. It fits the observed profiles of Hα through Hδ in such different stars as the Sun, Procyon, and the extremely metal-poor subdwarfs G41-41 and HD 140283. It turns out that the existence of the two different but equally important line-broadening mechanisms requires a temperature stratification near continuum optical depth unity which cannot be reconciled with a mixing-length l ≃ 1... 2 H p as is usually derived from stellar interior calculations