BALMER LINES IN COOL DWARF STARS .1. BASIC INFLUENCE OF ATMOSPHERIC MODELS

Based on horizontally homogeneous model atmospheres the analysis of th e first members of the hydrogen Balmer line series leads to a revision of the convective efficiency near the surfaces of cool dwarf stars. T he synthesis of the Balmer lines includes Stark and resonance broadeni ng. It fits the observed profiles of Halpha through Hdelta in such dif ferent stars as the Sun, Procyon, and the extremely metal-poor subdwar fs G41-41 and HD 140283. It turns out that the existence of the two di fferent but equally important line-broadening mechanisms requires a te mperature stratification near continuum optical depth unity which cann ot be reconciled with a mixing-length 1 congruent-to 1 ... 2 H(p) as i s usually derived from stellar interior calculations. With such high v alues of l the synthesis of the Balmer lines leads to effective temper atures differing by as much as 400 K. Comparison of Halpha and Hbeta p rofiles instead results in a much smaller mixing-length, l = (0.5 +/- 0.3) H(p), which seems to fit for all FG-type dwarfs even in the metal -poor limit. Thus, for the first time, spectroscopic observations of s tars lead directly to a consistent determination of the temperature st ratification in the lower photosphere.