THE FORMATION OF THE ALKALI RESONANCE LINES IN COOL ATMOSPHERES .1. NA-I AND K-I IN A SUNSPOT UMBRA

We have studied the formation of the Na I and K I resonance lines in a sunspot umbra looking at their diagnostic value. We have computed dif ferent cross-sections and rates for the excitation of the alkali atoms by collisions with neutral hydrogen atoms in addition to electrons, a nd we have evaluated their effects on the emergent line profiles. The main results of the line synthesis, which allowed for different temper ature structures, line blend, the Zeeman broadening produced by a cons tant vertical magnetic field, and a possible overcorrection for stray light of the observed spectrum, are that: i) the comparison between th e observed and the computed inner wings of the sodium profiles suggest s that the umbral upper photosphere is cooler than in a standard model ; ii) there is no satisfactory fit of the reversed and asymmetric line cores of the sodium and potassium resonance lines in the umbral spect rum.