The excitation of SI emission lines in chromospheres of late-type giant stars.

Abstract

Constraints on the structure of the chromospheres of Alpha Boo, Alpha Tau, and Beta Gru derived from high-resolution IUE spectra are used to investigate the excitation mechanisms of emission lines of neutral sulfur, which are prominent in IUE spectra of late-type giant and supergiant stars. Observational data for the S I lines from other studies are collected and discussed together with data for the prototype M supergiant Alpha Ori. The physical conditions under which the emission lines are excited are summarized, and the atomic processes and models required to interpret the line fluxes are addressed. The processes leading to emission in Alpha Tau, Alpha Boo, and Beta Gru are identified. The requirements for more accurate atomic and spectroscopic data are discussed, and the potential diagnostic value of the various emission lines is assessed.