The excitation of S I emission lines in chromospheres of late-type giant stars⋆

Abstract

Constraints on the structure of the chromospheres of α Boo (K2III), α Tau (K5III) and β Gru (M5III), derived from high-resolution IUE spectra in earlier papers, are used to investigate the excitation mechanisms of emission lines of neutral sulphur. The lines are prominent in IUE spectra of late-type giant and supergiant stars. An atomic model is developed to account for the important excitation processes. Radiative transfer calculations in a chromospheric model for α Tau reveal that chromospheric UV emission in both photoionization continua and strong lines governs the excitation of sulphur owing to the low particle densities encountered in low-gravity stars. Reasonable agreement of observed and computed fluxes is found for the case of α Tau, despite poorly determined atomic rate coefficients. The full potential of the S I lines as spectral diagnostics of the emitting regions will not be realized until better determinations of particular oscillator strengths, photoionization cross-sections and charge transfer ionization rates become available. Nevertheless, the processes controlling the emergent spectra are now broadly understood and the eventual value of the lines as diagnostics of emission-region structure is clear. A potentially valuable method of estimating ‘mean’ chromospheric electron densities is identified.