The Ultraviolet Spectra of Cool Star Chromospheres; Excitation Processes and Plasma Diagnostics

The advent of ultraviolet observations from space has led to rapid advances in our knowledge of the conditions in the chromospheres which surround cool stars. The solar spectrum has been studied at high spatial and spectral resolution for many years and can be used as a model for understanding the spectra of other high gravity (main sequence or dwarf) stars. The most recent spectroscopic developments in this area have come from the flights of US Naval Research Laboratory's High Resolution Telescope and Spectrograph (HRTS) on a series of rockets and on the Spacelab 2 Shuttle flight. Ultraviolet observations of other cool dwarf stars made with the International Ultraviolet Explorer (IUE) satellite have shown that their spectra are broadly similar to that of the sun and that the atomic excitation processes causing the emergent spectra are similar to those in the sun. The main requirement regarding atomic data is for electron-ion collision strengths.

In contrast to the solar chromosphere and corona, where electron temperatures reach T_e > 10⁶ K, observations with IUE have shown that stars with low surface gravities have only cool chromospheres with T_e ≲ 2 × 10⁴ K. The gas density regime is also lower and the excitation of many of the ultraviolet lines is by radiative processes rather than by ion/electron collisions as in the sun. In particular, higher opacities lead to multiple photon scattering and line leakage in semi-forbidden transitions. Several cases of accidental overlap with strong emission lines of H I and O I result in fluorescent excitation of species such as S I, Fe II and CO. Quantitative analyses of such spectra are at an early stage and a wide variety of atomic rate coefficients need to be known more accurately. Further laboratory spectroscopy in the VUV would be valuable for some species.