Ultraviolet spectroscopy of cool stars

Abstract

A review is given of some developments in the study of UV emission-line spectra of cool stars, including the Sun, in recent years. Although early work on solar-line identifications depended heavily on compilations of energy levels from laboratory studies [ Natl. Stand. Ref. Data Ser. Natl. Bur. Stand.35 ( 1971);Natl. Bur. Stand. (U.S.) Circ.488 ( 1950, 1952)], the astrophysical spectra have also led to progress in understanding atomic systems and level excitation processes. The differences in excitation conditions among various structures in the solar atmosphere have permitted the spatial resolution of solar spectra to aid line identifications. Observations of cool low-gravity stars (giants and supergiants) with the International Ultraviolet Explorer have led to the discovery of a variety of fluorescent excitation processes, some resulting in the determination of further energy levels. Some examples of line-identification problems and excitation processes are discussed. This mutually beneficial interaction can be expected to continue when stellar spectroscopy becomes possible from the Hubble Space Telescope and from missions extending to the far-UV and soft-x-ray regions.