Selective excitation of Fe II in the laboratory and late-type stellaratmospheres.

Abstract

Low resolution spectra of cool giants and supergiants observed with the International Ultraviolet Explorer (IUE) satellite contain emission features for which it has been difficult to establish unambiguous identifications. High resolution spectra of RR Tel and Beta Gru (M 3 II) show lines which can be definitely attributed to Fe II. The presence of particular transitions and absence of others suggests that the source of excitation for levels with excitation energy around 10 eV is emission in the H Ly alpha line which is strong and broad in the cool giants and supergiants. New laboratory spectra confirm the excitation route for Ly alpha pumping to a further 4G0 level at about 13 eV, which decays to give the strong feature at 1870 A observed at low spectral resolution in several cool stars.