Henrik Hartman (Lund Observatory, Lund University, Sweden)
Following the initial detection by Bowen in 1934 of the strong O III lines being due to accidental resonance with strong He II radiation, many strong spectral emission lines are explained as produced by fluorescence. Many of these are Fe II lines pumped by HLya, as a consequence of strong radiation from hydrogen and a favourable energy level structure for Fe II. The lines are observed in many types of object with low density plasma components. The Weigelt condensations in the vicinity of the massive star Eta Carinae is one location where these lines are observed and can be studied in detail, as well as been used for diagnostics.
These gas condensations do not only show a spectrum indicating a non-equilibrium excitation but also non-equilibrium ionization, where the strong hydrogen radiation plays a key role. Early studies identified certain strong lines being the result of Resonance Enhanced Two-Photon Ionization (RETPI). Further investigations suggest that RETPI can be the responsible mechanism for the ionization structure of gas condensation.
We will review the resonance processes, with emphasis on the Eta Carinae spectrum. Large spectral, spatial and temporal coverage is available for this fascinating object, allowing for detailed analysis.
Talk given at 3d CDAMOP, 14-16 December 2011, Delhi University, Delhi, India.