Resonance-Enhanced Two-Photon Ionization of Ions by Lyman alpha Radiation in Gaseous Nebulae

Abstract:
One of the mysteries of nebulae in the vicinity of bright stars is theappearance of bright emission spectral lines of ions, which imply fairlyhigh excitation temperatures. We suggest that an ion formationmechanism, based on resonance-enhanced two-photon ionization (RETPI) byintense H Lyman alpha radiation (wavelength of 1215 angstroms) trappedinside optically thick nebulae, can produce these spectral lines. Therate of such an ionization process is high enough for rarefied gaseousmedia where the recombination rate of the ions formed can be10<SUP>-6</SUP> to 10<SUP>-8</SUP> per second for an electron density of10<SUP>3</SUP> to 10<SUP>5</SUP> per cubic centimeter in the nebula.Under such conditions, the photo-ions formed may subsequently undergofurther RETPI, catalyzed by intense He I and He II radiation, which alsogets enhanced in optically thick nebulae that contain enough helium.