Photoionization and Raman-scattered He II features in young planetary nebulae

Raman-scattered He II features are known to be present in several young planetary nebulae (PNe) including NGC 7027, NGC 6302, IC 5117 and NGC 6790. These features provide a new spectroscopic window to probe both thick neutral regions and far UV regions near Lyman series. We carry out photoionization model calculations using 'CLOUDY' to explore He II emission strengths dependent on the physical conditions of the central star. The emission nebula is treated as a simple spherical shell with uniform density. It is found that detectable Raman-scattered He II are obtained for T∗ ~ 105 K in the presence of a thick neutral component. We present mock spectra exhibiting Raman He II features based on the photoionization calculations and compare them with observed data. We discuss effective strategies for searching young PNe with Raman-scattered He II emissivities.