2020ApJ...889....2C

We investigate line formation of Raman-scattered He II at 4851 Å in an expanding neutral, spherical shell that surrounds a point-like He II source located at the center. A new grid-based Monte Carlo code is used to take into consideration the H I density variation along each photon path. In the case of a monochromatic He II emission source, the resultant line profiles are characterized by an asymmetric double-peak structure with a tertiary peak and a significant red tail that may extend to the line centers of He IIλ4859 and Hβ. The peak separation corresponds to the expansion velocity, which we consider is in the range 20-40 km s^–1 in this work. Tertiary red peaks are formed as a result of multiple Rayleigh reflections at the inner surface of a hollow, spherical shell of H I. Due to a sharp increase of scattering cross section near resonance, the overall Raman conversion efficiency is significantly enhanced as the expansion speed increases. In the case of a He II line source with a Gaussian line profile with an FWHM of 30-70 km s^–1, we obtain distorted redward profiles that are due to the increasing redward cross section of H I. A simple application to the young planetary nebula IC 5117 is consistent with a neutral shell expanding with a speed ∼30 km s^–1.