Discovery of Raman-scattered He ii λ 6545 in the Planetary Nebulae NGC 6886 and NGC 6881

Young planetary nebulae (PNe) retain a large amount of neutral material that was shed in the previous asymptotic giant branch stage. The thick H i region in young PNe can be effectively probed by illuminating far-UV radiation that may be inelastically scattered to appear in the optical region. Raman-scattered features are unique spectroscopic tracers of neutral regions that can be used to investigate the mass-loss process in young PNe. We conduct high-resolution spectroscopy of young PNe using the Bohyunsan Observatory Echelle Spectrograph (BOES) and report the discovery of a Raman-scattered He ii feature at 6545 Å in NGC 6886 and NGC 6881. The Raman-scattered He ii features have been found in only five PNe so far, and in particular, it is the first direct detection of an H i component in NGC 6881. The Raman He ii λ6545 features in the two PNe are observed to be redshifted with respect to He ii λ6560, indicating that the neutral regions are expanding. We perform line profile analyses using the grid-based Monte Carlo code "STaRS" by assuming a neutral hydrogen region in the shape of a partial spherical shell expanding radially. The profiles are well fitted with the model parameters of covering factor CF = 0.3, H i column density NHI = 5 × 1020 cm−2, and expansion speed ${v}_{\exp }=25\,\,\mathrm{km}\,{{\rm{s}}}^{-1}$ for NGC 6886 and CF = 0.6, NHI = 3 × 1020 cm−2, and ${v}_{\exp }=30\,\mathrm{km}\,{{\rm{s}}}^{-1}$ for NGC 6881, respectively.