Discovery of Raman-scattered He II $\lambda$6545 in the Planetary Nebulae NGC 6886 and NGC 6881

TL;DR

This study reports the first detection of Raman-scattered He II at 6545 Å in NGC 6886 and NGC 6881, revealing expanding neutral regions and providing insights into mass-loss processes in young planetary nebulae.

Contribution

It presents the first direct detection of Raman-scattered He II in NGC 6881 and NGC 6886, and models the neutral hydrogen regions' expansion using high-resolution spectroscopy.

Findings

Raman He II 6545 features detected in two young PNe.

Neutral regions are expanding with velocities of 25-30 km/s.

Model fits suggest partial spherical shell geometry with specific column densities.

Abstract

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 BOES (the Bohyunsan Observatory Echelle Spectrograph) and report the discovery of a Raman-scattered He II feature at 6545 {\AA} 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 $\lambda 6545$ features in the two PNe are observed to be redshifted with respect to He II $\lambda6560$, 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 $N_{\rm HI} = 5 \times 10^{20}~{\rm cm^{-2}}$, and expansion speed $v_{\rm exp} = 25~\rm\ km~s^{-1}$ for NGC 6886 and $CF=0.6$, $N_{\rm HI} = 3 \times 10^{20}~{\rm cm^{-2}}$, and $v_{\rm exp} = 30\rm\ km~s^{-1}$ for NGC 6881, respectively.