Discovery of Raman-scattered He II $\lambda$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 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.
