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

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 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 $\lambda 6545$ features in the two PNe are observed to be redshifted with respect to He II $\lambda 6560$, 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 $N_{\mathrm{HI}}=5\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}$, and expansion speed $v_{\exp }=25\mathrm{km}{\mathrm{s}}^{-1}$ for NGC 6886 and $CF=0.6$, $N_{\mathrm{HI}}=3\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}$, and $v_{\exp }=30\mathrm{km}{\mathrm{s}}^{-1}$ for NGC 6881, respectively.

7 pages, 3 figures, 1 table, accepted for publication in ApJL

References in corpus (4)

• A census of symbiotic stars in the 2MASS, WISE and Gaia surveys
• The Unusual Distributions of Ionized Material and Molecular Hydrogen in NGC 6881: Signposts of Multiple Events of Bipolar Ejection in a Planetary Nebula
• A Survey of 3.3 Micron PAH Emission in Planetary Nebulae
• Line Formation of Raman-Scattered He II $\lambda$ 4851 in an Expanding Spherical H I Shell in Young Planetary Nebulae

Cited by in corpus (1)

• Distribution and Kinematics of H I through Raman He II Spectroscopy of NGC 6302