Abstract
We report the discovery of He II λλ4850 and 6545 Raman-scattered by atomic hydrogen in the high-resolution spectrum of the very compact and young planetary nebula IC 5117 obtained with the ESPaDOnS (Echelle Spectropolarimetric Device for the Observation of Stars) installed on the 3.6 m Canada-France-Hawaii Telescope. By adopting case B recombination atomic data and considering all the fine-structure emission components, we calculate the weighted averages for the line centers of the Raman-scattered features, as well as those for the emission lines He II λλ4859 and 6560 to determine redward velocity shifts of Δv4850 = +29 km s-1 and Δv6545 = +24 km s-1. We compute the Raman conversion efficiency by a Monte Carlo technique adopting a simple scattering geometry, in which the hot central star is surrounded by a hollow cylindrical H I region characterized by thickness T that is in turn parameterized by the H I column density. It is proposed that the central far-UV emission region is covered significantly by the neutral scattering region characterized by the H I column density N 4 × 1021 cm-2. The H I column density is higher by an order of magnitude than the value proposed from 21 cm radio observations. We briefly discuss the importance of Raman scattering as a probe of neutral material around a newly formed hot white dwarf and the mass-loss process occurring in the late stage of stellar evolution.
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