2000ApJ...530L..49L

The young and compact planetary nebula IC 4997 is known to exhibit very broad wings with a width exceeding 5000 km.s^–1 around Hα. We propose that the broad wings are formed through Rayleigh-Raman scattering that involves atomic hydrogen, by which Lyβ photons with a velocity width of a few 10² km.s^–1 are converted to optical photons and fill the Hα broad wing region. The conversion efficiency reaches 0.6 near the line center, where the scattering optical depth is much larger than 1, and rapidly decreases in the far wings. Assuming that close to the central star there exists an unresolved inner compact core of high density, n_H∼10⁹-10¹⁰ cm^–3, we use the photoionization code ``CLOUDY'' to show that sufficient Lyβ photons for scattering are produced. Using a top-hat-incident profile for the Lyβ flux and a scattering region with a H I column density N_HI_=2x10²⁰ cm^–2 and a substantial covering factor, we perform a profile-fitting analysis in order to obtain a satisfactory fit to the observed flux. We briefly discuss the astrophysical implications of the Rayleigh-Raman processes in planetary nebulae and other emission objects.