Broad H$\alpha$ Wing Formation in the Planetary Nebula IC 4997

Abstract

The young and compact planetary nebula IC 4997 is known to exhibit very broad wings with a width exceeding $5000 {\rm km s^{-1}}$ around H$\alpha$. We propose that the broad wings are formed through Rayleigh-Raman scattering involving atomic hydrogen, by which Ly$\beta$ photons with a velocity width of a few $10^2 {\rm km s^{-1}}$ are converted to optical photons and fill the H$\alpha$ 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, $n_H\sim 10^{9-10} {\rm cm^{-3}}$, we use the photoionization code `CLOUDY' to show that sufficient Ly$\beta$ photons for scattering are produced. Using a top-hat incident profile for the Ly$\beta$ flux and a scattering region with a H~I column density $N_{HI}=2\times 10^{20} {\rm cm^{-2}}$ and a substantial covering factor, we perform a profile fitting analysis 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.