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 $5000\mathrm{km}{\mathrm{s}}^{-1}$ 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 $10^2\mathrm{km}{\mathrm{s}}^{-1}$ 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, $n_H\sim 10^{9-10}\mathrm{c}{\mathrm{m}}^{-3}$, 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 $N_{HI}=2\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}$ 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.

10 pages with 2 figures, accepted for publication in ApJ Letters

References in corpus (3)

• Toward the Evidence of the Accretion Disk Emission in the Symbiotic Star RR Tel
• On the Profiles and Polarization of Raman Scattered Emission Lines in Symbiotic Stars:II. Numerical Simulations
• On the Polarization of H-alpha Lines Scattered by Neutral Hydrogen in Active Galactic Nuclei

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