The spectra of IC 418 and IC 5117 from 1.5 to 1.75 microns

Abstract

Near-infrared spectra of planetary nebulae IC 418 and IC 5117 in the wavelength range 1.5-1.75 μm observed with the United Kingdom Infrared Telescope (UKIRT) Cooled Grating Spectrometer (CGS2) are reported. More than 10 members of the Brackett line series are detected superposed on a strong continuum. Analysis of the spectra shows that the observed flux ratios of the Brackett lines are in good agreement with the recombination line theory after correction for extinction by the extinction law of Draine. A He I triplet line at 1.7 μm is identified in both nebulae. A Mg I line at 1.504 μm, and a few Si I lines are also seen in the spectra. After subtraction of the nebular and central star continua by modeling the spectral energy distributions, a near-infrared excess is revealed. After correction for contribution from line emission, this excess can be explained by a hot dust component. However, the steep slope of the H-band spectrum of IC 418 remains unexplained and could be due to an unknown broad feature.