Radiative Transfer Modelling of Dust in IRAS 18333–2357: The only planetary nebula in the metal-poor globular cluster M22

Abstract

We report results from our 1-D radiative transfer modelling of dust in the hydrogen-deficient planetary nebula IRAS 18333–2357 located in the globular cluster M22. A spectral energy distribution was constructed from archival UV, optical and IR data including Akari photometry at its 18-, 65-, 90-, 140- and 160-μm bands. An archival Spitzer spectrum shows several aromatic infrared bands indicating a carbon-rich dust shell. The spectral energy distribution is well fit by a model which considers a modified Mathis-Rumpl-Nordsiech grain size distribution and a radial density function which includes compression of the nebula by its interaction with the Galactic Halo gas. The model indicates a significant amount of cold dust, down to a temperature of 50K, is present at the outer edge of the nebula. At the inner edge the dust temperature is 97K. The dust shell has a size of 26 ± 6.3 arcsec. We find a large amount of excess emission, over the emission from thermal equilibrium dust, in the mid-IR region. This excess emission may have originated from the thermally fluctuating dust grains with size ∼ 12 °A in the UV field of the hot central star. These grains, however, come from the same population and conditions as the thermal equilibrium grains. The dust mass of this grain population is (1.2 ± 0.73) × 10−3 M⊙ and for the thermal equilibrium grains it is (1.4 ± 0.60) × 10−4 M⊙, leading to a total dust mass of (1.3 ± 0.91) × 10−3 M⊙. The derived dust-to-gas mass ratio is 0.3 ± 0.21.