Radiative transfer modelling of dust in IRAS 18333-2357: The only planetary nebula in the metal-poor globular cluster M22

We report results from our 1D radiative transfer modelling of dust in the hydrogen-deficient planetary nebula IRAS 18333-2357 located in the globular cluster M22. A spectral energy distribution wasconstructed 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 fitted by a model which considers a modified Mathis-Rumpl-Nordsieck 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 that 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 finda 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 grainswith size~12åin 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^-3M_odot and for the thermal equilibrium grains it is (1.4 ± 0.60) × 10^-4M_odot, leading to a total dust mass of (1.3 ± 0.91) × 10^-3M_odot. The derived dust-to-gas mass ratio is 0.3 ± 0.21. For a derived bolometric luminosity of (1700 ± 1230) L_odot and an assumed central star mass of (0.55 ± 0.02)M_odot, the surface gravity is derived to be log g = 4.6 ± 0.24. We propose that the progenitor of IRAS 18333-2357 had possibly evolved from an early stellar merger case and the hydrogen-deficient nebula results from a late thermal pulse. The hydrogenrich nebula, which was ejected by the progenitor during its normal asymptotic giant branch evolution, might have been stripped off by its strong interaction with the Galactic halo gas.