2017MNRAS.470..626M

We analyse the characteristics of dust and its distribution in the planetary nebula IRAS 18333-2357 located in M22 using a three-dimensional radiative transfer code HOCHUNK3D. The spectral energy distribution was constructed using ultraviolet, optical and infrared archival data. We also have used Spitzer 8-µm and Wide-field Infrared Survey Explorer (WISE) 22-µm images for our study. Taking into account that the dust shell is carbon-rich, models are presented for amorphous carbon and graphite grains. The spectral energy distribution and the thermal images are fit better by the amorphous carbon model than the graphite model. The stellar photospheric temperature is (50 000 ± 3000) K. IRAS 18333-2357 has a (40 ± 2)° inclined equatorial disc and a thin spherical shell around it, similar to the inner geometry of the born-again planetary nebula A30. Disc inner and outer radii are (2.8 ± 0.1) and (6.0 ± 0.6) arcsec, respectively. The inner and outer radii of the shell are (13.3 ± 1.5) and (25 ± 4) arcsec, respectively. Incorporating a very small grain population, we explain the excess emission in the region of 3-12 µm. The stellar bolometric luminosity is (2460 ± 800) L_☉ and the luminosity reprocessed by dust is (630 ± 200) L_☉. The masses of very small grain population and the classical dust grains are (9.4 ± 0.75) x 10^–4 M_☉ and (3.1 ± 0.24) x 10^–3 M_☉, respectively, resulting in a total dust mass of (4.1 ± 0.31)x 10^–3 M_☉. The derived gas-to-dust mass ratio is 7 ± 1. We discuss a possible origin of IRAS 18333-2357 from a born-again event. The faint envelope seen in the WISE 22-µm image may contain H-rich matter ejected before the H-deficient nebula.