Three-dimensional Monte Carlo dust radiative transfer study of the H-poor planetary nebula IRAS located in M22

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1 doi: /mnras/stx1071 Three-dimensional Monte Carlo dust radiative transfer study of the H-poor planetary nebula IRAS located in M22 C. Muthumariappan Indian Institute of Astrophysics, Bangalore , India Accepted 2017 May 2. Received 2017 April 28; in original form 2016 February 10 1 INTRODUCTION IRAS is a halo planetary nebula (PN) located in the metal-poor ([Fe/H] = 1.7; Lee, Demarque & Zinn 1994) globular cluster M22 (NGC 6656) and it was discovered as a strong farinfrared source by Gillett et al. (1986). M22 is one of the nearest globular star clusters and it is located in Sagittarius, 400 pc below the Galactic plane. It has an angular diameter of about 32 arcmin and a core diameter of 2.66 arcmin (Hartwick, Cowley & Grindlay 1982) and its main sequence turn-off mass is 0.83 M, implying an age of 12 Gyr (Sippel & Hurley 2013). IRAS is located in the core of M22, about 1 arcmin from the centre of the cluster, which has a well-determined distance of 3.1 kpc (Frogel, Cohen & Persson 1983). PNe are very rare objects to see in globular clusters as their lowmass stars do not evolve fast enough to ionize the ejected envelopes before their dissipation into the interstellar medium (ISM). There muthu@iiap.res.in ABSTRACT We analyse the characteristics of dust and its distribution in the planetary nebula IRAS 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 ( ± 3000) K. IRAS 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) 10 4 M and (3.1 ± 0.24) 10 3 M, respectively, resulting in a total dust mass of (4.1 ± 0.31) 10 3 M. The derived gas-to-dust mass ratio is 7 ± 1. We discuss a possible origin of IRAS 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. Key words: radiative transfer stars: AGB and post-agb stars: evolution dust, extinction planetary nebulae: individual: IRAS have been only four PNe detected in approximately 150 globular clusters known in our Galaxy (Jacoby et al. 1997). These peculiar PNe help us to understand the evolution of low-mass stars at very low metallicity and the evolution of stellar mergers. It is now known that dust forms even in an extremely metal-poor environment (Evans et al. 2003; Boyer et al. 2006). PNe located in globular clusters help us to understand the nature of the grains ejected from metal-poor stellar systems. Unlike other globular cluster PNe, IRAS is bright, large in size and provides a unique opportunity to study the geometrical distribution of dust required to probe its origin. M22 is moving with a space velocity of 158 km s 1 with respect to the Galactic Centre (Cudworth 1990). The ram pressure of the Galactic halo gas (Halo gas, hereafter) is causing a strong asymmetry in the PN morphology (Borkowski, Sarazin & Soker 1990; Borkowski &Harrington1991, hereafter BH91) stripping the nebular gas away from the star. The presence of large oxygen abundance with respect to hydrogen in the optical nebula was proposed by Gillett et al. (1989), whereas an analysis of the Spitzer infrared spectrograph (IRS) spectrum by Muthumariappan, Parthasarathy & C 2017 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society

2 3D MC dust radiative transfer study of IRAS Ita (2013; MPI13 hereafter) showed many aromatic infrared bands showing a carbon-rich dust shell. The central star of this PN was suggested to be H-deficient and He-rich with T eff of K by Cohen & Gillett (1989, CG89 hereafter). However, Harrington & Paltoglou (1993, hereafter HP93) showed that its spectrum closely resembles the optical spectrum of SdO star KS 292 and suggested that the star is H-rich with T eff of K. There is still no detailed study to confirm the photospheric temperature and H-deficiency of the central star of IRAS Another interesting aspect of this PN comes from its spectrum, which shows the absence of H and He emission lines but the presence of [O III] and[ne III] lines (Gillett et al. 1989). Photoionization of H and He or other elements cannot provide the required energy for the free electrons to account for the observed forbidden line fluxes of [O III] and[ne III] (Gillett et al. 1989). An additional source of energy should indeed be present for this PN (BH91). BH91 and, later, Dopita & Sutherland (2000) presented self-consistent models of PNe by including the effect of dust heating through photoelectric emission. Dopita & Sutherland (2000) found a strong influence of dust on the thermal structure and the emission-line spectra of PNe and they showed that the efficiency of dust heating depends on the total area projected by grains to the stellar ultraviolet (UV) field. A study of the dust-heated PN Abell 30 (A30 hereafter) was performed later by Ercolano et al. (2003). A detailed dust emission model of IRAS was carried out by BH91 using IRAS fluxes at its four far-infrared (FIR) bands. They claimed that the photoelectric emission from dust grains heats the nebular gas to a temperature of 10 4 K and accounts for the [O III]and[NeIII] line fluxes. MPI13 extended the study of BH91 using a one-dimensional (1D) radiative transfer modelling of the spectral energy distribution (SED) of IRAS constructed with a longer wavelength baseline (from far-uv to 160 μm in the FIR). They derived the nebular and the central star parameters, addressed the nature of different grain populations and discussed the evolutionary nature of IRAS With the advent of Spitzer IRAC narrow-band images and Wide-field Infrared Survey Explorer (WISE) broad-band images, this study can further be extended to three dimensions. The distribution of dust grains can be examined in more detail to better constrain the proposed evolutionary nature of this object. We perform this study using a three-dimensional (3D) Monte Carlo radiative transfer code by fitting the SED from the far-uv to the FIR and we also compare the model images with the observed images at the Spitzer and WISE bands. We also include a transiently heated polycyclic aromatic hydrocarbon (PAH) and very small grain (VSG) population in the radiative transfer calculation, which has not been done before for this PN. We derive the nebular physical and geometrical parameters and we constrain the masses of classical dust and PAH/VSG. We further discuss the influence of these grain populations on the nebular thermal balance. 2 ARCHIVAL DATA The SED of IRAS was constructed from the photometric and spectroscopic archival data in the UV to the FIR wavelength region, which were obtained using ground-based and space-based observations. We took the same data points as those used by MPI13 for their SED. The UV data were obtained from International Ultraviolet Explorer (IUE) short-wavelength-prime (SWP) and long-wavelength-prime (LWP) and Hubble Space Telescope (HST) Goddard High-Resolution Spectrograph (GHRS) observations, which are available at the Mikulski Archive for Space Telescopes 1 (MAST). The optical and near-ir data were from the Deep Near Infrared Survey (DENIS; Epchtein et al. 1994) and the Two-Micron All-Sky Survey (2MASS;Cutri et al.2003) archive, and also from Gillett et al. (1989). A Spitzer IRS spectrum at short-low (SL, with wavelength coverage from 5.13 to μm) and longlow (LL, with wavelength coverage from 13.9 to 39.9 μm) modules was obtained from the Spitzer Heritage Archive. 2 The InfraRed Astronomical Satellite (IRAS; Neugebauer 1984) fluxes at its 12-, 25-, 60- and 100-μm bands and the WISE (Wright et al. 2010; Cutri et al. 2012) fluxes at its 3.4-, 4.6-, 12- and 22-μm bands were used to span the mid-ir and FIR emission. We also used the archived fluxes from Akari (Ishihara et al. 2010) at its 18-, 65-, 90-, 140- and 160-μm bands to trace the emission from cold dust. The DENIS, 2MASS, WISE and Akari data are available at the NASA/IPAC Infrared Science Archive. 3 See table 1 of MPI13 for the data summary and the text for more details. IRAS is an extended source located near the centre of the cluster and it is likely to suffer from flux contamination by other sources in the cluster. For details on the data calibrations and a discussion on source confusion and flux contamination, see section 2 of MPI13. In addition to the photometric and spectroscopic data, we have also obtained the archival images of IRAS at the Spitzer IRAC 8-μm band from the Spitzer Heritage Archive to trace the distribution of PAH/VSG emission. The WISE image at the 22-μm band was obtained from the NASA/IPAC Infrared Science Archive to trace the cooler dust distribution. The WISE images at 3.4, 4.5 and 12 μm show several cool stellar objects, which are unresolved by IRAS, which contaminate the nebular flux significantly at these bands. The flux contamination at the WISE 12-μm band was estimated as 11 per cent by MPI13. The nebular flux increases and the contamination decreases with wavelength. At 22 μm, the stellar contamination is much lower than the nebular contribution, as seen from the WISE 22-μm image. The Spitzer IRAC 8-μm image also does not suffer from contamination, because of its narrow bandwidth and the presence of VSG continuum emission and PAH line emission at 8.33 and 8.6 μm, which are dominant in this band. Hence, we chose 8- and 22-μm images for modelling. 3 THREE-DIMENSIONAL RADIATIVE TRANSFER MODELLING 3.1 Three-dimensional Monte Carlo radiative transfer code Hochunk3D To model the SED of IRAS and to reproduce its observed mid-ir images, we have used a 3D Monte Carlo dust radiative transfer code HOCHUNK3D (Whitney et al. 2013). It solves the radiation transfer equations for dust that is in radiative equilibrium and is illuminated by a central source. HOCHUNK3D allows two separate discs, a shell and a bipolar cavity for geometry. The discs and the shell can coexist, each with different grain properties. The code was updated from its earlier version (see Whitney et al. 2003a,b; Whitney et al. 2004) with the inclusion of greater physics enhancement (Whitney et al. 2013). This includes the option of 1 Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS

3 628 C. Muthumariappan simple power-law envelope geometry and the consideration of transiently heated grains (with a radius smaller than 200 Å) and external illumination from the ISM. The grid structure was also modified to allow multiple dust species in each cell. For this study, we used the option of power-law radial density distribution for the shell and we considered the presence of the transiently heated PAHs/VSG in the shell and in the disc. An opacity file for the circumstellar matter with dust is made separately and is supplied to HOCHUNK3D for the radiative transfer computation. The opacity file contains averaged values of scattering coefficients, of extinction coefficients and of mass absorption coefficients (opacity) for different wavelengths. The opacity file used for the PAH/VSG population (draine_opac_new.dat) was computed by Bruce Draine (Draine & Li 2007) and is available at It was computed taking two lognormal size distributions, one with grain radii below 20 Å and another for grains with radii from 20 to 200 Å. PAHs constitute a mass fraction of 4.5 per cent of this grain population and they have the size-dependent fractional ionization of the diffuse ISM (Wood et al. 2008). The presence of the 8.6-μm featureinthespitzer IRS spectrum of MPI13 implies the existence of ionized PAHs. The PAH/VSG grain population is not in thermal equilibrium with the radiation field. However, their emissivities can be computed based on specific energy absorbed in each grid cell and assuming that the emissivity is the function of the mean intensity of the radiation field, which approximates the spectral shape to the first order (Robitaille et al. 2012; Whitney et al. 2013). The code uses the pre-computed emissivity tables of Draine & Li (2007) for different specific energy. As a Monte Carlo energy pocket is absorbed by a PAH/VSG, the pocket is reprocessed, sampling a new frequency from precomputed emissivity tables. A detailed discussion on this can be found in Wood et al. (2008). The updated version of HOCHUNK3D is hence quite useful in modelling the circumstellar shells around evolved stars, which have thermally fluctuating dust. The code uses the method of Lucy (1999) to calculate the dust temperature very efficiently by summing up photon path-lengths. The temperature of the grid cell is updated when an iteration of simulation is completed, and the temperature converges in three to five iterations; this efficiency is due to the fact that flux is conserved exactly across all surfaces (Lucy 1999). By constructing Monte Carlo radiation fields that are rigorously divergence-free, rapid convergence is achieved with the temperature correction procedure in this method. We have performed the Lucy temperature correction of dust with five iterations. The Spitzer 8-μm image of the PN shows an arc-like structure around its central star (see Fig. 1). The Spitzer IRS spectrum analysed by MPI13 shows strong PAH features at 8.33 and 8.61 μm located on the top of the VSG continuum emission, which were coveredbythe8-μm band. This indicates the presence of PAHs in an arc-like structure. The WISE 22-μm image shows a bright, nearly spherical structure around this, showing the presence of cooler dust distributed in a spherical shell. For comparison, we have taken the Spitzer 8-μm and the WISE 22-μm images of a well-known PN, A30. There is a striking similarity between these two PNe in their mid-ir morphologies, as shown in Figs 1 and 2. The inner region of A30 is H-poor and it has an equatorial ring with knots and filaments distributed around it in a spherical geometry (Borkowski et al. 1994). The equatorial ring is seen as an arc-like structure in the 8-μm image and the spherical shell is seen in the 22-μm image of A30 (see Fig. 2). Hence, the arc-like structure seen in the 8-μm image of IRAS is possibly a disc that is surrounded by a shell seen in its 22-μm image. There is a faint envelope in the WISE 22-μm image of IRAS , which consists of a mildly elliptical structure near the inner shell and a highly irregular structure in the outermost regions (see Fig. 1 and also see Section 5.1 for a discussion). The identified morphological components of the PNe IRAS and A30 are shown in the schematic diagrams in Figs 1 and 2, respectively. 3.2 Model description We have invoked an axisymmetrical geometry in the code to model the morphological structures of IRAS and to simulate the observed SED and the mid-ir images. We employ a spherical shell and a disc inside the shell with the illuminating star at the centre of the geometry in our model. The outer envelope seen in the 22-μm image is not included in the model. The input stellar radiation field is defined by a model atmosphere taken from the ATLAS9 grids of models (Castelli & Kurucz 2003). The model atmosphere spectrum was taken, corresponding to T eff = K, log g = 4.5 and [Fe/H] = 1.5 (close to the metallicity of the cluster). The shell has an inner radius of R sh,in and an outer radius of R sh,out. The density in the shell falls radially outward with a ρ 0 /r n function, where ρ 0 is the fiducial density at 1 au (in g cm 3 )and n is the density exponent. The inner and outer radii of the disc are R disc,in and R disc,out. The disc density structure is parametrized with a radial power law and a vertical Gaussian structure (Shakura & Sunyaev 1973; Lynden-Bell & Pringle 1974). The density in the disc is described by ρ = ρ 0 (R /ω) α e 1/2(z/hω)2.0 (1) where ω is the radial coordinate of the disc mid-plane and the scale height increases with radius as h = h 0 (ω/r ) β. For our models, we adopt flaring parameter β = 1.25 based on the accretion disc models at hydrostatic equilibrium (D Alessio et al. 1999), which was used to describe the structure of the accretion disc (Wood et al. 2001;Thi, Woitke & Kamp 2011; Muthumariappan & Parthasarathy 2012). We have taken the value of α = 3(β 0.5) = 2.25 (Shakura & Sunyaev 1973). The scale height of the disc, at a radial distance of 100 au, is an input variable parameter. For a given dust type (i.e. amorphous carbon or graphite), the opacity file for the circumstellar matter was made using a Mie scattering code following the methods described by Bohren & Huffman (2004) (bhmie_v0.1.0; available at hyperion-rt/bhmie), assuming that the grains are spherical. The optical constants of amorphous carbon were obtained from Zubko et al. (1996) for their ACAR sample, which has a featureless opacity curve and a slope that is similar to the opacity curves commonly used for C-rich circumstellar envelopes (Hanner 1988; Rouleau & Martin 1991). For graphites, the optical constants were taken from Draine & Lee (1984). Grains follow a modified MRN size distribution function (Mathis, Rumpl & Nordsieck 1977), which is described by a minimum grain radius a min, a maximum grain radius a max and a power-law exponent q.we have considered 100 grain radii between a min and a max, spaced in equal intervals in the logarithmic scale. Dust parameters were calculated for 500 wavelengths from 0.01 to 1200 μm, equally spaced in the logarithmic scale and for 181 scattering angles. The opacity (κ, in cm 2 g 1 ) of the circumstellar matter at different wavelengths was calculated for a gas-to-dust mass ratio, which is an input variable parameter. The circumstellar matter has both gas and dust where the mass is dominated by gas and the opacity is dominated by dust. As the gas provides negligible opacity compared to the dust, the chemical composition of the gas does not play a role here. For a given

4 3D MC dust radiative transfer study of IRAS Figure 1. The left panels show observed images of IRAS at 5007 Å taken from Gillett et al. (1989) (top), at the Spitzer IRAC 8-µm band (middle) and at the WISE 22-µm band (bottom). The right panels show a schematic diagram of the model (top) and the amorphous carbon (amc) model images of IRAS at the Spitzer IRAC 8-µm band (middle) and at the WISE 22-µm band (bottom). The morphological components of the PN are marked in the observed images and in the schematic diagram. The WISE 22-µm image shows the direction of the transverse velocity (V T ) of the PN. The asterisk marks the location of the central star.

5 630 C. Muthumariappan Figure 2. The left panels show observed images of the PN A30 at 5007 Å taken from Borkowski et al. (1994) (top) and at the Spitzer IRAC 8-µm band (bottom). The right panels show a schematic diagram of the morphology of A30 (top) and the observed image at the WISE 22-µm band (bottom). The morphological components of the PN are marked in the observed images and in the schematic diagram. The asterisk marks the location of the central star. circumstellar mass, the gas-to-dust mass ratio decides the amount of dust present in it and hence its opacity. We have understood that the optical nebula is the disc traced by PAH/VSG emission (see Section 5.1 for a discussion). The total mass of the disc (m disc )is fixed at 10 2 M, which is the maximum possible mass of the optical nebula estimated by BH91, and our mass estimations rely on this. The gas-to-dust mass ratio of the disc can thus be constrained from the input opacity. We assume that the shell also has the same gas-to-dust mass ratio, which helps us to derive the shell mass (see Section 4.1.2). The fraction of total dust mass residing in the PAH/VSG population (mf VSG in per cent) is an input variable parameter, which strongly influences the fitting of the PAH emission features and mid-ir excess emission. The SED and the thermal images of the nebula were calculated for an angle of disc inclination (θ incl,90 is edge-on viewing of the disc). We have explored the possible ranges in the parameters T eff, n, the disc scale height, the gas-to-dust mass ratio, mf VSG, R sh,in, R sh,out, R disc,in, R disc,out and θ incl to obtain the best model fit to the observed SED. Approximate values of the geometrical parameters such as the disc and shell radii and θ incl were initially estimated from the thermal images (see Table 1). The shell outer radius was estimated from the 22-μm image. Assuming that the disc is circular in shape and is inclined to the line of sight, the disc radii and the disc inclination θ incl were estimated by tracing the ellipse to the 8-μm image. These estimates were then used by the code as input variable parameters to derive their best values. Flux in the N and Q

6 3D MC dust radiative transfer study of IRAS Table 1. Parameters of the amorphous carbon and graphite models of IRAS Parameter values estimated from thermal images are given in the table. See text for details. Parameter The amc model The graphite model Observation Input parameters T eff (K) ± ± (CG89) log g E(B V) (CG89); 0.48 (HP93) Grain radius (µm) a min = 0.02 a min = 0.02 a max = 0.11 a max = 0.15 Power-law exponent q = 3.5 q = 3.5 Disc radius (arcsec) R disc,in = 2.8 ± 0.1 R disc,in = 0.76 R disc,in = 3 R disc,out = 6.0 ± 0.6 R disc,out = 3.33 R disc,out = 7.0 Shell radius (arcsec) R sh,in = 13.3 ± 1.5 R sh,in = R sh,out = ± 4 R sh,out = R sh,out = Inclination (40 ± 2) Shell radial density 1/r 1.7 ± /r 1.7 Floor density (g cm 3 ) ρ 0 (g cm 3 ) scale height at 100 au (au) 10 ± 2 10 Derived parameters Disc grain temperature (K) T in = 115 T in = 160 T out = 68 T out = 90 Shell grain temperature (K) T in = 74 T in = 103 T out = 45 T out = 60 L d (L ) 630 ± L total (L ) ± mf VSG (%) 25 ± 1 13± 1 M d,classical (M ) (3.1± 0.24) M d,vsg (M ) (9.4± 0.75) M d,disc (M ) M d,env (M ) (2.7± 0.31) M d,total (M ) (4.1± 0.31) M env (M ) (1.86 ± 0.22) M total (M ) (2.86 ± 0.22) Gas-to-dust mass ratio 7 ± 1 10 Grid parameters Number of radial cells Number of θ cells bands is dominated by the disc and hence strongly constrains the disc parameters. Shell contribution becomes significant after 20 μm and the flux at longer wavelengths of the SED is dominated by the shell and is used to derive shell parameters. If the acceptable geometrical parameters could not reach a good fit to the SED, then a min and a max were varied. The model SED was calculated for 500 wavelengths from 0.01 to 1200 μm with a uniform spacing of in the logarithmic wavelength scale. This spacing is sufficient to sample the PAH spectral features in the 10-μm region. In addition to the computed SED, the code also returns component model flux originating from the disc and from the shell. The computed fluxes are scaled by a factor L /4πd 2,whered = 3.1 kpc. A blackbody curve for an effective temperature of 4100 K was appropriately scaled and added to the computed SED to account for the observed excess emission in the near-ir region, which is due to the M0-type field star. The model SED was then subjected to an interstellar extinction with a wavelength-dependent extinction curve given by Fitzpatrick & Massa (2007) fora V = 1.4 mag (which corresponds to E(B V) = 0.45 mag for R V = 3.1). This was required to make a good fit at the far-uv region. An additional extinction of A V = 0.6 mag was included for the field star to account for the extinction due to the Galactic foreground ISM and due to the PN (see MPI13). Images at different IR bands are also synthesized and are given by the code. For our study, we have taken the model images at the Spitzer 8-μm and WISE 22-μm bands. While the 8-μm image traces the flux from the disc, the 22-μm image has fluxes from both disc and shell (the bandpass of the WISE 22-μm band extends up to 28 μm). As noted earlier, because of contamination, the WISE 12-μm band image was not modelled. Further, the 22-μm image can better trace the cool dust distribution (there are no resolved images taken at further longer wavelengths available in the archive). The synthesized images were convolved with their respective filter transmission functions. The WISE 22-μm band transmission function is not inbuilt with the code. We have taken this from Wright et al. (2010) and, as a user-defined filter transmission function, we have used a facility that is available with the code. The images were then convolved with their respective telescope point spread functions (PSFs). The PSF of the Spitzer IRAC image at its 8-μm band has a FWHM of 2.05 arcsec. The FWHM of the WISE 22-μm image is 12 arcsec. The nebular and grain parameters were constrained by making the best possible fit to the observed SED as well as to

7 632 C. Muthumariappan Figure 3. Variation of mass absorption coefficients of the circumstellar matter for the amc model (solid line) and the graphite model (dashed line) with wavelengths. the thermal images at 8 and 22 μm. There is no degeneracy in the parameters that can fit both the SED and the images together, within the attempts we have made. Stellar mass could not be constrained by our models, which considered a power-law density function for the shell. 4 RESULTS The aromatic infrared bands seen in the Spitzer IRS spectrum analysed by MPI13 indicate a carbon-rich dust chemistry for IRAS Hence, we have worked out 3D radiative transfer models taking the classical dust either as amorphous carbon grains or as graphites. We discuss these two models in detail in the following subsections. 4.1 Amorphous carbon model For the radiative transfer model with amorphous carbon grains (i.e. the amc model), the opacity file was computed with grain radii a min = 0.02 μm anda max = 0.11 μm, and for a grain density of 2.1 g cm 3. The power-law exponent q is fixed at 3.5. An increase in the upper limit of the grain radius results in the mid-ir flux peaking at a longer wavelength than observed. The lower limit of the grain radius is fixed at the radius of the largest grain in the PAH/VSG population. For the amc model, the gas-to-dust mass ratio is 7 ± 1. Fig. 3 shows the plot of mass absorption coefficient, κ, as a function of wavelength for this dust model. In addition, to account for the emission from the PAH/VSG population, the opacity file, draine_opac_new.dat, was supplied to the code. We have run models by taking the PAH/VSG population to be present only in the disc and also present both in the disc and in the shell. The best fit to the Spitzer IRS spectrum can be achieved if this grain population is present both in the disc and in the shell with nearly equal mass fraction. Also, the closest region from the central star at which these grains are located is at R disc,in. The disc inclination angle is (40 ± 2). The Monte Carlo radiation transfer was computed initially with three million photons from the star (each model takes a CPU time of 830 s on an Intel-5 computer with a single processor having 2-GHz clock speed), exploring the full scope of the input parameter space. When the best-fitting model parameters were achieved, the radiative transfer was computed with 100 million photons to obtain the final SED and the mid-ir images with reasonably low noise. The final model takes a CPU time of about s. Table 1 lists the values of the input and the output parameters corresponding to the best-fitting amc model. The model SED is plotted in Fig. 4(a) against the observations. Flux coming from the shell and from the disc is also shown in the figure, along with the scaled blackbody flux of the field star. As can be seen in this figure, the model SED fits the observations well from the far-uv to the FIR region, and the spectral features of Spitzer IRS spectrum are also reproduced reasonably well. However, the plateau features seen in the 8 12 μm region and around 15 μm are not well fit. Carriers of these plateau features are not known and a discussion on the possible carriers of the IR plateau features can be found in Kwok, Volk & Bernath (2001). The synthesized mid-ir images at 8 and 22 μm, convolved with their respective PSFs, are shown along with the observed images at these wavebands in Fig. 1. The disc is tilted in the north south direction. The spatial locations of emission from the disc and from the inner shell are not resolved in the WISE 22-μm image, as found from our model. Fig. 5 shows the plots of flux distribution across the observed and the amc model images at the WISE 22-μm band. They match well in their inner region up to a 12-arcsec radius and the model curve deviates from the observation afterwards the maximum deviation, with the model flux half of the observed flux, is seen at a 23-arcsec radius where the observed flux is 12 per cent of its peak value. This is unlikely because of the faint, highly asymmetrical outer envelope seen in the WISE 22-μm image, as the inner shell radius is about 30 arcsec (see Section 4.1.1). More likely, the enhanced brightness at the edge of the shell could be a result of its interaction with the slowly expanding outer envelope Stellar and nebular parameters The wavelength-integrated flux derived from the model SED (before applying the interstellar extinction correction) is erg cm 2 s 1, which corresponds to a luminosity of 2460 L at a distance of 3.1 kpc. Thermal emission originating from both PAH/VSG and classical grains is erg cm 2 s 1, which corresponds to a luminosity of 630 L. Emission from the thermally fluctuating PAH/VSG grains dominates over the classical dust emission approximately in the 3 12 μm wavelength region of the SED, as found from our models that include and do not include PAH/VSG. The SED fit and the synthesized image at the Spitzer 8-μm band for the amc model without PAH/VSG population are shown in Figs 6(b) and (e), and can be compared with Figs 6(a) and (d) that show the amc model. Classical dust emission dominates after around 20 μm of the SED. In the amc model, the luminosity reprocessed by the classical dust and the PAH/VSG population is roughly 18 and 7 per cent of the total luminosity, respectively. A significant contribution to the uncertainty in the derived luminosities comes from the uncertainty associated with the stellar photospheric temperature, some of which comes from the adopted interstellar extinction to fit the observed SED. The central star photospheric temperature is not yet known accurately, and it has been suggested that it has a possible value between and K (CG89). However, taking the photospheric temperature to be much larger than or much smaller than K

8 3D MC dust radiative transfer study of IRAS Figure 4. (a) The amc model SED fit to the observation (left) and (b) the graphite model SED fit to the observation (right). Data description: the diagonal crosses represent the HST FOS spectrum, the open triangles represent the IUE spectrum, the open stars are the optical and IR photometry from Gillett et al. (1989), the filled stars are the DENIS photometry, the filled squares are the WISE photometry, the filled triangles represent the Spitzer IRS spectrum, the open hexagons are the IRAS photometry and the filled hexagons are the Akari photometry. We also show the model SED fit to the observed data with amorphous carbon grains (solid line) and the component model spectra originating from the disc (long-dashed line) and from the shell (short-dashed line). The blackbody curve of the field star is shown by the dotted line. Flux short-ward of 1 µm is the central star flux attenuated by the nebula. Figure 5. Normalized flux distribution across the IRAS image at the WISE 22-µm band. The observed curve is shown by the solid line, the long-dashed line represents the amc model curve and the dotted line is for the graphite model. does not fit the SED well, both in the IR and in the optical regions. An admissible variation in T eff is about ±3000 K, within which the models can find a reasonably good fit. This uncertainty in T eff and the possible uncertainty in E(B V) result in an uncertainty of ±800 L in the derived total luminosity. An additional uncertainty can come from the uncertainties associated with the optical constants of the grains, which is taken to be insignificant. The distance is assumed to be accurate and causes no significant uncertainty in the values of the derived parameters. Hence, we arrive at a bolometric luminosity of (2460 ± 800) L for IRAS The total luminosity reprocessed by PAH/VSG and classical dust grains is hence (630 ± 200) L. This means that the stellar luminosity unprocessed by the nebula is (1830 ± 820) L. The luminosity of the [O III] line is 0.1 L (BH91), which with the luminosity reprocessed by thermally fluctuating grains gives the efficiency at which these grains heat the nebula by photoelectric emission. This efficiency, η,is , which is close to the typical value it can have ( 10 3 to 10 2 ; BH91). The bolometric luminosity of IRAS from our 3D model is larger than the value obtained from the 1D analysis of MPI13 (1700 ± 1230) L. The luminosities reprocessed by classical dust and by PAH/VSG, as derived by MPI13, are (510 ± 370) and (330 ± 240) L, respectively. The inner and outer shell radii of IRAS , R sh,in and R sh,out, have their respective values of (13.3 ± 1.5) and (25 ± 4) arcsec. The measured outer radius of the spherical shell, where the intensity count meets the 3σ background level in the WISE 220 μm image (with a FWHM of the telescope PSF of 12 arcsec), is about 30 arcsec. The outer radius from the model is 25 arcsec (see Table 1), which after convolving with the telescope PSF function has a value of 29 arcsec. Uncertainties associated with these derived values were obtained from their possible ranges, resulting in acceptable fits to the SED and the radial flux distribution. We note that the value of R disc,in is very sensitive to fit the SED in the mid-ir region. The nebular optical size was reported as 10 7 arcsec (Gillett et al. 1989) andbh91 adopted a diameter of 12 arcsec for their modelling. MPI13suggested that the nebula is extended to an outer edge diameter of (26 ± 6.3) arcsec. In our 3D model, we propose that the shell is further extended to a 50-arcsec diameter. The faint halfmoon shaped envelope seen in the WISE 22-μm image surrounds the shell. In this study, we take that the optical nebula detected by BH91 is located in the disc (see Section 5.1 for a discussion).

9 634 C. Muthumariappan Figure 6. The top panels show the Spitzer IRS spectrum (filled triangles) fitted by (a) the presented amc model, (b) the amc model without PAH/VSG population and (c) the amc model with PAH/VSG population outside the optical nebula. The model SED (solid line) is shown with the disc component (long-dashed line) and the shell component (short-dashed line). The bottom panels show the Spitzer 8-µm image of (d) the presented amc model, (e) the amc model without PAH/VSG population and (f) the amc model with PAH/VSG population outside the optical nebula. The asterisk marks the location of the central star Gas-to-dust mass ratio and dust mass As noted earlier, the derived gas-to-dust mass ratio for the bestfitting amc model is 7 ± 1. This is two to three times larger than the value derived using the 1D model by MPI13 and is about 30 times smaller than the typical value found for a PN, 200. The gas-to-dust mass ratio and the maximum possible disc mass give an upper limit for the dust mass in the disc. The shell mass was derived from the fiducial density, its inner and outer radii and the radial density function. It was calculated by integrating the mass present in a spherical shell with an infinitesimal radial thickness for integration limits from R sh,in to R sh,out. The possible ranges in R sh,in, R sh,out and n (± 0.03) give an estimate of the uncertainty for the shell mass. The shell mass and the gas-to-dust mass ratio give the dust mass in the shell. From this, the masses of the PAH/VSG population and the classical dust present in the disc and in the shell were derived. The masses of different nebular components are given in Table 1. The mass fraction of PAH/VSG in the disc (and in the shell) is larger than the value suggested for the diffuse ISM (19.2 per cent) by Draine & Li (2007). The total mass of the system is (2.86 ± 0.22) 10 2 M. A good fraction of the total mass resides in the shell but could not be observed in optical wavelengths (see Section 5.1 for more discussion). For a comparison to our estimates, the total mass of classical dust obtained by MPI13 is (1.4 ± 0.60) 10 4 M and their estimated mass of the thermally fluctuating grains from a classical approach is (1.2 ± 0.70) 10 3 M. We derive the mass fractions of the classical dust and the PAH/VSG population to be 77 and 23 per cent, respectively. The fractions of stellar luminosity reprocessed by these grain populations are 18 and 7 per cent. From the estimates of MPI13, the mass fractions of classical dust and the PAH/VSG population are 92 and 8 per cent and the fractions of stellar luminosity reprocessed by them are 30 and 19.5 per cent, respectively. The 1D model of MPI13 and our 3D model arrive at similar conclusion that the classical grains dominate in reprocessing the stellar flux over the PAH/VSG population. However, the mass fraction of the PAH/VSG population and its reprocessed stellar luminosity fraction were overestimated by the classical approach of MPI13, which used spherical geometry for grain distribution. Our 3D model, which considers this grain population for radiative transfer calculations in a more realistic way, should give more accurate values. 4.2 Graphite model For the graphite model, the opacity file was made for graphite grains with a min = 0.02 μm anda max = 0.15 μm and for a grain density of 2.26 g cm 3. The power-law exponent q is 3.5. We need to increase a max in this case as the geometrical parameters could not yield a good fit to the observations in the FIR region. A relatively better fit is found by increasing a max and shifting the peak to a longer wavelength. The lower limit of the grain size is fixed at

10 3D MC dust radiative transfer study of IRAS the largest grain size of the PAH/VSG population. The opacity of circumstellar matter was calculated for a gas-to-dust mass ratio of 10. Fig. 3 shows the variation of mass absorption coefficient as a function of wavelength for the graphite model. To account for the emission from PAH/VSG, the opacity file draine_opac_new.dat was supplied to the code, similar to the amc model. The Lucy temperature correction for the graphite model was performed with five iterations. We have explored the full scope of the parameter space and we have run each model with three million photons. For the presented model, the radiative transfer was computed with 100 million photons to obtain the SED and the mid- IR images with reasonably low noise. We reach a better fit to the Spitzer IRS spectrum if we take that PAH/VSG are present both in the disc and in the shell with nearly equal mass fraction, similar to a result we found for the amc model. Also, the minimum radius at which these grains are located is R in,disc and the disc inclination angle is 40. We list the values of input and output model parameters corresponding to the best possible graphite model in Table 1. The model SED is plotted in Fig. 4(b) along with the observed SED. Flux components representing the shell and the disc are also shown in this figure. The mass of the shell was derived, in the same way as for the amc model, to be M. This is 3.4 times larger than the disc mass (0.01 M ) implying a total mass of M. A gas-to-dust mass ratio of 10 for the graphite model implies a total dust mass of M. PAH/VSG constitute 12 per cent of the dust mass in the disc (and also in the shell) which is significantly smaller than the ISM value. The derived masses of different components corresponding to the graphite model can be seen in Table 1. The disc radii derived from the graphite model are too far from the approximate values one can infer from the Spitzer 8-μm image (model values are R disc,in = 0.37 arcsec and R disc,out = 3.33 arcsec, whereas their expected values are about 3 and 7 arcsec, respectively). With an increase in the stellar temperature to K, the disc still needs to be much closer to the star than expected. The SED of the graphite model does not fit the observations well compared to the amc model. The IR part of the SED deviates significantly from the observation, as shown in Fig. 4(b). The flux distribution across the graphite model image at 22 μm is compared with the observation in Fig. 5. As can be seen in the figure, the model curve differs significantly from the observed one. Hence, our study argues that the grains in IRAS are amorphous in nature and graphites do not explain the observed IR characteristics. This was also suggested earlier by BH91 and MPI13 from their 1D studies. As the SED fit of the graphite model is not satisfactory, an acceptable range for the derived parameters cannot be justified, and hence we cannot calculate their uncertainty. 5 DISCUSSION 5.1 Shell, disc and envelope of IRAS The WISE 22-μm image of IRAS traces the cooler dust distribution of the PN. As noted earlier, this image shows a faint, half-moon shaped envelope located in the north-west direction; however, the bright inner shell is nearly spherical (see Fig. 1). A proper motion study of the cluster implied that the nebula is moving in a direction, which has a sky-projected angle of 25 from east to south (Cudworth 1986). The direction of the asymmetry of the envelope is close to the direction of the stellar motion. This indicates that the nebula is strongly interacting with the Halo gas and the envelope asymmetry is caused by this interaction. It appears that the interaction does not affect the shell and the disc. The mild elliptical inner rim of the inner envelope could be a result of the interaction of the expanding shell with the envelope, which needs further investigation. The envelope is not seen in the images taken at the shorter wavebands of WISE. Deep imaging in optical with 4-m or 8-m telescopes might bring out this envelope and its interaction with the Halo gas. The size of the optical nebula is 10 7arcsec 2 with the longer size along the north south direction (see fig. 2 of Gillett et al. 1989). The projected size of a 12-arcsec diameter circular disc (or the region of PAH/VSG emission seen in the Spitzer 8-μm image) at an inclination angle of 40 in the east west direction is arcsec 2, with the longer size positioned along the north south direction. Their spatial locations show that the optical nebula observed by Gillett et al. (1989) is the gas distributed inside the disc and the PAH/VSG emitting region has good spatial overlap with the optical nebula. To strengthen this finding, we have run a model considering the PAH/VSG population located outside the optical nebula (R disc,in = 5 arcsec). The model SED and the synthesized image at the Spitzer 8-μm band for this case are shown in Figs 6(c) and (f), which can be compared with the amc model results in Figs 6(a) and (d). As can be seen, they do not match with the observations, indicating the presence of an overlapping region of PAH/VSG emission with the optical nebula. It is not possible to disentangle the PAH emitting region from the Spitzer 8-μm image as there is no N-band continuum image to trace the emission distribution because of VSG. However, in addition to VSG, it is expected that a significant amount of PAH might reside inside the optical nebula. The survival of PAHs in the optical nebula was known earlier for an H-rich PN BD (Bernard et al. 1994), which has a similar stellar photospheric temperature (Sandin et al. 2016). However, Siebenmorgen, Zijlstra & Krügel (1994) suggested that PAHs are partly destroyed in the ionized region by nebular Lyα photons in this PN. In the H-poor ejecta of A30, where there is a lack of nebular Lyα photons because of the absence of hydrogen, the survival of grains with a radius of 7.2 Å and above in the premises of an even hotter central star ( K) was discussed earlier by Borkowski et al. (1994). They claim that grains with a radius less than 7.2 Å were destroyed by harsh stellar radiation in about 1000 yr, and grains with a 7.6-Å radius can survive for yr. For IRAS , the estimated kinematic age is 4000 yr (see Section 5.2) and the central star is much cooler; thus, here one can expect the presence of grains with similar sizes close the central star. MPI13 suggested a mean grain radius of 12 Å for the PAH/VSG population using a classical approach. Together, the spatial coexistence of PAH/VSG with the optical nebula and the absence of hydrogen in IRAS imply that the PAH/VSG population plays an important role in heating the optical nebula by photoelectric emission. This, in turn, leads to the optical emission lines arising from the collisional excitation process, as discussed earlier by BH91 and MPI13. The mass fractions of the PAH/VSG population in the disc and in the shell are the same, and we suggest that the PAH/VSG grain population, and possibly the H-poor material, are also present in the shell. Our estimated mass of the shell in Section is larger than the disc mass. This is similar to the case argued for the PN A30 by Borkowski et al. (1994). They suggested that the dense H-poor gas and dust of A30 are ejected along an equatorial ring and suffer little acceleration by the stellar wind, whereas more tenuous ejecta would be strongly accelerated and most of the H-poor gas and dust would be carried outward by the stellar wind. The shell in IRAS had also likely originated from the interaction of the stellar wind with the tenuous

11 636 C. Muthumariappan H-poor ejecta from the central star. Though the mass of the shell is larger than the disc mass, it is not observable in optical. A possible explanation may come from the low number density in the shell and the dilution of the stellar radiation field, which together result in less efficient photoelectric heating. In an H-poor environment such as IRAS , one can expect pure, non-hydrogenated carbon clusters such as fullerene (C 60 ). However, the spectral signatures of fullerene (e.g. at 7, 8.5, 17.4 and 18.9 μm) were not found for this PN in its Spitzer IRS spectrum (see table 2 of MPI13). 5.2 The evolutionary nature of IRAS Thermal images of IRAS and A30 are similar at the WISE 22-μm band as well as at the Spitzer IRAC 8-μm band. One difference is that faint emission is seen from the shell at the 8-μm image of A30, which is not observed for IRAS As argued earlier, the optical nebula of IRAS is a disc component and is known to be H-poor, and it is likely that the shell is also H-poor. The IR morphology of the H-poor disc and shell of IRAS is similar to the IR morphology of the H-poor ejecta of A30. From these similarities, we propose that, very likely, the disc and the shell of IRAS had formed from the same process as in the case of A30. A born-again PN is expected to have an inner ejecta with large IR excess, a low gas-to-dust mass ratio with highly processed H-poor material, a C-rich dust chemistry and a PAH/VSG population (Borkowskietal.1994; Kimeswenger, Kerber & Weinberger 1998; Ercolano et al. 2003). The central star, which has undergone a very late-thermal pulse (VLTP), sheds highly processed material in its stellar wind. This enhances the dust formation process and hence the circumstellar shell has a larger amount of dust than observed in normal PNe, as proposed by Borkowski et al. (1994). A textbook example for a born-again PN is A30 (Guerrero et al. 2012). IRAS shows many of these characteristics and is similar to A30 in many aspects, in addition to the morphology. As seen earlier, the derived gas-to-dust mass ratio of this PN is much lower than that found for a normal PN. Hence, there is a high possibility that IRAS has also originated from a born-again scenario, as suggested earlier by BH91 and also discussed by MPI13. However, PNe descended from born-again AGB stars often show high-velocity outflows; for example, the presence of high-velocity (60 km s 1 ) collimated outflows in A30 was shown by Tajitsu & Otsuka (2006). There is no high-velocity outflow component known for IRAS The central stars of born-again PNe are H-deficient, but the photospheric composition of the central star of this PN is not yet well constrained. It should be noted that the gas in the inner ejecta of A30 is O-rich in nature with enhanced N and Ne abundances, as discussed by Wesson, Liu & Barlow (2003), and similarly for another born-again PN Abell 58 (see Wesson et al. 2008). The optical nebula of IRAS also shows enhanced Ne abundance (BH91). These observations are not expected from a PN originated from VLTP alone, which should eject C-rich material. A possible formation scenario may be sought from a binary origin (De Marco 2008). It is unknown if an H-rich envelope is present in IRAS around its H-poor shell, like the presence of an H-rich nebula surrounding the H-poor inner ejecta of A30. Its strong interaction with the Halo gas might have stripped off most of the (old, tenuous) H-rich material that was ejected during the AGB evolution of the progenitor before the occurrence of the H-poor ejection. It is possible that the faint, fragmented envelope seen in the WISE 22-μm image has this H-rich matter. Optical observation of the faint envelope is not available to test this possibility. As stated earlier, deep imaging in H α and in collisionally excited lines using 4-m or 8-m class telescopes may help to trace the H-rich nature of the envelope. PAH emission can originate from the edge of the optical nebula by the photodissociation process, as proposed for the PNe with dualdust chemistry in the Galactic bulge by Guzman-Remirez et al. (2014). The possible coexistence of the PAH emitting region with the optical nebula in IRAS constrains the fact that the origin of PAH should be sought from a different pathway in this case. A likely pathway is that a companion played an important role in ejecting C-rich, H-poor material from the progenitor along an equatorial disc. Following the VLTP that occurs in a close binary system, it is possible that the presence of a companion close to the H-deficient born-again central star facilitates PAH formation by providing H-rich gas to the C-rich wind, as suggested by De Marco (2008). However, if the central star is H-rich, as proposed by HP93, then it forms a unique case, as none of the PNe that are known to have H-poor material show a H-rich central star. In such a case, the star had also likely experienced an accretion of H-rich material from its companion, which re-coated the central star after ejecting the H- poor nebula (as hypothesized by De Marco 2008). Determining the photospheric composition of the central star, testing its binary nature and detecting the proposed H-poor shell and H-rich envelope of IRAS are the suggested key observations to confirm its origin and its evolutionary nature. If we say that the mass loss occurred during VLTP in the equatorial regions forming an expanding disc, then the disc dimensions and its expansion velocity can give an estimation for its expansion age. CG89 reported the expansion velocity of the nebula to be 11 km s 1. Correcting for an inclination angle of 40 for the disc, the nebular expansion is 14 km s 1. This means that the mass ejection began about 7500 yr ago, and occurred for 3600 yr until it was terminated 3900 yr ago. The total mass of M and the duration of mass loss during the born-again AGB phase give a mass-loss rate of M yr 1 for IRAS PNe show radial-dependent velocity law, with the inner region moving slower than the outer region (Gesicki & Zijlstra 2000). However, there has been no spatio-kinematic study of IRAS to derive the velocity law of the nebula, so the given expansion velocity is an average value. Hence, the expansion time-scales would be different from the dynamical time-scales. However, considering the slow expansion of the disc and the considerably smaller value of (R disc,out R disc,in )/(R disc,out + R disc,in ), the expansion ages are not expected to be very different from the dynamical ages. 6 CONCLUSIONS We have carried out a 3D Monte Carlo radiative transfer study of dust in the PN IRAS located in M22. The SED was constructed from the far-uv to FIR observations taken from the archival data. We have included reprocessing of stellar radiation by the PAH/VSG population in addition to the classical dust in our radiative transfer. Thermal images of the PN at Spitzer 8 μm andwise 22 μm were also used for this study. Our model with amorphous carbon grains fits the SED and reproduces the thermal images better than the graphite model. The central star is hot with a photospheric temperature of ( ± 3000) K. The wavelength-integrated flux of the model SED gives a luminosity of (2460 ± 800) L. A large mid-ir excess emission observed with this PN is attributed to the PAH/VSG population. The disc is bright in PAH/VSG emission and it has inner and outer radii of (2.8 ± 0.1) and (6.0 ± 0.6) arcsec, respectively. With an inclination