We present a three-dimensional photoionisation and dust radiative transfer
model of NGC 6302, an extreme, high-excitation planetary nebula. We use the 3D
photoionisation code Mocassin} to model the emission from the gas and dust. We
have produced a good fit to the optical emission-line spectrum, from which we
derived a density distribution for the nebula. A fit to the infrared coronal
lines places strong constraints on the properties of the unseen ionising
source. We find the best fit comes from using a 220,000 K hydrogen-deficient
central star model atmosphere, indicating that the central star of this PN may
have undergone a late thermal pulse.
  We have also fitted the overall shape of the ISO spectrum of NGC 6302 using a
dust model with a shallow power-law size distribution and grains up to 1.0
micron in size. To obtain a good fit to the infrared SED the dust must be
sufficiently recessed within the circumstellar disk to prevent large amounts of
hot dust at short wavelengths, a region where the ISO spectrum is particularly
lacking. These and other discoveries are helping to unveil many properties of
this extreme object and trace it's evolutionary history.Comment: 8 pages, 4 figures; for the proceedings of "Asymmetric Planetary
  Nebuale IV," R. L. M. Corradi, A. Manchado, N. Soker ed

Barlow, M. J.

Ercolano, B.

Rauch, T.

Wright, N. J.

English

arXiv

We present a three-dimensional photoionisation and dust radiative transfer model of NGC 6302, an extreme, high-excitation planetary nebula. We use the 3D photoionisation code Mocassin} to model the emission from the gas and dust. We have produced a good fit to the optical emission-line spectrum, from which we derived a density distribution for the nebula. A fit to the infrared coronal lines places strong constraints on the properties of the unseen ionising source. We find the best fit comes from using a 220,000 K hydrogen-deficient central star model atmosphere, indicating that the central star of this PN may have undergone a late thermal pulse. We have also fitted the overall shape of the ISO spectrum of NGC 6302 using a dust model with a shallow power-law size distribution and grains up to 1.0 micron in size. To obtain a good fit to the infrared SED the dust must be sufficiently recessed within the circumstellar disk to prevent large amounts of hot dust at short wavelengths, a region where the ISO spectrum is particularly lacking. These and other discoveries are helping to unveil many properties of this extreme object and trace it's evolutionary history

Wright, NJ

Barlow, MJ

Ercolano, B

Rauch, T

UCL Discovery

arXiv:0709.2122v1  [astro-ph]  13 Sep 20073D Photoionisation Modelling of NGC 6302N.J. Wright1, M.J. Barlow1, B. Ercolano2, and T. Rauch31 Department of Physics and Astronomy, University College London, GowerStreet, London, WC1E 6BT. email: nwright@star.ucl.ac.uk2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,MA 02138, USA3 Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076Tübingen, GermanySummary. We present a three-dimensional photoionisation and dust radiativetransfer model of NGC 6302, an extreme, high-excitation planetary nebula. Weuse the 3D photoionisation code mocassin to model the emission from the gas anddust. We have produced a good fit to the optical emission-line spectrum, from whichwe derived a density distribution for the nebula. A fit to the infrared coronal linesplaces strong constraints on the properties of the unseen ionising source. We findthe best fit comes from using a 220, 000 K hydrogen-deficient central star modelatmosphere, indicating that the central star of this PN may have undergone a latethermal pulse.We have also fitted the overall shape of the ISO spectrum of NGC 6302 usinga dust model with a shallow power-law size distribution and grains up to 1.0µmin size. To obtain a good fit to the infrared SED the dust must be sufficientlyrecessed within the circumstellar disk to prevent large amounts of hot dust at shortwavelengths, a region where the ISO spectrum is particularly lacking. These andother discoveries are helping to unveil many properties of this extreme object andtrace it’s evolutionary history.Key words: planetary nebulae: individual: NGC 63021 IntroductionNGC 6302 is a massive planetary nebula (PN) with a dense circumstellardisk. Estimations of the mass of the ionised nebula (∼ 0.5 − 3 M⊙ [17])and the high nebular abundance of nitrogen [1] are consistent with it’s type Istatus and imply a massive progenitor [5]. The nebula shows a strongly bipolarmorphology with a massive equatorial disk, the density of which is sufficientto completely obscure the unobserved central star [17].Infrared spectroscopy of the central region of NGC 6302 has shown manyemission lines from high-ionisation species, in particular that of [Si ix] 3.934 µm[5], the highest ionisation species found in a PN, as well as lines of [Mg viii]2 N.J. Wright, M.J. Barlow, B. Ercolano, and T. Rauchand [Al vi]. These observations point towards a very hot central star withestimates of the surface temperature of up to 430,000 K [2].NGC 6302 has a large dust component calculated to have a total massof 0.05 - 0.05 M⊙ [17, 12]. The ISO spectrum of NGC 6302 [3] also shows awealth of emission features from crystalline dust species, including crystallinesilicates [24], crystalline water ice [3] and the first extra-solar identification ofcarbonate features [12]. Despite the large number of oxygen-rich solid statespecies detected, bands attributed to polycyclic aromatic hydrocarbons (PAH)have been detected [21], usually indicative of carbon-rich material. This di-chotomy between O-rich and C-rich material makes NGC 6302 an importantcase in stellar evolution. Only by compiling a fully three-dimensional pho-toionisation model of NGC 6302 can a full understanding of this enigmaticobject be possible.2 The Modelmocassin is a 3D photoionisation code that uses Monte Carlo techniquesto solve the thermal equilibrium and ionisation balance equations across anentire grid of arbitrary geometry and density distribution [7]. The code allowsmultiple non-central ionising sources, multiple gas chemistries and has a fulldust model incorporating all the gas-dust coupled processes [8].2.1 The Nebula Density StructureThe 3-dimensional nebula model was constructed by fitting the optical emission-line spectrum, particularly the density- and temperature-sensitive line ratios.The chosen optical emission-line spectrum was that of Tsamis et al. (2003)[23], a deep optical spectrum made with a fixed slit that mocassin can simu-late when producing model line fluxes for comparison with observations. TheIR spectrum was taken from the ISO LWS analysis of Liu et al. (2001) [14]and the infrared grating and echelle spectroscopy of Casassus et al. (2000) [5].We started our models with an hourglass-shaped bipolar nebula with di-mensions determined from observations of the nebula and the recently deriveddistance of 1.04 kpc [17, 16]. This was complemented by a thin circumstellardisk with a radial density dependence [19]. From there we varied the free pa-rameters to achieve a best fit to the observations. Figure 1 shows an image ofthe model nebula showing the diffuse bipolar lobes and the dense circumstel-lar disk. The nebula lobes are filled and at a constant density of 1000 cm−3,while the circumstellar disk follows a radial density dependence, ρ ∝ r−1,which rises to a density of 700,000 cm−3 at the inner edge of the disk. Thedimensions of the bipolar lobes and the disk are given in Table 1.Figure 2 shows the current best fit model results for some of the density-and temperature-sensitive line ratios from the optical and IR line spectra. Toachieve this fit, a high density contrast between the diffuse bipolar lobes and3D Photoionisation Modelling of NGC 6302 3Fig. 1. Image of the 3D nebula structureused to model NGC 6302.Fig. 2. Ratios of model-to-observedline fluxes and line ratios for the cur-rent best-fit model of NGC 6302.the dense circumstellar disk was required. The nebula densities derived byTsamis et al. (2003) for NGC 6302 range from 1380 cm−3 from the [O iii] 52µm/88 µm line ratio to 22,450 cm−3 from the [Cl iii] λ5538/λ5518 line ratio[23]. Our good fits to both these line ratios show that our extreme densitiesare a viable method of reproducing the wide range of density-diagnostic lineratios measured in NGC 6302. Binette & Casassus (1999) [4] measure electrondensities of 1.4×105 cm−3 and 2.3×105 cm−3 from the density diagnostic lineratios [Ne v] 24.3 µm/14.3 µm and [Ar v] 13.1 µm/7.91 µm respectively. Thelatter of which is a good density diagnostic in the range 10,000-300,000 cm−3and is highlighting the extremely high densities in the circumstellar disk whichwe have modelled and also achieve a good match for.The derived density structure that offers us the best fit to the observationsleads us to a total nebula mass of 0.428 M⊙, with approximately 40 % of themass in the circumstellar disk. This disk mass of 0.160 M⊙ is far short of the3 M⊙ derived by Matsuura et al. (2005) from dust masses from the infraredSED and using a dust-to-gas ratio of 0.01 . It should be noted that our modelmasses are only those that are necessary to reproduce the observations, andthat, because the disk is optically thick when viewed radially, we are obviouslyunderestimating this mass by potentially an order of magnitude. If the outerradius of the disk is extended to 3.0×1017 cm (increasing the mass by a factorof 5.0) we find no appreciable difference in the produced spectrum, indicatingthat large amounts of neutral or molecular material could be hidden withinthe disk, without affecting the ionisation structure of the nebula.Initially we have not attempted to model any density inhomogeneities orto reproduce the many small-scale structures seen in the lobes of NGC 6302[17]. This is mainly due to a lack of spatial information on the gas properties4 N.J. Wright, M.J. Barlow, B. Ercolano, and T. RauchTable 1. Dimensions used in the 3D density distribution for the model of NGC 6302Lobe dimensions Disk dimensionsParameter Value Parameter ValueLength of lobe 6.8 ×1017 cm Inner radius 1.0 ×1016 cmWaist radius of lobe 1.0 ×1016 cm Outer radius 1.0 ×1017 cmMaximum radius of lobe 3.1 ×1017 cm Disk height 5.0 ×1015 cmVolume of lobes (both) 3.2 ×1053 cm3 Volume of disk 2.24 ×1051 cm3Density of lobes 1000 H-atoms cm−3 Density profile ρ ∝ r−1Mass of lobes (both) 0.268 M⊙ Mass of disk 0.160 M⊙but also because of the importance of compiling as simple a model as possible.We have also maintained a completely homogeneous gas chemistry throughoutthe model nebula. The chemical abundances used in the model are taken fromthe analysis of Tsamis et al. (2003) [23] and Casassus et al. (2000) [5] andincorporates 14 elements.2.2 The IR Coronal LinesWhile the central ionising source of NGC 6302 has never been observed, theemission from the IR coronal lines puts a constraint on the ionising flux. Ofall the high-ionisation lines observed, those of Mg+7 and Si+8 [5] are of thehighest stages and, if predominantly photo-ionised, will put a strong constrainton the high-energy end of the ionising spectrum.The first observations of IR coronal lines in NGC 6302 were the prominent[Si vi] and [Si vii] lines [2]. Previous attempts with one-dimensional photoioni-sation codes have been unable to accurately reproduce these lines [13] leadingto suggestions that these lines may arise from collisionally-excited material[22]. Many of the arguments for shock-excited material originated from theobservation of ∼500 kms−1 wings on the [Ne v] λ3426 line [15]. However,similar wings were not seen by Casassus et al. (2000) in their observationsof infrared coronal lines, despite similar excitation potentials. The origin ofthe [Ne v] λ3426 wings is now thought to be instrumental [16]. We are there-fore attempting to fully reproduce all the high-excitation lines by means ofphotoionisation alone.To model emission from the central star we use non-LTE stellar atmo-spheres calculated using the Tübingen NLTE Model Atmospheres Package[20]. We have used both the solar abundance models that incorporate all el-ements up to nickel, and the hydrogen-deficient models which use ”typical”PG 1159 abundance ratios of He:C:N:O = 33:50:2:15 by mass. For both ofthese we have varied both the effective temperature and the gravity of thecentral star in attempting to fit the coronal lines. We also varied certain neb-ula parameters such as the disk density and height, that might effect theionisation structures of these elements.3D Photoionisation Modelling of NGC 6302 5Table 2. Observed and modelled line strengths for the high-ionisation IR coronallines of NGC 6302. All line strengths given relative to Hβ×10−3. Observations fromCasassus et al. (2000).Solar abundances H-deficientID λ Observations 220 kK 220 kK[Mg v] 5.60µm 2.19 59.2 5.23[Mg vii] 5.51µm 1.68 1.06 0.60[Mg viii] 3.03µm 0.234 0.0 0.58[Si vi] 1.96µm 3.04 34.0 2.23[Si vii] 2.48µm 2.66 0.09 0.22[Si ix] 3.93µm 0.00389 0.0 0.0318Our models reveal that the majority of the nebula parameters have lit-tle effect on these highly-ionised ions. The effective temperature of the centralstar has by far the strongest influence on the ionisation structure, yet by usinga solar abundance model we were unable to reproduce the highest ionisationstages without reaching temperatures in excess of 400,000 K, where many ofthe model atmosphere calculations may not be applicable [20]. Even at lowertemperatures, the ionisation structure that a solar abundance atmosphere cre-ates does not allow accurate reproduction of even the lower-ionisation lines.This can be seen in Table 2 for the ionisation structures of magnesium and sili-con, where the lower ionisation lines (from Mg+4 and Si+5) are over-predicted,while no flux is seen in the high-ionisation lines (from Mg+7 and Si+8).However, models using H-deficient central star atmospheres produced ion-isation structures that matched the observations much better and were ableto reproduce the highest ionisation lines at much lower temperatures (see Ta-ble 2). Order of magnitude matches can be obtained for nearly all the linesusing a 220,000 K model atmosphere, reproducing the ionisation structures ofthese elements much better.The difference between a solar abundance stellar atmosphere and a hydrogen-deficient stellar atmosphere is most apparent at high energies where the lackof opacity due to hydrogen causes a greater flux of high-energy photons. Thiscan be seen in Figure 3 where we show the spectral energy distributions of asolar-abundance and a hydrogen-deficient stellar atmosphere. The ionisationpotentials of Mg+6 (225eV) and Si+7 (303eV), are also indicated, and it canbe seen that a hydrogen-deficient stellar atmosphere has a significantly higherflux at these energies than a solar-abundance stellar atmosphere. It is thisdifference in the spectral energy distribution of the ionising spectrum thatallows a much better fit to the ionisation structure apparent in NGC 6302.3 The Dust Modelmocassin has a complete dust model, with the dust competing with the gasfor UV photons and interacting directly with it via gas-grain collisions [8]. It6 N.J. Wright, M.J. Barlow, B. Ercolano, and T. RauchFig. 3. Spectral energy distributions of two of the solar abundance and hydrogen-deficient stellar atmospheres used in our models. Both stellar atmospheres haveT⋆ = 220, 000 K and log g = 7. The ionisation potentials of the Mg+6 and Si+7 ionsare indicated. A 220,000 K blackbody spectrum is shown for comparison.allows any mix of dust species and grain sizes in multiple density distributions.NGC 6302 is bright in the infrared and the dust distribution has beeen shownto be complex [17], with the overall broad shape of the SED indicating a largerange of dust temperatures. These two factors make it important to model thedust using a full 3-dimensional dust radiative transfer code such as mocassin.We have initially used a model with a single dust species using the as-tronomical silicates model of Draine (2003), which includes optical constantscovering the full wavelength range of X-rays to the sub-mm [6]. In a full treat-ment of the radiative transfer of dust it is important to have a continuouscoverage of optical constants to accurately treat the absorption by the grainsof UV photons and the re-emission in the IR. We started with a simple MRNgrain-size distribution using amin = 0.04µm and amax = 0.4µm and a slope of−3.5. We found however that a better fit could be obtained with larger grainsand a shallow slope, producing more larger grains and less small grains. Fig-ure 4 shows our current single-species fit to the ISO SED.This fit to the SED uses a power-law grain size distribution with Amin =0.06µm and Amax = 1.0µm with a slope of −2.5. The presence of large grainsup to 10µm in size has been implied from the sub-mm continuum [11] andthe presence of such a dense circumstellar disk provides a stable environmentwhere such large grains could be built up.A better fit to the shape of the SED could be gained by modelling twoseparate populations of dust grains, one consisting of large grains in the cir-3D Photoionisation Modelling of NGC 6302 7Fig. 4. mocassin fit to the ISO SED of NGC 6302.cumstellar disk and the other with a more standard grain-size distributionthat might exist in the bipolar lobes of the nebula. The inclusion of multi-ple grain distributions, with different grain sizes and different chemistries inseparate 3-dimensional density distributions is a unique and versatile featureof mocassin and will allow the observations of NGC 6302 to be much moreaccurately reproduced and studied.The ISO spectrum of NGC 6302 [3] also shows a wealth of emission fea-tures from crystalline dust species, including crystalline silicates [24, 18] andcrystalline water ice [3]. Two previously unidentified features at 62 and 90 µmwere also identified as belonging to the carbonates calcite and dolomite [12],though it is thought that carbonates might not be able to form in these envi-ronments [10].We are in the process of compiling a full database of dust species suitablefor use in photoionisation and radiative transfer codes such as mocassin. Weintend to gather optical constants for all dust species that have been observedin NGC 6302 or could possibly exist in such environments and attempt toreproduce the detailed emission features seen in the ISO spectrum. A fit tothe spectral energy distribution will allow us to derive dust masses for all thespecies present and also determine if the oxygen-rich and carbon-rich speciesare spatially separate, as some evolutionary scenarios have suggested.4 ConclusionsWe have presented initial results from our three-dimensional photoionisationand dust radiative transfer model of NGC 6302 using the 3D photoionisa-tion code mocassin. Our fit to the emission-line spectra has resulted in ahigh density contrast between a large pair of diffuse bipolar lobes and a thin8 N.J. Wright, M.J. Barlow, B. Ercolano, and T. Rauchcircumstellar disk where densities reach 700, 000 cm−3. This density distribu-tion supports the multiple density diagnostics of NGC 6302 that cover a largerange of densities.Our fit to the infrared coronal lines comes using a 220, 000 K hydrogen-deficient stellar atmosphere model. This is one of the lowest temperaturesderived for the unseen central star of this object, which is only possible us-ing a hydrogen-deficient central star model, implying that the central star ofNGC 6302 may have undergone some sort of late thermal pulse.Our dust model includes a fit to the overall shape of the SED using adust grain size distribution that extends up to 1.0µm. The future inclusion ofmultiple crystalline silicate species in separate dust density distributions willallow us to reproduce the ISO spectrum and determine chemical properties ofthe dust grains around this object. These and other discoveries are helping tounveil many interesting properties of this interesting object.References1. L.H. Aller, J.E. Ross, B.J. Omara et al: MNRAS 197, 95 (1981)2. 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3D Photoionisation Modelling of NGC 6302

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3D Photoionisation Modelling of NGC 6302

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