Three-Dimensional Monte Carlo Simulations of Ionised Nebulae

Ercolano, Barbara; Morisset, Christopher; Barlow, M. J.; Liu, Xiaowei; Storey, P. J.

doi:10.1017/s0074180900209182

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(3 citation statements)

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“…During the same Lexington meeting, a new R-matrix calculation of [Ne V] was independently published by Griffin & Badnell (2000) who also obtained similar results. In the current work the decision to use the collision strengths of Lennon & Burke (1991), although producing results which are in disagreement with C87's observations, was finally taken, since the benchmarking carried out for the Lexington optically thick planetary nebula, which has a central star with effective temperature, T ef f = 150, 000 K (similar to the central star of NGC 3918), clearly showed that most photoionization codes (all but Sutherland's Mappings) use the new collision strengths (Ercolano et al 2002).…”

Section: Appendix A: Analytical Description Of Spindle-like Model Bmentioning

confidence: 99%

“…In fact, although both codes treat the diffuse field exactly, the C87 model could not allow for transfer of diffuse radiation between the thick and the thin sectors. As shown by the scatter of values obtained for the benchmark problems (Péquignot et al 2001;Ercolano 2002;Ercolano et al 2002), even amongst onedimensional classical codes it is usual for different codes to return slightly different predictions for the same problem, even in the extremely simplified case of homogeneous, spherically symmetric nebulae. Figure 2 shows plots of the ionization structure of oxygen (middle panels) and carbon (bottom panels) as a function of radius along the optically thin polar direction (left panels) and along the optically thick equatorial direction (right panels).…”

Section: The Ionization Structurementioning

confidence: 99%

“…Given the large amount of observational data available for this object and the existence of the two different models described above, NGC 3918 seemed an excellent candidate for a detailed three-dimensional photoionization study using the Mocassin code The abundances are given by number, relative to H described by Ercolano (2002) and Ercolano et al (2002). Three photoionization models were constructed, using different density distribution descriptions, in order to try to reproduce the main spectroscopic features, as well as the projected maps published by C99.…”

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Three-dimensional photoionization modelling of the planetary nebula NGC 3918

Ercolano

Morisset

Barlow

et al. 2003

Monthly Notices of the Royal Astronomical Society

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The three‐dimensional Monte Carlo photoionization code mocassin has been applied to construct a realistic model of the planetary nebula NGC 3918. Three different geometric models were tried, the first being the biconical density distribution already used some years ago by Clegg et al. In this model the nebula is approximated by a biconical structure of enhanced density, embedded in a lower‐density spherical region. Spindle‐like density distributions were used for the other two models (models A and B). Model A used a mass distribution slightly modified from one of Mellema's hydrodynamical models that had already been adopted by Corradi et al. for their observational analysis of NGC 3918. Our spindle‐like model B instead used an analytical expression to describe the shape of the inner shell of this object as consisting of an ellipsoid embedded in a sphere. The effects of the interaction of the diffuse fields coming from two adjacent regions of different densities were investigated. These are found to be non‐negligible, even for the relatively uncomplicated case of a biconical geometry. We found that the ionization structure of low‐ionization species near the boundaries is particularly affected. It is found that all three models provided acceptable matches to the integrated nebular optical and ultraviolet spectrum. Large discrepancies were found between all of the model predictions of infrared fine‐structure line fluxes and ISO SWS measurements. This was found to be largely due to an offset of ≈14 arcsec from the centre of the nebula that affected all of the ISO observations of NGC 3918. For each model, we also produced projected emission‐line maps and position–velocity diagrams from synthetic long‐slit spectra, which could be compared to recent HST images and ground‐based long‐slit echelle spectra. This comparison showed that spindle‐like model B provided the best match to the observations. Although the integrated emission‐line spectrum of NGC 3918 can be reproduced by all three of the three‐dimensional models investigated in this work, the capability of creating projected emission‐line maps and position–velocity diagrams from synthetic long‐slit spectra was found to be crucial in allowing us to constrain the structure of this object.

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Section: Appendix A: Analytical Description Of Spindle-like Model Bmentioning

confidence: 99%

Section: The Ionization Structurementioning

confidence: 99%

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