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Photoionization models of Planetary Nebulae
Proceedings of the International Astronomical Union, 2016
The understanding of astronomical nebulae is based on observational data (images, spectra, 3D data-cubes) and theoretical models. In this review, I present my very biased view on photoionization modeling of planetary nebulae, focusing on 1D multi-component models, on 3D models and on big database of models.
The giant HII region NGC588 as a benchmark for 2D photoionisation models
Astronomy & Astrophysics, 2014
Aims. We use optical integral field spectroscopy and 8µm and 24µm mid-IR observations of the giant H ii region NGC 588 in the disc of M33 as input and constraints for two-dimensional tailor-made photoionisation models under different geometrical approaches. We do this to explore the spatial distribution of gas and dust in the interstellar ionised medium surrounding multiple massive stars. Methods. Two different geometrical approaches are followed for the modelling structure: i) Each spatial element of the emitting gas is studied individually using models which assume that the ionisation structure is complete in each element to look for azimuthal variations across gas and dust. ii) A single model is considered, and the two-dimensional structure of the gas and the dust are assumed to be due to the projection of an emitting sphere onto the sky. Results. The models in both assumptions reproduce the radial profiles of Hβ surface brightness, the observed number of ionising photons, and the strong optical emission-line relative intensities. The first approach produces a constant-density matter-bounded thin shell of variable thickness and dust-to-gas ratio, while the second gives place to a radiation-bounded thick shell sphere of decreasing particle density. However, the radial profile of the 8µm/24µm IR ratio, depending on the gas and dust geometry, only fits well when the thick-shell model is used. The resulting dust-to-gas mass ratio, which was obtained empirically from the derived dust mass using data from Spitzer, also has a better fit using the thick-shell solution. In both approaches, models support the importance of the low surface-brightness positions on the integrated spectrum of the nebula, the chemical homogeneity, the ionisation-parameter radial decrease, and the robustness of strong-line methods to derive the equivalent effective temperature in extended regions. These results must be taken with care in view of the very low extinction values that are derived from the IR, as compared to that derived from the Balmer decrement. Besides, the IR can be possibly contaminated with the emission from a cloud of diffuse gas and dust above the plane of the galaxy detected at 250 µm Herschel image.
Photoionization model analysis of the planetary nebula Hu1-2
Astronomy and Astrophysics, 2004
We have obtained high resolution optical spectra of the planetary nebula Hu 1-2 in the wavelength region of 3700 Å-10 050 Å, with the Hamilton Echelle Spectrograph (HES) at Lick Observatory. Diagnostic analysis indicates that the nebular gas can be represented by inhomogeneous shells of electron density N 4000-10 000 cm −3 , and a gas temperature of 12 000-18 000 K. Using the spherically symmetric photoionization model with appropriate abundances, we tried to accommodate the observed physical conditions and high electron temperatures. The chemical composition of the nebula was derived from calculations using a photoionization model which predicts the observed IUE, HES and ISO line intensities; and the composition was then compared to previous determinations. Model analysis confirms the semi-empirically determined abundance derivations carried out in earlier studies. He and N abundances are high, but those of C, O, Ne and S are very low.
Modelling of aspherical nebulae - I. A quick pseudo-3D photoionization code
Monthly Notices of The Royal Astronomical Society, 2005
We describe a pseudo-3D photoionization code, NEBU 3D and its associated visualization tool, VIS NEB3D, which are able to easily and rapidly treat a wide variety of nebular geometries, by combining models obtained with a 1D photoionization code. The only requirement for the code to work is that the ionization source is unique and not extended. It is applicable as long as the diffuse ionizing radiation field is not dominant and strongly inhomogeneous. As examples of the capabilities of these new tools, we consider two very different theoretical cases. One is that of a high excitation planetary nebula that has an ellipsoidal shape with two polar density knots. The other one is that of a blister HII region, for which we have also constructed a spherical model (the spherical impostor) which has exactly the same Hβ surface brightness distribution as the blister model and the same ionizing star.
Three-Dimensional Photoionization Structure and Distances of Planetary Nebulae. II. Menzel 1
Astrophysical Journal, 2005
Continuing our series of papers on the three-dimensional (3D) structure and accurate distances of planetary nebulae (PNe), we present here the results obtained for PN NGC 40. Using data from different sources and wavelengths, we construct 3D photoionization models and derive the physical quantities of the ionizing source and nebular gas. The procedure, discussed in detail in the previous papers, consists of the use of 3D photoionization codes constrained by observational data to derive the 3D nebular structure, physical and chemical characteristics, and ionizing star parameters of the objects by simultaneously fitting the integrated line intensities, the density map, the temperature map, and the observed morphologies in different emission lines. For this particular case we combined hydrodynamical simulations with the photoionization scheme in order to obtain self-consistent distributions of density and velocity of the nebular material. Combining the velocity field with the emission-line cubes we also obtained the synthetic position-velocity plots that are compared to the observations. Finally, using theoretical evolutionary tracks of intermediate-and low-mass stars, we derive the mass and age of the central star of NGC 40 as (0.567 ± 0.06) M and (5810 ± 600) yr, respectively. The distance obtained from the fitting procedure was (1150 ± 120) pc.
Photoionized and Photodissociated Regions around Main‐Sequence Stars
The Astrophysical Journal, 1998
Within a molecular cloud, the strong ultraviolet radiation Ðeld produced by newly formed stars dissociates and ionizes the surrounding molecular gas. The radiative Ñux depends on the e †ective temperature and metallicity of the star. Using the most recent line-blanketed atmosphere models from Kurucz, we obtain the rates of ionizing and dissociating photons from stars with e †ective temperatures of 7.5 ] 103 to 5 ] 104 K, and for metallicities between 0.01 times solar and solar. With a radiative transfer model, we then compute the basic structures and sizes of the photoionized and photodissociated regions produced by stars embedded in a molecular gas with uniform densities. Absorption of the UV Ñux by dust decreases the mass of H II and H I produced within the cloud, and its e †ects are taken into account in our model. We also discuss the constraints imposed by photodissociated regions on the number of intermediate-and high-mass stars that can form in molecular clouds.
Monthly Notices of the Royal Astronomical Society, 2011
We analyse the reliability of oxygen abundances and ionization parameters obtained from a number of diagnostic diagrams. To do this, we used the literature to compile the observational emission-line intensities and oxygen abundances of 446 star-forming regions whose O/H abundances were determined by direct estimation of the electron temperature. These compiled abundances were compared with the values calculated in this work using various diagnostic diagrams in combination with results from a grid of photoionization models. We found that the [O III]/[O II] versus [N II]/[O II], [O III]/Hβ versus [N II]/[O II] and ([O III]/Hβ)/([N II]/Hα) versus [S II]/[S III] diagnostic diagrams gave O/H values close to those obtained using the electron temperature, with differences of about 0.04 dex and a dispersion of about 0.3 dex. Similar results were obtained by detailed models, but with a dispersion of 0.08 dex. The origin of the dispersion found with the use of diagnostic diagrams is probably the differences between the real N/O-O/H relation of the sample and the one assumed in the models. This is confirmed by the use of detailed models that do not have a fixed N/O-O/H relation. We found no correlation between the ionization parameter and metallicity for the objects of our sample. We conclude that the combination of two line ratios predicted by photoionization models, one sensitive to the metallicity and the other sensitive to the ionization parameter, which takes into account the physical conditions of star-forming regions, gives O/H estimates close to the values derived using direct detections of electron temperature.
Photoionization analysis of chemodynamical dwarf galaxies simulations
Monthly Notices of the Royal Astronomical Society, 2015
Photoionization modelling allows us to follow the transport, the emergence, and the absorption of photons taking into account all important processes in nebular plasmas. Such modelling needs the spatial distribution of density, chemical abundances, and temperature, that can be provided by chemodynamical simulations (ChDS) of dwarf galaxies. We perform multicomponent photoionization modelling of the ionized gas using 2D ChDSs of dwarf galaxies. We calculate emissivity maps for important nebular emission lines. Their intensities are used to derive the chemical abundance of oxygen by the so-called T e-and R 23-methods. Some disagreements are found between oxygen abundances calculated with these methods and the ones coming from the ChDSs. We investigate the fraction of ionizing radiation emitted in the starforming region which is able to leak out the galaxy. The time-and direction-averaged escape fraction in our simulation is 0.35-0.4. Finally, we have calculated the total Hαluminosity of our model galaxy using Kennicutt's calibration to derive the star formation rate. This value has been compared to the 'true' rate in the ChDSs. The Hα-based star formation rate agrees with the true one only at the beginning of the simulation. Minor deviations arise later on and are due in part to the production of high-energy photons in the warm-hot gas, in part to the leakage of energetic photons out of the galaxy. The effect of artificially introduced thin dense shells (with thicknesses smaller than the ChDSs spatial resolution) is investigated, as well.
Compact planetary nebulae MaC 2-1 and Sp 4-1: photoionization models and dust characteristics
Monthly Notices of the Royal Astronomical Society, 2021
We study the characteristics of the planetary nebulae (PNe) MaC 2-1 and Sp 4-1. We use our optical spectra taken with the 2 m Himalayan Chandra Telescope, Spitzer mid-infrared (mid-IR) spectra, HST images, and IR photometric data. These PNe have not previously been individually studied in detail. Both are in the low- to moderate-excitation class. MaC 2-1 shows the presence of silicon carbide (SiC) and magnesium sulphide (MgS) dust. Sp 4-1 hosts polycyclic aromatic hydrocarbon (PAH) molecules. We obtain the plasma properties of the PNe from the optical and mid-IR emission-line fluxes. We compute photoionization models of the PNe for the self-consistent estimation of physical parameters associated with the central star and the nebula, including nebular abundances. From the modelling of the IR data, we obtain the characteristics of dust and molecules formed in the nebulae. From our study, we estimate that the progenitors of MaC 2-1 and Sp 4-1 had masses of 1.2 and 1.55 M⊙, respectively...
Ionization Models of Planetary Nebulae
Symposium - International Astronomical Union
Our knowledge of the ionization structure of gaseous nebulae may be dated from the classic paper of Strömgren (1939), who showed that the transition between ionized and neutral hydrogen is sharp. In fact the very sharpness of the transition region has at last become a difficulty. Another milestone was the paper of Hummer and Seaton (1964) which clarified the structure of nebulae containing helium as well as hydrogen. Then, just a decade ago, the first models of planetary nebulae were constructed which detailed the changes in the temperature and in the ionization of the heavy elements as the stellar radiation was progressively diminished by absorption in the nebular gas (Goodson, 1967 Harrington, 1968; Williams, 1968; Flower, 1969a). These first models were of a general nature, but soon models were constructed to match specific nebulae (Flower, 1968, 1969b; Harrington, 1969; Kirkpatrick, 1970).