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Register a new userLow-ionization structures in planetary nebulae: confronting models with observations
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Denise Rocha Goncalves
Publication date
2000
fields
Physics
and research's language is
English
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Around 50 PNe are presently known to possess small-scale low-ionization structures (LISs). We consider here jets, jet-like, symmetrical and non-symmetrical LISs and present a detailed comparison of the existing model predictions with the observational morphological and kinematical properties. We find that nebulae with LISs appear indistinctly spread among all morphological classes of PNe, indicating that the processes leading to the formation of LISs are not necessarily related to those responsible for the asphericity of the large-scale morphological components of PNe. We show that both the observed velocities and locations of most non-symmetrical LISs can be reasonably well reproduced assuming either fossil condensations originated in the AGB wind or in-situ instabilities. The jet models proposed to date (HD and MHD interacting winds or accretion-disk collimated winds) appear unable to account simultaneously for the kinematical ages and the angle between the jet and the symmetry axes of the nebulae. The linear increase in velocity observed in several jets favors MHD confinement compared to pure HD interacting wind models. On the other hand, we find that the formation of jet-like systems characterized by relatively low expansion velocities cannot be explained by any of the existing models. Finally, the knots which appear in symmetrical and opposite pairs of low velocity could be understood as the survival of fossil (symmetrical) condensations formed during the AGB phase or as structures that have experienced substantial slowing down by the ambient medium.
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We present new results of a program aimed at studying the physical properties, origin and evolution of those phenomena which go under the somewhat generic definition of low-ionization, small-scale structures in PNe. We have obtained morphological and kinematical data for 10 PNe, finding low-ionization structures with very different properties relative to each other, in terms of expansion velocities, shapes, sizes and locations relatively to the main nebular components. It is clear that several physical processes have to be considered in order to account for the formation and evolution of the different structures observed. We present here some results that are illustrative of our work - on IC 4593, NGC 3918, K 1-2, Wray 17-1, NGC 6337, He 2-186 and K 4-47 - and some of the questions that we try to address.
Individual distances to planetary nebulae are of the utmost relevance for our understanding of post-asymptotic giant-branch evolution because they allow a precise determination of stellar and nebular properties. Also, objects with individual distances serve as calibrators for the so-called statistical distances based on secondary nebular properties. With independently known distances, it is possible to check empirically our understanding of the formation and evolution of planetary nebulae as suggested by existing hydrodynamical simulations. We compared the expansion parallaxes that have recently been determined for a number of planetary nebulae with the trigonometric parallaxes provided by the Gaia Data Release 2. Except for two out of 11 nebulae, we found good agreement between the expansion and the Gaia trigonometric parallaxes without any systematic trend with distance. Therefore, the Gaia measurements also prove that the correction factors necessary to convert proper motions of shocks into Doppler velocities cannot be ignored. Rather, the size of these correction factors and their evolution with time as predicted by 1-D hydrodynamical models of planetary nebulae is basically validated. These correction factors are generally greater than unity and are different for the outer shell and the inner bright rim of a planetary nebula. The Gaia measurements also confirm earlier findings that spectroscopic methods often lead to an overestimation of the distance. They also show that even modelling of the entire system of star and nebula by means of sophisticated photoionization modeling may not always provide reliable results. The Gaia measurements confirm the basic correctness of the present radiation-hydrodynamics models, which predict that both the shell and the rim of a planetary nebula are two independently expanding entities.
The hydrogen ionization and dissociation front around an ultraviolet radiation source should merge when the ratio of ionizing photon flux to gas density is sufficiently low and the spectrum is sufficiently hard. This regime is particularly relevant to the molecular knots that are commonly found in evolved planetary nebulae, such as the Helix Nebula, where traditional models of photodissociation regions have proved unable to explain the high observed luminosity in H_2 lines. In this paper we present results for the structure and steady-state dynamics of such advection-dominated merged fronts, calculated using the Cloudy plasma/molecular physics code. We find that the principal destruction processes for H_2 are photoionization by extreme ultraviolet radiation and charge exchange reactions with protons, both of which form H_2^+, which rapidly combines with free electrons to undergo dissociative recombination. Advection moves the dissociation front to lower column densities than in the static case, which vastly increases the heating in the partially molecular gas due to photoionization of He^0, H_2, and H^0. This causes a significant fraction of the incident bolometric flux to be re-radiated as thermally excited infrared H_2 lines, with the lower excitation pure rotational lines arising in 1000 K gas and higher excitation H_2 lines arising in 2000 K gas, as is required to explain the H_2 spectrum of the Helix cometary knots.
The Spitzer Space Telescope has three science instruments (IRAC, MIPS, and IRS) that can take images at 3.6, 4.5, 5.8, 8.0, 24, 70, and 160 microns, spectra over 5--38 microns, and spectral energy distribution over 52--100 microns. The Spitzer archive contains targeted imaging observations for more than 100 PNe. Spitzer legacy surveys, particularly the GLIMPSE survey of the Galactic plane, contain additional serendipitous imaging observations of PNe. Spitzer imaging and spectroscopic observations of PNe allow us to investigate atomic/molecular line emission and dust continuum from the nebulae as well as circumstellar dust disks around the central stars. Highlights of Spitzer observations of PNe are reviewed in this paper.
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.
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