A 3D Look at Planetary Nebulae: Their Structure and Distance

Abstract

The research activities proposed here will enable us to determine: a) the full 3D structure of planetary nebulae (PNe); b) characteristics of their central ionizing star; c) their distance. By combining all these we can link PN evolution to that of their progenitors. We will be able to construct their 3-D distribution in the Galaxy, and trace the evolution of their precursors, leading to a better understanding of Galactic evolution and structure. This research will have an important impact in these fields of research.

1 Introduction

Many questions concerning PNe structure formation and evolution are still unanswered. For instance, assuming that Asymptotic Giant Branch (AGB) stars lose their matter in a spherical manner, when does this wind become asymmetrical? In order for these AGB stars to be progenitors of PNe they must, at some point of their evolution, change to asymmetrical mass loss if one expects them to explain the many morphological types of PNe found. In fact evidence that stars show some asymmetry as early as the end of red giant phase has been given by Johnson & Jones. Of course, there is no question as to the importance of wind interactions in the shaping of these objects, first described in Kwok, Purton, & Fitzgerald. However, how they actually become asymmetrical and what type of interactions produce which morphologies is still an open issue. To explain this, many theories have been presented, from binary (or even triple) central star systems to magnetic fields, fast rotation, and various combinations of mechanisms. The importance of any given process, or if they all play a certain role in the shaping of PNe is still unknown.

The Hubble Space Telescope (HST) has played a major role in solving some old paradigms and opening new ones, boosting PNe research to a new level. The incredible images made available by HST have changed the way we view these objects giving an unprecedented detailed look at their structure. Objects before classified as simple bipolar or elliptical revealed a wealth of internal structural features such as blobs, filaments and polysymmetric structures as well as incredibly symmetric concentric rings that challenged our classical view of these objects. This wealth of data from HST has been one of the main motivations of recent research in the PNe field, especially for the theoretical efforts. Photo-ionization codes as well as MHD codes are just now becoming sophisticated enough to tackle some of the paradigms opened by HST data. The available data also has revealed the evolving aspect of these objects unraveling their internal motions as can be seen, for example, in Fernández, Monteiro, & Schwarz, where the proper motions of the ansae of NGC 7009 have been studied for the first time, using HST archival data.

Although much progress has been made on observational and theoretical grounds, many of the works in PNe research still base their conclusions on results using 2D morphologies and distance determinations that are plagued by limiting assumptions such as spherical symmetry, ionized mass and filling factors, to cite a few.

2 Previous Research and PhD Dissertation

The area of computational modeling is where I have spent most of my research efforts, working with 3-D photo-ionization models supervised by Dr. Ruth Gruenwald, at the University of Sao Paulo, Brazil. During this time we studied the PN NGC 3132 in detail, using 3D photoionization models and observations carried out by us as well as data in the literature. We found that a bipolar structure was necessary to reproduce all the available data. This was a peculiar result since this nebula had been considered a closed ellipsoidal shell based on its observed morphology showing a nice elliptical shape. In our work we showed that an “hour-glass” structure was able to reproduce all of the observed quantities as well as the morphology, if oriented in a specific way (Monteiro, Morisset, Gruenwald, & Viegas, 2000).

This led the way for my PhD project on the spatial structure of PNe using our 3-D photo-ionization code and observations. The last 3 years I collected data and ran models for specific objects to try and understand if there is a common characteristic in PNe structure or if they are all truly unique. We approached this by modeling precisely three objects, namely NGC 6369, NGC 6781 and Menzel 1, determining their full 3D structure and central source characteristics. We also analyzed a sample of about 50 PNe observed with long-slit spectroscopy, obtaining their density profiles from the [SII] dublet lines. We identified, using these methods, which nebulae were misclassified according to their 2-D morphology and how this possible misclassification changes the percentages of a given morphology in a sample. This percentage is important since it is closely correlated to the progenitor star mass Corradi & Schwarz. We showed that many PNe were indeed misclassified and in particular that many PNe with ellipsoidal morphology were in fact, hour-glass structures oriented in such a way as to appear ellipsoidal.

Perhaps one of the most important aspects of the work was the recognition of the 3D photoionization modeling as a precise distance determination method. By modeling the nebulae and allowing the distance to be a free parameter in the process, we were able to determine not only the 3D structure,chemical composition, central source characteristics but also the distance, self-consistently. The method is discussed in detail in Monteiro, Schwarz, Gruenwald, Guenthner & Heathcote. With this a complete picture of many PNe parameters including distance, were obtained, self-consistently, for the first time. This raised some important questions concerning previous studies of PNe (see discussion in Sec. 3 and in Fig. 1) and our understanding of their formation and evolution as well as their central sources. These are the questions we intend to answer with this proposal.

3 Proposed Research

A problem for PNe research is the fact that what we observe is always the 2-D projection of 3-D objects. This can give rise to false interpretations and can only be avoided by doing a full 3-D study. Additionally, the distances to PNe are badly determined, typically with factors of 3-5 uncertainties for individual objects. Classical distance determination methods are statistical or individual in nature and all assume constancy of one or more physical parameters of the PNe. Our method differs fundamentally from the classical methods, in which one or more of the observable parameters is assumed to be constant or have some simple relationship. By modeling PNe with 3-D matter distributions one can determine the spatial structure of the matter ejected precisely. This removes ambiguities due to orientation effects, assumptions of ionized masses, filling factors, etc., usually present in previous studies.

Our method uses a photoionization model constrained by the quantities derived from detailed spectra, and spectral images. We constrain simultaneously with: several line images, several line fluxes, complete projected density map, and the velocity structure (when available), obtaining the best overall fit by adjusting the central star luminosity, spectral distribution, temperature, average chemical abundances, and the distance, also obtaining the complete and detailed 3-D structure of the nebula. All the above mentioned parameters are therefore known much more precisely, and the distance determination is correspondingly better. The model precision is therefore defined by the observations. Typical long-slit spectroscopic mapping yields parameters good to about 10-20%, including the all-important distance.

In Monteiro et al. (2004) and Monteiro et al. (2005), we obtained with our 3D photo-ionization code, the 3D structure and the distance to the nebulae NGC 6369 and Menzel I. In these papers the models were constrained with full spectro-photometric mapping of the objects, yielding precise total fluxes, images for many emission lines as well as the most important diagnostic ratios obtained from these images. In the case of Menzel I, literature observations of position-velocity diagrams were also used to further constrain the structure used. The result of this careful model fitting is a self-consistent, precise determination of structure and ionization source characteristics and distance. These results allowed us, for the first time, to place these objects on the HR diagram (see Fig. 1) with parameters determined self-consistently from the model fitting procedure. Using available evolutionary tracks for central stars of PNe taken from Bloecker, we also derived core and progenitor masses as well as ages for these objects. We are also on the final steps of the modeling of NGC 6781 and its preliminary position in the HR diagram is shown in Fig. 1. It is clear from this figure that the assumptions usually made in general PNe studies are very limiting and can lead to wrong conclusions. In the case of NGC 6369 for example, the values from Stanghellini, Corradi, & Schwarz, using distances obtained from statistical methods (with all the assumptions discussed above), differ drastically from our self-consistently determined value. We believe our results are closer to the true values since our model results are consistent with every observational constraint available within the errors. As an added support to our results, the age we determine from the evolutionary tracks and our model is also consistent with the dynamical age given by Hony, Waters, & Tielens. The HR diagram position given by Stanghellini, Corradi, & Schwarz for NGC 6369 would require a very low mass central star, wich in turn would lead to much longer evolutionary time scales, making it inconsistent with the value of Hony, Waters, & Tielens.

Figure 1: Position of PNe modeled with 3D photo-ionization code on the HR diagram (points with error bars). The evolutionary tracks are taken from Bloecker and Vassiliadis, & Wood. Also included are points (triangles) obtained from the literature values for the same objects. These points are labeled with the name of the object and a letter: (a) for values from Stanghellini, Corradi, & Schwarz and (b) for values from Stanghellini, Villaver, Manchado, & Guerrero.

This method gives us the ability to look at the connections between PNe structure and evolution in an unprecedented way. The fact that no limiting assumptions are made, such as filling factors, ionized masses, etc., is a major step forward in understanding the intricacies of PNe formation and evolution. This in turn can have major impact in areas where PNe research has been used, such as galactic chemical evolution. In fact, the procedure described in the works cited above are not limited to the study of PNe, but can be applied to any ionized nebula that can be sufficiently resolved, such as the many HII regions known.

The determination of the 3-D spatial structure can also be used in conjunction with extinction maps and IR observations to give important information on the dust content of these objects, another open issue. Dust models can also be used to study the structure of disks as a possible shaping agent of asymmetrical objects. This can be done in the same manner as carried out in Schwarz & Monteiro and Monteiro, Schwarz, & Peterson, where the orientation effects in bipolar nebula were studied. They showed that for a sample of 30 nebulae with observational data from the literature, the fractional fluxes in the FIR, NIR and visual varied as a function of the inclination angle, pole-on objects being ‘bluer‘, and edge-on nebulae ‘redder‘. Modeling with randomly varying stellar and disk parameters produced a realistic sample to compare with observations. It was shown that disks of reasonable size and density can explain the observed effects. There are many unresolved issues however, and a combination of these modeling efforts and modern infra-red observations might help to pin-point disk structure and its importance as a collimator of winds and in the shaping of PNe as well as the importance of binarity.

It is clear from the discussion above that that this 3D study of PNe is of high relevance to the field of PNe structure and formation as well as stellar evolution, and will have major impact in these and other fields of research such as galactic evolution as well.

References

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