---
author:
- 'Letizia Stanghellini, R. A. Shaw,, and M. Mutchler,'
title: 'Magellanic Clouds Planetary Nebulae: an updated view on stellar evolution and populations'
---

Open questions on Planetary Nebula Morphology and Evolution
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During the past decades, significant progress has been made toward the understanding of the late phases of stellar evolution of low- and intermediate-mass stars through the study of Galactic and Extra-galactic Planetary Nebulae (PNs). Planetary Nebulae are the gaseous relics of the evolution of stars in the 1–8 M$_{\odot}$ mass range, and they carry a wealth of information on the physical status of their progenitors.

The morphology of PNs is an essential physical parameter to know in order to construct reliable models of nebulae and stars. Pioneering studies on the connections between PN morphology and the evolution of the stars, and the progenitor populations, started in the early 1970s with the work of Greig (1972), and continued with Peimbert (1978), and Peimbert and Torres–Peimbert (1983). The work done by the Peimbert’s group set the stage for more recent studies, by showing that PNs in the Galaxy are the progeny of different stellar populations, and that there is a connection between the scale height distribution of Galactic PNs and their chemical content. In fact, Peimbert (1978) found that PNs in the Galactic disk are underabundant in carbon, and overabundant in nitrogen, as expected from stellar evolution of stars more massive than about 2–3 M$_{\odot}$. Morphology is another link between Galactic spatial distribution and chemistry of PNs: the nitrogen overabundant PNs in the Galactic disk tend to be Bipolar in shape.

The early results have been later confirmed on the basis of larger databases. Stanghellini et al. (1993) suggested that the Bipolar nebulae host central stars with higher masses than those hosted by Elliptical and Round nebulae. Manchado et al. (2000) performed a similar analysis with data in the IAC catalog of northern PNs (Manchado et al. 1996). The analysis basically confirms the earlier results, (Manchado et al. 2000) with the added bonus of the reliability of a complete and homogeneous PN sample.

Despite the large amount of important work on planetary nebulae and their evolution, there are still fundamental questions that remain unanswered. The main question related to PN morphology is the quest for the mechanism(s) that produce symmetric or asymmetric PNs, and how the appropriate mechanisms can account for the observed scenarios. An excellent description of the present understanding of the link between morphology and evolution has been presented by Guillermo Garcia–Segura at this conference (Garcia–Segura et al., this volume). It is more and more evident that measuring absolute physical parameters of PNs is essential in order to constraint the models.

A thorough analysis and modeling of the Magellanic Cloud PNs is very important to achieve the next level of understanding of PNs and their environment. Toward this end, it is essential to minimize the distance (and stellar luminosity) uncertainties. Studies of PNs in the Magellanic Clouds are of fundamental importance in answering the next set of open questions:

1.  [Is PN morphology related to the progenitor Populations, and its chemistry?]{}

2.  [How does PN morphology relate to the galaxian properties, such as metallicity, star formation history, and distribution of stellar populations?]{}

The importance of Magellanic Cloud Planetary Nebulae
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Background
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The spatial resolution achieved with [*HST*]{} observations allows to explore Extra-galactic PN morphology, and its relations to the physics of the central stars and to nebular evolution. Planetary Nebulae in the Magellanic Clouds are exceptionally suited for this type of studies, for their known distances and their low field reddening.

Since the mid 1950s, planetary nebulae have been identified and confirmed spectroscopically in the Magellanic Clouds (e. g., Lindsay 1955; Webster 1969). The Magellanic Cloud (MC) PNs are typically spatially unresolved from the ground. For this reason, the earliy science results are mainly based on PN spectra, used to determine the nebular abundances, the plasma diagnostics, and the motion of the PNs within the host galaxies (Dopita et al. 1985; Aller & Keyes 1987; Peimbert 1987; Boroson & Liebert 1989; Barlow 1991; Torres-Peimbert 1993).

The inability to resolve the MC PNs spatially with ground-based astronomy did not prevent studying the correlations between the PNs and their central stars, using stellar physical parameters inferred from the physics of the host nebulae. Kaler & Jacoby (1990) found correlations between the central star masses and the N/O and C/O abundances of the PN shells in the MC. Their technique was to determine stellar temperature via the cross–over temperature method, then get the central star masses from their location on the HR diagram. Among other findings, this work revealed the nitrogen depletion of the low-mass central stars.

The availability of the [*HST*]{} imagery naturally induced a renassance of PN studies in the Magellanic Clouds. Even with the earlier, optically-aberrated [*HST*]{} observations, studies of LMC and SMC PNs led to morphological insight (Blades et al. 1992; Dopita et al. 1996; Vassiliadis et al. 1998; Stanghellini et al. 1999). As WFPC2 and STIS became available on [*HST*]{}, the level of accuracy of the morphological studies of MC PNs reached the level of detail of Galactic PN morphology via ground-based observations.

Our HST/STIS Morphological Program
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In this paper, we show the early results of a large project on the morphology, evolution, and populations of the Magellanic Clouds PNs. Our project aims at studying the correlations between morphology and stellar evolution and populations in PNs in the Magellanic Clouds. To this end, we use our own [*HST*]{} observations, as well as data from the [*HST*]{} Data Archive. The archived data consists of MC PN images including 29 PNs. Most of these images are compromised by spherical aberration, thus their resolution is limited. Our own [*HST*]{} images are acquired within two [*HST*]{} snapshot programs: 8271 (LMC PNs) and 8663 (SMC PNs). The LMC program is complete with 29 PN observed to date. The SMC program is still active, counting so far about 20 PNs. Here we describe the results obtained from the LMC program, but the importance of the SMC PN observations and a preview of the early results is also discussed.

Our MC PNs are observed with STIS slitless spectroscopy (Shaw et al. 2001). This method produces a series of [*narrowband*]{} images in the prominent nebular lines, and achieve spatial and spectral resolution at once. Together with the slitless dispersion, a [*clear filter*]{} image is also taken for each nebula, as a reference for the general morphology and to locate the central star, if visible, and to determine its magnitude. The archived PN images, together with our newly observed images, where classified morphologically as Round (R), Elliptical (E), Bipolar (B), Bipolar Core (BC), Quadrupolar (Q), and Pointsymmetric (P). All types are discussed in Manchado et al. (1996), except the BC class, discussed by Stanghellini et al. (1999). The spatial resolution that we achieved with the STIS images of LMC PNs is as good as the typical ground based resolution for Galactic PNs that are 3 kpc away. This means that we can acquire a reliable morphological classification of our sample MC PNs. Nonetheless, it is impossible with the existing technology to detect the fine details in MC PNs. In particular, the pointsymmetry, when associated to the Bipolarity, can be hard to detect.

We found that the individual morphological types in LMC PNs are similar to those of Galactic PNs. In Figure 1 we show a sampler of LMC PNs from our survey. We show the images as observed through the clear filter, and the \[O III\] $\lambda$5007 Å  contour plots. We found in our sample Round, Elliptical (e.g., SMP 4), Bipolar (e.g., SMP 16), Quadrupolar (e.g., SMP 27), and Pointsymmetric (e.g. SMP 10) PNs. The ratio of symmetric-to-asymmetric PNs[^1] is higher in the Galaxy than in the LMC. This is an indication that morphology traces the metallicity of the PN progenitors.

![Morphological sampler of LMC PNs.](mexiconf1.eps){width="14truecm"}

In Figure 2 we show the \[O III\] $\lambda$5007Å  surface brightness evolution, as a function of the physical radius. The LMC PNs are indicated with different symbols, depending on their shell morphology (see Figure legend). We can see a clear morphological separation accordingly to the evolutionary rates of the different types. Round PNs show a slow surface brightness decline, while Bipolar (and, in general, asymmetric) PNs evolve fast. This is true if the physical radius is a good measure of the dynamical time of the PNs. In effect, ther nebular evolution also depends moderately on the velocity of the shell expansion (see also Shaw et al. 2001).

By studying the chemical content of symmetric and asymmetric PNs in the LMC, Stanghellini et al. (2000) found that asymmetric PNs derive from the evolution of the youngest of the PN-producing stellar population. In Figure 3 we illustrate this point by showing the segregation of the LMC PN morphological types on the basis of their neon and sulphur abundances. The evolution of stars in the PN progenitor mass range do not alter the abundances of sulphur and neon, thus the plot clearly shows that there is a separation in the populations of the progenitors of the PNs with different morphological types. This finding bears on the question of formation mechanisms for asymmetric PNs: the genesis of PNs structure should relate strongly to the population type, and by inference the mass of the progenitor star, and less strongly on whether the central star is a member of a close binary system, as previously believed.

![The evolution of the \[O III\] $\lambda$5007Å  surface brightness in LMC PNs.](mexiconf2.eps){width="11truecm"}

Summary, and Future Projects
============================

Despite the large number of excellent studies of Galactic PNs, several questions on PN morphology and its formation still remain unanswered. The study of the Magellanmic Clouds PNs has the great advantage to produce absolute physical parameters, to constraint and probe the existing evolutionary and hydrodynamical models. Our program on LMC PNs has already shown that, indeed, there is population and mass segregation among planetary nebulae of different morphology, and that symmetric PNs seem to evolve slower than asymmetric PNs. Furthermore, there are hints that the overall morphological distribution in a galaxian PN population depends strongly on the galaxian type and metallicity. A follow up on these studies bears not only on the understanding of PN morphology and evolution, but also on the stellar populations in galaxies, and on the planetary nebula luminosity function as a secondary indicator of the Extra-galactic distance scale. We will strive to obtain quantitative results on solid statistical grounds to enlighten these astrophysical aspects.

A similar study in the Small Magellanic Cloud (SMC) is planned, to extend the [*metallicity baseline*]{} of the above findings. The LMC and SMC PN images acquired by Stanghellini and collaborators will form a database of Extra-galactic PN images that will far exceed in number the Galactic PNs observed with [*HST*]{}, providing an homogeneous sample for testing the implications of metallicity variations in stellar evolution.

Thanks are due to J. Franco and the other organizers for the opportunity to participate to an important celebration for the Peimberts. It was a pleasure to be at the Conference, and to be able to discuss scientific and non-scientific issues with J. Franco, G. García-Segura, M. Dopita, S. Oey, M. Richer, and many others.

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[^1]: Symmetric PNs are Round and Elliptical PNs, Asymmetric PNs are Bipolar, Bipolar Core, and Quadrupolar PNs. We do not include Pointsymmetric PNs here, for homogeneity of the Galactic and Extra-Galactic samples.
