Planetary Nebulae

Involved people at OAB: Stanghellini.

During the past year large progress has been made on the Magellanic Cloud Planetary Nebula (PN) project, a major effort started in 1997 to understand PN evolution in different environments. In collaboration with D. Shaw, C. Blades, M. Mutchler and L. Cawley (STScI), and B. Balick (U. of Washington), a series of observing programs to probe PN morphology in extra-galactic environments have been successfully executed . The analysis also include HST Data Archived images.

In Cycle 8 we have observed about 30 LMC PNe (few more are planned) with an innovative method involving slitless STIS/HST spectroscopy, capable of reveal the morphology of LMC PNe in all the major optical recombination and forbidden lines. We obtained the largest sample of extra-galactic PN morphology ever observed with this much detail. A first set of images has been published in a STScI press release, and presented at international meetings. The early science results from this study can be summarized as follows:

1. The LMC morphological types are similar to their Galactic counterparts. Nonetheless, the ratio of symmetric-to-asymmetric PNe is higher in the Galaxy than in the LMC. Future completion of the LMC sample will allow a sound comparison between the two samples, to confirm the important conclusion that morphology is related to the metallicity of the population. We also show that the surface brightness of LMC PNe declines with physical photometric radius, as expected, and that the asymmetric PNe are typically low surface brightness objects. Given that all LMC PNe are at approximately the same distance from us, we can infer that the dynamical evolution also depends on morphological type.

2. The LMC PN morphology correlates tightly with the progenitor abundance of the primordial elements, i.e., those elements that are not affected by stellar evolution (e.g., Ne, S, Ar). This finding bears on the question of formation mechanisms for asymmetric PNe: the genesis of PNe 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.

The spectroscopic follow-up of our LMC targets with NTT/ESO has also been started, with the aim of obtaining an homogeneous database for LMC PN abundances.

We will also observe in more depth the central stars of the faintest LMC PN nuclei with WFPC2 photometry (20 HST orbits awarded in Cycle 9, PI: R. Shaw). The aim is to correlate PN shapes and central star evolution in a distance-bias free environment.

Finally, our study will be extended to SMC PNe (55 HST orbits awarded in Cycle 9, PI: L. Stanghellini) in order to determine the late evolutionary paths of the most common stars in a galaxy that, in its chemical content, mimics a young galaxy.

In the future, our LMC and SMC PN slitless spectra `images' will form a database of extra-galactic PNe that will far exceed in number the Galactic PNe observed with HST, providing an homogeneous sample for testing the evolutionary implications of metallicity variations in stellar evolution.

• Morphology of Galactic Planetary Nebulae
• Post-Asymptotic Giant Branch Evolution