The program on the Magellanic Cloud Planetary nebula (PN) continues to produce results. This project is a major effort started in 1997, to understand PN evolution in different environments. In collaboration with Shaw (NOAO),Blades, and Mutchler (STScI), and 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.
We have observed about 30 LMC PNe with an innovative method involving slitless STIS/HST spectroscopy, capable of revealing the morphology of LMC PNe in all the major optical recombination and forbidden lines. We obtained the largest sample of extragalactic 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. An invited review on this subject was given at the Planetary Nebula IAU Symposium in November 2001. Three refereed papers have been published on this subject, and several others are in preparation. An invited science highlight article in the Annual Report for the Space Telescope Science Institute was written on this results. The early science results from this study can be summarized as follows:
i) 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 dynamic evolution also depends on morphological type.
ii) The LMC PN morphology correlates tightly with the progenitor abundance of 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 to 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 a homogeneous database for LMC PN abundances.
We also observed in more depth the central stars of the faintest LMC PN nuclei with WFPC2 photometry (20 HST orbits), to correlate PN shapes and central star evolution in a distance-bias free environment.
Our study has been extended to SMC PNe (55 HST orbits) 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. We find that the morphological distribution of SMC PNe is different from that of the Galaxy or the LMC, further indicating that morphology depends on galaxian metallicity.
In Cycle 10, two other MC programs with STIS/HST have been approved, to complete the LMC survey (224 orbits, PI: Shaw) and to measure carbon abundances (28 prime orbits, PI: Stanghellini). The UV spectra of the LMC PNe have been acquired for the most part, and the monochromatic images show very good signatures of the carbon and neon lines, strongest in the UV for PNe.
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 a homogeneous sample for testing the evolutionary implications of metallicity variations in stellar evolution. We have produced a public web page to collect all our results, as a part of the STScI MAST archive: http://archive.stsci.edu/hst/mcpn/.