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R.A. Shaw (NOAO), L. Stanghellini, M. Mutchler (STScI), B. Balick (U. Washington), J.C. Blades, E. Villaver (STScI)
We present an initial analysis of images and slitless spectroscopy of a large sample of LMC and SMC planetary nebulae (PNs), which were obtained in three SNAP surveys with the Space Telescope Imaging Spectrograph over the past 3 years. These surveys have increased the number of Magellanic Cloud PNe that have been imaged with HST by a factor of more than four. The data permit us to determine the nebular dimensions and morphology in the monochromatic light of several emission lines, including those that have traditionally been used for morphological studies in the Galaxy (H-alpha, [N II] 6583 and [O III] 5007), plus others of varying ionization, such as [O I], He I, and [S II]. We find the decline in surface brightness with nebular radius for the aggragate sample can be well characterized by an R-3 law for all observed emission lines, and for both the LMC and SMC PNs. Thus, this relationship holds some promise for constructing a useful distance scale for Galactic PNs.
The differing metallicity of the host galaxies makes our comparative study of these PN samples well suited to illuminating the question of Population effects on nebular and stellar evolution. Interestingly, the incidence of asymmetric PNs (which is an indicator of Population I ancestry) in the SMC (~30%) is significantly lower than that found in the LMC (~50%), but is more comparible with the fraction found in the Galaxy (26%). The ionized gas in the SMC PNs in our sample are on average denser by a factor of 1.5, and more massive by nearly that same factor than the LMC PNs, which is at least broadly consistent with a slower evolution for SMC PNs. Finally, we examine the curious tendency of I([O III] 5007)/I(H-beta) for PNs in the LMC to exceed that of the SMC by about 65%. One might naively expect the ratio to be reversed, owing to the lower average chemical abundances (and therefore higher average Te) in the SMC PNs. Photoionization modelling suggests that other emission lines, such as C IV 1550, may provide the dominant cooling mechanism for these nebulae, which would then allow for a lower emissivity in 5007.
Support for this work was provided by NASA through grants GO-08271, GO-08663, and GO-09077 from ST~ScI.
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The author(s) of this abstract have provided an email address for comments about the abstract: shaw@noao.edu
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.