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R.A. Shaw, L. Stanghellini, J.C. Blades (STScI), B. Balick (Univ. Washington)
We present images and slit-less spectroscopy from a survey of LMC planetary nebulae (PNe), which is now underway. These data on 27 targets were obtained within the last half-year with HST using the Space Telescope Imaging Spectrograph. 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~{\sc ii}] 6583 and [O~{\sc iii}] 5007), plus others of varying ionization, such as [O~{\sc i}], He~{\sc i}, and [S~{\sc ii}]. The broad-band images will allow us to determine the central star magnitudes (for the brighter stars), which will yield the evolutionary state of the central stars. Our LMC sample is ideal for studying the co-evolution of PNe and their central stars, in that the debilitating uncertainties of the Galactic PN distance scale and the selection effects of interstellar dust do not apply, and the nebular dynamical ages can be used with greater confidence.
Together with the ~30 LMC PNe for which monochromatic images exist in the HST archive, our data show that the incidence of non-symmetric nebulae (including bi-polar nebulae, which is an indicator of Population I ancestry) is much higher than that reported for the Galaxy. Indeed, the published abundances for these nebulae suggest that nebular morphology may be an excellent discriminator of the Population type of the progenitor star. In addition, the onset of asymmetric features appears even in very young nebulae (<2000 yr), suggesting that at least the gross features of the nebular morphology are more directly tied to PN formation rather than subsequent shaping by the radiation field and wind from the central star. Although the broad morphological class seldom varies with the ionization potential of the emitting ion, there are a few cases where the ionization structure is quite complex.
Support for this work was provided by NASA through grant GO-08271.01-97A from ST~ScI.