Morphology and Evolution of the Large Magellanic Cloud Planetary Nebulae^*

The LMC is ideal for studying the coevolution of planetary nebulae (PNs) and their central stars in that the debilitating uncertainties of the Galactic PN distance scale and selection biases from attenuation by interstellar dust do not apply. We present images and analyze slitless spectra that were obtained in a survey of Large Magellanic Cloud PNs. These data on 29 targets were obtained with the Hubble Space Telescope (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: Hα, [N II] λ6583, and [O III] λ5007, plus others of varying ionization including [O I], He I, and [S II]. Together with the 31 resolved LMC PNs for which monochromatic images exist in the HST archive, these data show that the incidence of nonsymmetric nebulae, including bipolar nebulae (which is an indicator of Population I ancestry in the Galaxy), is significantly higher than that reported for the Galaxy. The onset of asymmetric features appears even in very young nebulae (with dynamical ages of ~1400 yr), suggesting that at least the gross features of the nebular morphology may be more closely tied to PN formation and that subsequent shaping of the expanding envelope by the radiation field and wind from the central star may play the lesser role of amplifying these gross features. There is some evidence of evolution between two morphological types in the sense that bipolar core nebulae may evolve to pure bipolars late in the PN lifetime.