HUBBLE SPACE TELESCOPE OBSERVATIONS OF PLANETARY NEBULAE IN THE MAGELLANIC CLOUDS. Ⅳ. [O Ⅲ] IMAGES AND EVOLUTIONARY AGES

摘要

The Planetary Camera of the Hubble Space Telescope has been used to obtain images in the [O Ⅲ] λ5007 line for a sample of 15 planetary nebulae in the Magellanic Clouds chosen to eliminate any selection bias in either excitation class or in flux [for log F(Hβ) > -13.7 ergs cm~(-2) s~(-1)]. These images are used to derive the physical dimensions, the spatial structure, and the kinematic ages of the nebulae. The raw images were deconvolved using the Richardson-Lucy image restoration algorithm, and based on extensive tests of model images, a limit of 100 iterations of the algorithm was adopted. The restored images show clear evidence for size evolution across the H-R diagram. The younger, low-excitation, compact planetary nebulae tend to be systematically smaller than photoionization models based on ground-based data would predict, suggesting that these planetary nebulae have a central reservoir of dense atomic and molecular gas. This gas lies close to the central star and is undergoing ionization and being accelerated into outflow. Planetary nebulae previously classified as nitrogen-rich objects with massive central stars (Peimbert type Ⅰ) show the bipolar "butterfly" symmetry that is also a characteristic of their Galactic counterparts. The derived kinematic ages range from less than 1000 yr up to almost 5000 yr but show little sign of systematic increase along the evolutionary tracks. The true ages of the larger objects are systematically underestimated because of acceleration of the nebular shell during its lifetime. Using both the empirical fit that we had previously derived for the expansion velocity as a function of the position on the H-R diagram, and the theoretical evolutionary tracks of the central star, we have derived two semiempirical estimates for the evolutionary timescales based upon the nebular size and the measured dynamical age. If these evolutionary timescales are to be consistent with the evolutionary age derived from theory, then He burners outnumber H burners in the approximate ratio 2:1.