Radio and optical images of three compact planetary nebulae, NGC 6543, IC 3568, and NGC 40, are found to reveal a variety of morphological and ionization signatures of wind shaping and heating. Both NGC 6543 (one of the most morphologically complex planetaries in the sky) and IC 3568 (one of the simplest) exhibit large density changes on size scales corresponding to sound-crossing times of about a century, arguing that the density gradients are sustained by shocks driven by stellar winds. All of the available data show the structure and ionization of IC 3568 to conform to simple hydrodynamic models of nebulae that are shaped by interior stellar winds. NGC 6543 also shows very local changes in optical line ratios that are best understood in the same manner.
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Balick et al. (1987) studied this question.