A hydrodynamical study of multiple-shell planetaries I. NGC 2438

We obtained deep imaging and high-resolution spectroscopy of the planetary nebula NGC 2438. In addition to the bright inner rim and the faint halo-lik e shell already known from previous imagery, our observations also highligh t the existence of an even fainter large halo of nearly spherical shape. An alysis of the surface-brightness profiles and the line splitting indicates a clear ionization stratification within the bright rim, with the flow velo cities increasing with distance from the central star, reaching 37 km s(-1) We compared the surface-brightness profiles and kinematical data of NGC 243 8 with state-of-the-art radiation-hydrodynamics models, and obtained a surp risingly good qualitative agreement. By means of this modelling we show tha t the faint, halo-like shell has formed by recombination from a once well-d eveloped photoionized shell in response to a fast luminosity drop of the ce ntral star. The large, very faint halo is interpreted as being the relic of the now fully ionized AGE wind. Also the velocity/ionization stratificatio n and the thickness of the bright rim are fully consistent with the predict ion of the models for such an evolved planetary nebula. The properties of NGC 2438 can be explained by assuming that the planetary nebula formation started about 45 000 years after a thermal pulse on the AG E, and that the remnant star continued to burn hydrogen in a shell until bu rning has stopped quite recently.