The Acceleration of the Nebular Shells in Planetary Nebulae in the Milky Way Bulge^*

We present a systematic study of line widths in the [O III] λ5007 and Hα lines for a sample of 86 planetary nebulae in the Milky Way bulge based on spectroscopy obtained at the Observatorio Astronómico Nacional in the Sierra San Pedro Mártir (OAN-SPM) using the Manchester Echelle Spectrograph. The planetary nebulae were selected with the intention of simulating samples of bright extragalactic planetary nebulae. We separate the planetary nebulae into two samples containing cooler and hotter central stars, defined by the absence or presence, respectively, of the He II λ6560 line in the Hα spectra. This division separates samples of younger and more evolved planetary nebulae. The sample of planetary nebulae with hotter central stars has systematically larger line widths, larger radii, lower electron densities, and lower Hβ luminosities. The distributions of these parameters in the two samples all differ at significance levels exceeding 99%. These differences are all in agreement with the expectations from hydrodynamical models, but for the first time confirmed for a homogeneous and statistically significant sample of Galactic planetary nebulae. We interpret these differences as evidence for the acceleration of the nebular shells during the early evolution of these intrinsically bright planetary nebulae. As is the case for planetary nebulae in the Magellanic Clouds, the acceleration of the nebular shells appears to be the direct result of the evolution of the central stars.