SPECTROSCOPY OF BRIGHT EXTRAGALACTIC PLANETARY NEBULAE

Abstract

The spectroscopic properties of bright extragalactic planetary nebulae are reviewed, considering primarily their chemical abundances and their internal kinematics. Low-resolution spectroscopy is used to investigate how the precursor stars of bright planetary nebulae modify their original composition through nucleosynthesis and dredge up. At present, the evidence indicates that oxygen and neon abundances usually remain unchanged, helium abundances are typically enhanced by less than 50%, while nitrogen enhancements span a very wide range. Interpreting these changes in terms of the masses of their progenitor stars implies that the progenitor stars typically have masses or order <TEX>$1.5M_{\bigodot}$</TEX> or less, though no models satisfactorily explain the nitrogen enrichment. High-resolution spectroscopy is used to study the internal kinematics of bright planetary nebulae in Local Group galaxies. At first sight, the expansion velocities are remarkably uniform, with a typical expansion velocity of 18 km/s and a range of 8-28 km/s, independent of the progenitor stellar population. Upon closer examination, bright planetary nebulae in the bulge of M31 expand slightly faster than their counterparts in M31's disk, a result that may extend generally to the planetary nebulae arising from old and young stellar populations. There are no very strong correlations between expansion velocity and global nebular properties, except that there are no large expansion velocities at the highest <TEX>$H{\beta}$</TEX> luminosities (i.e., the youngest objects never expand rapidly). These results independently suggest that bright planetary nebulae arise from a similar mass range in all galaxies. Nonetheless, there are good reasons to believe that bright planetary nebulae do not arise from identical progenitor stars in all galaxies.