Properties of Some Old Planetary Nebulae

Eighty-six objects identified on the original plates taken for the Palomar Sky Survey are provisionally classed as planetary nebulae. Most of these objects are large and faint and are probably at an advanced stage in their evolution as planetary nebulae. On the assumption that the nebulae are optically thin and all of the same mass, distances are calculated for them by a variation of the method used by Shklovsky. The galactic distribution of the nebulae studied suggests that they have a disk population, a result in agreement with earlier studies. Comparison of the magnitudes of the nebulae and their central stars leads to estimates of lower limits to the temperatures of the stars, and hence of lower limits to their bolometric corrections. Most of the stars have very high temperatures, and in general they seem to lie near the extreme left end of the horizontal branch on the Hertzsprung-Russell diagram. Many of the stars have dimensions of white dwarfs. The evidence for evolution of the stars as their nebular shells expand is critically examined.