Einstein X-Ray Observations of Planetary Nebulae and Their Implications

Central stars of 19 planetary nebulas were observed for X-ray emission using the Einstein Observatory and in four of them X-rays were detected. High-resolution observation with the Einstein Observatory indicates that the X-ray source in NGC 246 is a point source. These planetary nebulas with positive observation turn out to be the nearest, have the least extinction, and also have the largest size of the nebulas in their vicinity. It is possible that X-ray emission is observed from these planetary nebulas with larger ages because of the smaller extinction by the nebulas and also because of the settling of heavy elements in the central star, which would otherwise prevent escape of X-rays by providing opacity.