X-rays from Planetary Nebulae

Two sources of X-ray emission are expected from planetary nebulae: the hot central stars with $T_{eff} > 10^5$ K, and shocked fast stellar winds at temperatures of 10$^6 - 10^7$ K. The stellar emission and nebular emission differ in spatial distribution and spectral properties. Observations of X-ray emission from PNe may provide essential information on formation mechanisms and physical conditions of PNe. X-ray emission from PNe has been detected by Einstein and EXOSAT, but significant advances are made only after ROSAT became available. The ROSAT archive contains useful observations of ~80 PNe, of which 13 are detected. Three types of X-ray spectra are seen. Only three PNe are marginally resolved by the ROSAT instruments. In the near future, Chandra will provide X-ray observations with much higher angular and spectral resolution, and help us understand the central stars as well as the hot interiors of PNe.