X - rays from Planetary Nebulae

Two sources of X-ray emission are expected from planetary nebulae: the hot central stars with Teff > 10 5 K, and shocked fast stellar winds at temperatures of 10 − 10 K. The stellar emission and nebular emission differ in spatial distribution and spectral properties. Observations of Xray 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. 1. Origins of X-ray Emission from Planetary Nebulae Two sources of X-ray emission are expected in planetary nebulae (PNe). First, the hot central stars may reach temperatures as high as 100,000 – 200,000 K and emit in soft X-rays. Second, in a wind-wind interaction model of PNe, the fast (1,000 – 3,000 km s) stellar wind can be shock-heated to 10 − 10 K and emit X-rays. These two types of X-ray emission have different spatial extent and spectral properties. X-ray emission from a hot central star should be a point source; its spectral properties ought to reflect the photospheric emission expected from the star. X-ray emission from the shocked fast stellar wind, on the other hand, should be distributed and extend toward the inner wall of the dense PN shell; its spectrum ought to be characterized by thin plasma emission, which consists of both lines and bremsstrahlung emission. X-ray emission from a PN, if detected, may provide essential information on PN formation mechanisms. For example, the spatial distribution and spectral properties of diffuse X-ray emission from a PN tell us the location and physical conditions of hot, shocked gas in the PN interior. Diffuse X-ray emission from a PN interior extending toward the inner shell walls would lend strong support to the wind-wind interaction model. A point source centered on the PN nucleus with a hard X-ray spectrum or an extraordinarily high X-ray luminosity may indicate a different emission mechanism, such as an X-ray binary.