On the asymmetries of extended X-ray emission from planetary nebulae

Chandra X-Ray Observatory (CXO) images have revealed that the X-ray-emitting regions of the molecule-rich young planetary nebulae (PNe) BD + 30degrees3639 and NGC 7027 are much more asymmetric than their optical nebulosities. To evaluate the potential origins of these X-ray asymmetries, we analyze X-ray images of BD + 30degrees3639, NGC 7027, and another PN resolved by CXO, NGC 6543, within specific energy bands. Image resolution has been optimized by subpixel repositioning of individual X-ray events. The resulting subarcsecond resolution images reveal that the soft (E < 0.7 keV) X-ray emission from BD + 30°3639 is more uniform than the harder emission, which is largely confined to the eastern rim of the optical nebula. In contrast, soft X-rays from NGC 7027 are highly localized and this PN is more axially symmetric in harder emission. The broadband X-ray morphologies of BD + 30°3639 and NGC 7027 are highly anticorrelated with their distributions of visual extinction, as determined from high-resolution, space-and ground-based optical and infrared imaging. Hence, it is likely that the observed X-ray asymmetries of these nebulae are due in large part to the effects of nonuniform intranebular extinction. However, the energy-dependent X-ray structures in both nebulae and NGC 6543-which is by far the least dusty and molecule-rich of the three PNe and displays very uniform intranebular extinction suggest that other mechanisms, such as the action of collimated outflows and heat conduction, are also important in determining the detailed X-ray morphologies of young PNe.