X‐Ray Emission from Planetary Nebulae. I. Spherically Symmetric Numerical Simulations

(abridged) The interaction of a fast wind with a spherical Asymptotic GiantBranch (AGB) wind is thought to be the basic mechanism for shapingPre-Planetary Nebulae (PPN) and later Planetary Nebulae (PN). Due to the largespeed of the fast wind, one expects extended X-ray emission from these objects,but X-ray emission has only been detected in a small fraction of PNs and onlyin one PPN. Using numerical simulations we investigate the constraints that canbe set on the physical properties of the fast wind (speed, mass-flux, openingangle) in order to produce the observed X-ray emission properties of PPNs andPNs. We combine numerical hydrodynamical simulations including radiativecooling using the code FLASH with calculations of the X-ray properties of theresulting expanding hot bubble using the atomic database ATOMDB. In this firststudy, we compute X-ray fluxes and spectra using one-dimensional models.Comparing our results with analytical solutions, we find some agreements andmany disagreements. In particular, we test the effect of different timehistories of the fast wind on the X-ray emission and find that it is determinedby the final stage of the time history during which the fast wind velocity hasits largest value. The disagreements which are both qualitative andquantitative in nature argue for the necessity of using numerical simulationsfor understanding the X-ray properties of PNs.Comment: 17 pages, accepted for publication in ApJ (July 27, 2006), uses emulateap