X-ray emission from planetary nebulae. I. Spherically symmetric numerical simulations

The interaction of a fast wind with a spherical asymptotic giant branch (AGB) wind is thought to be the basic mechanism for shaping pre-planetary nebulae (PPNs) and later planetary nebulae (PNs). Due to the large speed 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 only in one PPN. Using numerical simulations we investigate the constraints that can be set on the physical properties of the fast wind (speed, mass flux, opening angle) in order to produce the observed X-ray emission properties of PPNs and PNs. We combine numerical hydrodynamic simulations including radiative cooling using the code FLASH with calculations of the X-ray properties of the resulting expanding hot bubble using the atomic database ATOMDB. In this first study, we compute X-ray fluxes and spectra using one-dimensional models. Comparing our results with analytical solutions, we find some agreements and many disagreements. These disagreements, which are both qualitative and quantitative in nature, argue for the necessity of using numerical simulations for understanding the X-ray properties of PNs. We apply our spherical models to the objects BD +30 3639 and NGC 40. We find that the model values of the X-ray temperature and luminosity for these objects are significantly higher than observed values and discuss several mechanisms for resolving the discrepancies.