Comparing Shocks in Planetary Nebulae with the Solar Wind Termination Shock

We show that suprathermal particles, termed pick-up ions (PUIs), might reduce the postshock temperature of the fast wind and jets in some planetary nebulae (PNs) and in symbiotic systems. The goal is to explain the finding that the temperature of the 'hot bubble' formed by the postshock gas in some PNs and symbiotic nebulae is lower, sometimes by more than an order of magnitude, than the value expected from simple hydrodynamical calculations. Although various explanations have been proposed, there is as yet no preferred solution for this 'low temperature problem.' PUIs have been invoked to explain the low temperature behind the termination shock of the solar wind. While in the case of the solar wind the neutral atoms that turn into PUIs penetrate the preshock solar wind region from the interstellar medium, in PNs the PUI source is more likely slowly moving clumps embedded in the fast wind or jets. These clumps are formed by instabilities or from backflowing cold gas. Our estimates indicate that in young PNs these PUIs will thermalize before leaving the system. Only in older PNs whose sizes exceed ~5000 AU and for which the fast wind mass loss rate is do we expect the PUIs to be an efficient carrier of energy out of the postshock region (the hot bubble).