SIMBAD references

We calculate the X-ray emission from both constant and time-evolving shocked fast winds blown by the central stars of planetary nebulae (PNe) and compare our calculations with observations. Using spherically symmetric numerical simulations with radiative cooling, we calculate the flow structure and the X-ray temperature and luminosity of the hot bubble formed by the shocked fast wind. We find that a constant fast wind gives results that are very close to those obtained from the self-similar solution. We show that in order for a fast shocked wind to explain the observed X-ray properties of PNe, rapid evolution of the wind is essential. More specifically, the mass-loss rate of the fast wind should be high early on when the speed is ∼300-700 km/s, and then it needs to drop drastically by the time the PN age reaches ∼1000 yr. This implies that the central star has a very short pre-PN (post-asymptotic giant branch) phase.