Galactic orbits of Planetary Nebulae unveil thin and thick disk populations and cast light on interaction with the interstellar medium

We report reliable proper motion values for the central stars of four Planetary Nebulae (PNe). The proper motions have been compiled from existing optical catalogues i.e. Tycho-2, UCAC2, USNO-B and GSC-II, which are the product of large scale surveys. Results from the different sources have been compared and excellent agreement has been found in all cases reported here. Using known PNe distances and radial velocities, we then computed their Galactic velocity components which, by using a simplified model of the Galactic gravitational field, have allowed us to derive, for the first time, their Galactic orbits. These have provided the first kinematic evidence of the existence of thin and thick Galactic disk PNe populations. This approach, extended to a larger sample of PNe, will result in a statistically more solid basis. For our four individual objects though, the determination of the spatial velocity vector has already provided useful insights into their interaction with the ambient interstellar medium (ISM), confirming that the motion of the central star and the nebular shell through the ISM is the root cause for the interaction process. Our results therefore show how a more quantitative understanding of the interaction process can be achieved in the future. This would provide new insight into the return of metal enriched matter to the ISM, which in turn is in no small part responsible for the chemical evolution of galaxies.