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N. Pirzkal, F. Kerber (ESO/ST-ECF), M. Roth (Las Campanas Observatory, Carnegie)
Distances to Planetary Nebulae (PNe) are notoriously unreliable. It is known that statistical methods such as the different versions of the Shklovsky--derived distance scales can not be applied to individual objects with any confidence.
Various methods to estimate the distance of individual PN have been successfully applied in the past, e.g. using trigonometric parallaxes, radio and optical expansion rates, the central star's atmosphere, or the nature of companions in binary systems. However, these have the drawback of relying on some special properties of a PN, and require various combinations of very high S/N observations, of large time bases, and of extensive modeling. In the end, these methods can only be applied to a very small number of PNe.
As a result, after more than 30 years of research, accurate and reliable distances are known for less than 5 2000 or so Galactic PNe. This is the largest single obstacle to a better understanding of the fundamental properties of PNe.
One method which does not require us to make any assumptions about the PN and which can be applied to all PNe close to the Galactic plane is the extinction distance method. This method relies on the fact that most of the stars surrounding a target PN are either Main-Sequence stars or giants with known intrinsic colors. Using these, together with an a-priori dust extinction law, an extinction vs. distance relation can be determined for the field. We have applied this method to several PN fields which were observed in the B,V,Rc, and Ic bands. We report here on the first results and present extinction vs. distance relations in the direction of our target PNe. A discussion about the accuracy of the method and the derived distances is included.