The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale

Abstract

Galactic planetary nebula (PN) distances are derived, except in a small number of cases, through the calibration of statistical properties of PNs. Such calibrations are limited by the accuracy of individual PN distances, which are obtained with several nonhomogeneous methods, each carrying its own set of liabilities. In this paper we use the physical properties of the PNs in the Magellanic Clouds and their accurately known distances to recalibrate the Shklovsky/Daub distance technique. Our new calibration is very similar (within 1%) to the commonly used distance scale by Cahn et al., although there are important differences. We find that neither distance scale works well for PNs with classic ("butterfly") bipolar morphology, and while the radiation-bounded PN sequences in both the Galactic and the Magellanic Cloud calibration have similar slopes, the transition from optically thick to optically thin appears to occur at higher surface brightness and smaller size than in that adopted by Cahn et al. The dispersion in the determination of the scale factor suggests that PN distances derived by this method are uncertain by at least 30%, and that this dispersion cannot be reduced significantly by using better calibrators. We present a catalog of Galactic PN distances using our recalibration, which can be used for future applications, and compare the best individual Galactic PN distances to our new and several other distance scales, both in the literature and newly recalibrated by us, finding that our scale is the most reliable to date.

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