On the distances of planetary nebulae

Abstract

We reconsider calibration of statistical distance scales for planetary nebulae, examining precision and systematic error for various distance methods used as well as the scales themselves. A different calibration strategy, one based on precise trigonometric parallaxes by Harris et al. (2007; some improved by Benedict et al. 2009), is presented. Most statistical scales have an overall scale error; in addition, all four tested show dependence of distance ratio [scale/actual] on nebular radius. However, the H-alpha scale of Frew (2008; slightly revised in Frew et al. 2014) has only a small radius dependence at large R and overall is consistent with these parallaxes. Some methods used for calibration have systematic errors, and the extinction method is found to be mostly unreliable. Comparison of Hipparcos parallaxes for a few large planetaries with our "best estimate" distances for those confirms the overestimation of 2.5 found by Harris et al. for PHL 932. We suggest a possible connection with the smaller overestimation found by Melis et al. (2014) when testing Hipparcos parallaxes for some of the Pleiades.