2015MNRAS.449.2980S

Past calibrations of statistical distance scales for planetary nebulae have been problematic, especially with regard to `short' versus `long' scales. Reconsidering the calibration process naturally involves examining the precision and especially the systematic errors of various distance methods. Here, we present a different calibration strategy, new for planetaries, that is anchored by precise trigonometric parallaxes for 16 central stars published by Harris et al. of USNO, with four improved by Benedict et al. using the Hubble Space Telescope. We show how an internally consistent system of distances might be constructed by testing other methods against those and each other. In such a way, systematic errors can be minimized. Several of the older statistical scales have systematic errors that can account for the short-long dichotomy. In addition to scale-factor errors, all show signs of radius dependence, i.e. the distance ratio [scale/true] is some function of nebular radius. These systematic errors were introduced by choices of data sets for calibration, by the methodologies used, and by assumptions made about nebular evolution. The statistical scale of Frew and collaborators is largely free of these errors, although there may be a radius dependence for the largest objects. One set of spectroscopic parallaxes was found to be consistent with the trigonometric ones while another set underestimates distance consistently by a factor of 2, probably because of a calibration difference. `Gravity' distances seem to be overestimated for nearby objects but may be underestimated for distant objects, i.e. distance dependent. Angular expansion distances appear to be suitable for calibration after correction for astrophysical effects. We find extinction distances to be often unreliable individually though sometimes approximately correct overall (total sample). Comparison of the Hipparcos parallaxes for large planetaries with our `best estimate' distances confirms that those parallaxes are overestimated by a factor 2.5, as suggested by Harris et al.'s result for PHL 932. Assuming the problem arises from the presence of nebulosity, we suggest a possible connection with the much smaller overestimation recently shown for the Hipparcos Pleiades parallaxes by Melis et al.