On the distances of planetary nebulae

Abstract

Past calibrations of statistical distance scales for planetary nebulae have been problematic, especially with regard to 'short' vs. '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 sixteen central stars published by Harris et al. (2007) 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 (2008, 2014) 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 two, 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 (e.g. Mellema 2004). We find extinction distances to be often unreliable individually though sometimes approximately correct overall (total sample).Comparison of the Hipparcos parallaxes (van Leeuwen 2007) 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. There may be negative implications for Gaia parallaxes for these objects. We suggest a possible connection with the much smaller overestimation recently shown for the Hipparcos Pleiades parallaxes by Melis et al. (2014).

The H surface brightness-radius relation: a robust statistical distance indicator for planetary nebulae

“…We further note that Smith (2015) identified and discussed the scale error at large radii that affects the SSV distance scale. Recall that SSV used a constant mass assumption for all PNe larger than a radius of 0.06 pc.…”

“…In this section we briefly review the standard statistical techniques previously used in the literature. The reader is referred to the review of Smith (2015) for a fuller discussion of the limitations and biases of each distance technique.…”

“…Harris et al 1997Harris et al , 2007, the Hipparcos satellite van Leeuwen 2007), or the Hubble Space Telescope (HST; Benedict et al 2003Benedict et al , 2009). The ground-based US Naval Observatory (USNO) CCD parallaxes of Harris et al (2007) form an homogenous sample of accurate distances for several nearby PNe, and Smith (2015) has shown that they form a reliable, internally consistent dataset. Owing to uncertain systematics, we have not used the ground-based data from Gutiérrez-Moreno et al (1999).…”

“…The Hipparcos parallaxes (van Leeuwen 2007) have also been shown to be problematic (e.g. Smith 2015), especially for compact PNe where subtle surface brightness variations across the PN may have had an undue influence on the astrometric reductions, therefore the Hipparcos parallaxes have not been used as calibrating data (cf. F08).…”

“…Indeed, several independent lines of evidence point to problems with some of the published determinations, especially some of the older ones (see Pottasch 1996;Smith 2015). More specifically, the log g values are often underestimated, especially at low to moderate surface gravities.…”