The Population of Galactic Planetary Nebulae: a Study of Distance Scales and Central Stars Based on the Second Gaia Release

We matched the astrometry of central stars (CSs) of spectroscopically confirmed Galactic planetary nebulae (PNe) with DR2 Gaia parallaxes (p), finding 430 targets in common with p > 0 and Gaia parallaxes is presented in Table 1. We compared DR2 parallaxes with those in the literature, finding good correlation between the two samples. We used PNe parallaxes to calibrate the Galactic PN distance scale. Restricting the sample to objects with 20% parallax accuracy, we derive the distance scale log (R-pc) = -(0.226 +/- 0.0155) x log(S-H beta) - (3.920 +/- 0.215), which represents a notable improvement with respect to previous ones. We found that the ionized mass versus optical thickness distance scale for Galactic PNe is not as constrained by the Gaia calibrators, but gives important insight into the nature of the PNe, and is essential to define the domain for our distance scale application. We placed the CSs whose distance has been determined directly by parallax on the HR diagram, and found that their location on the post-asymptotic-giant-branch (AGB) H-burning evolutionary tracks is typical for post-AGB stars.