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 $|\sigma_{\rm
p}/p|<1$. A catalog of PNe whose CSs have DR2 Gaia parallaxes is presented in
Table 1. We compared DR2 parallaxes with those in the literature, finding a
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 ${\rm log}(R_{\rm
pc})=-(0.226\pm0.0155)\times{\rm log}(S_{\rm H\beta})-(3.920\pm0.215)$, which
represents a notable improvement with respect to previous ones. We found that
the ionized mass vs. optical thickness distance scale for Galactic PNe is not
as well constrained by the Gaia calibrators, but gives important insight on 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-AGB
H-burning evolutionary tracks is typical for post-AGB stars.
