Age distribution of the central stars of galactic disk planetary nebulae
  The determination of ages of central stars of planetary nebulae (CSPN) is a
complex problem, and there is presently no single method that can be generally
applied. We have developed several methods to estimate the ages of CSPN, based
both on the observed nebular properties and in some properties of the stars
themselves.Our aim is to estimate the ages and the age distribution of CSPN and
to compare the derived results with mass and age determinations of CSPN and
white dwarfs based on empirical determinations of these quantities. We discuss
several methods to derive the age distribution of CSPN, namely, (i) the use of
an age-metallicity relation that also depends on the galactocentric distance,
(ii) the use of an age-metallicity relation obtained for the galactic disk, and
(iii) the determination of ages from the central star masses obtained from the
observed nitrogen abundances. We consider a sample of planetary nebulae in the
galactic disk, most of which ($\sim$ 69%) are located in the solar
neighbourhood, within 3 kpc from the Sun. We estimate the age distribution of
CSPN with average uncertainties of 1-2 Gyr, and compare our results with the
expected distribution based both on the observed mass distribution of white
dwarfs and on the age distribution derived from available mass distributions of
CSPN. We conclude most CSPN in the galactic disk have ages under 6 Gyr, and
that the age distribution is peaked around 2-4 Gyr.
