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AGES OF CENTRAL STARS OF 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. Our group has pioneered in the treatment of this problem, and we have developed several methods to estimate the ages of the PN progenitor stars, based both on the observed nebular properties and in some properties of the stars themselves. In principle, the traditional methods to derive the ages of galactic stars can be applied to CSPN, such as the use of theoretical isochrones. However, the physical properties of these objects are not as well known as in the case of normal stars, so that the derived isochrones are generally uncertain, leading to the need of alternative methods. In this work, we will discuss several methods developed so far by our group, such as (i) the use of an age-metallicity relation that also depends on the galactocentric radius, (ii) the determination of ages from the central star masses obtained from the observed nitrogen abundances, and (iii) the use of an age-metallicity relation obtained for the galactic disk. Also, theoretical isochrones for A G B s t a r s h a v e b e e n u s e d t o d e r i v e a g e s o f C SPN in the SMC. We estimate the expected age distribution of the CSPN based on the observed distribution of white dwarf stars, and compare the results with the distributions obtained by the methods mentioned above and with available mass distributions of CSPN.