Abstract
We have developed several methods to estimate the ages of central stars of planetary nebulae (CSPN), which are based either on observed nebular properties or on data from the stars themselves. Our goal is to derive the age distribution of these stars and compare the results with empirical distributions for CSPN and white dwarfs. We have initially developed three methods based on nebular abundances, using (i) an age-metallicity relation which is also a function of the galactocentric distance; (ii) an age-metallicity relation obtained for the galactic disk, and (iii) the central star masses derived from the observed nitrogen abundances. In this work we present two new, more accurate methods, which are based on kinematic properties: (I) in this method, the expected rotation velocities of the nebulae around the galactic centre at their galactocentric distances are compared with the predicted values for the galactic rotation curve, and the differences are attributed to the different ages of the evolved stars; (II) we determine directly the U, V, W, velocity components of the stars, as well as the velocity dispersions, and use the dispersion-age relation by the Geneva-Copenhagen survey. These methods were applied to two large samples of galactic CSPN. We conclude that most CSPN in the galactic disk have ages under 5 Gyr, and that the age distribution is peaked around 1 to 3 Gyr.
Highlights
Why do we need stellar ages? The main motivation of this work is the study of the time variation of the abundance gradients in the galactic disk
We have developed several methods to estimate the ages of central stars of planetary nebulae (CSPN), which are based either on observed nebular properties or on data from the stars themselves
We have initially developed three methods based on nebular abundances, using (i) an age-metallicity relation which is a function of the galactocentric distance; (ii) an age-metallicity relation obtained for the galactic disk, and (iii) the central star masses derived from the observed nitrogen abundances
Summary
Why do we need stellar ages? The main motivation of this work is the study of the time variation of the abundance gradients in the galactic disk. Some recent theoretical models predict a time flattening of the observed radial abundance gradients, while other models predict the opposite behaviour. This can be analyzed on the basis of abundances of planetary nebulae and open cluster stars, and in both cases the results depend on the ages of the objects considered. CSPN and white dwarfs are the offspring of intermediate mass stars with main sequence masses between 0.8 and 8 solar masses, approximately As a consequence, their properties reflect different physical conditions depending on the stellar masses, and ages, which makes these objects extremely important in the study of the chemical evolution of the Galaxy. We discuss two kinematic methods based on the age-velocity dispersion relation obtained by the Geneva-Copenhagen survey
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