Planetary nebulae as tracers of galaxy stellar populations

The main conclusions of our study can be summarized as follows. (i) According to the post-asymptotic giant branch (AGB) stellar core mass, PN lifetime in a SSP is constrained by three relevant regimes, driven by the nuclear (M_core>~ 0.57M_solar), dynamical (0.57M_solar>~M_core>~ 0.55M_solar) and transition (0.55M_solar>~M_core>~ 0.52M_solar) time-scales. The lower limit for M_core also sets the minimum mass for stars to reach the AGB thermal-pulsing phase and experience the PN event. (ii) Mass loss is the crucial mechanism to constrain the value of α, through the definition of the initial-to-final mass relation (IFMR). The Reimers mass-loss parametrization, calibrated on Pop II stars of Galactic globular clusters, poorly reproduces the observed value of α in late-type galaxies, while a better fit is obtained using the empirical IFMR derived from white dwarf observations in the Galaxy open clusters. (iii) The inferred PN lifetime for LG spirals and irregulars exceeds 10000yr, which suggests that M_core<~ 0.65M_solar cores dominate, throughout. (iv) The relative PN deficiency in elliptical galaxies, and the observed trend of α with galaxy optical colours, support the presence of a prevailing fraction of low-mass cores (M_core<~ 0.55M_solar) in the PN distribution and a reduced visibility time-scale for the nebulae as a consequence of the increased AGB transition time. The stellar component with M_core<~ 0.52M_solar, which overrides the PN phase, could provide an enhanced contribution to hotter HB and post-HB evolution, as directly observed in M 32 and the bulge of M 31. This implies that the most UV-enhanced ellipticals should also display the lowest values of α, as confirmed by the Virgo cluster early-type galaxy population. (v) Any blue-straggler population, invoked as progenitor of the M_core>~ 0.7M_solar PNe in order to preserve the constancy of the bright luminosity-function cut-off magnitude in ellipticals, must be confined to a small fraction (a few per cent at most) of the whole galaxy PN population.