Abstract. We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to ∼0.15 M . This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a disk control field were used to decontaminate the bulge CMDs from foreground disk stars, and hence to set a stronger constraint on the bulge age, which we found to be as large as that of Galactic globular clusters, or > ∼ 10 Gyr. No trace is found for any younger stellar population. Synthetic CMDs have been constructed to simulate the effect of photometric errors, blending, differential reddening, metallicity dispersion and depth effect in the comparison with the observational data. By combining the near-IR data with optical ones, from the Wide Field Imager at the ESO/MPG 2.2 m telescope, a disk-decontaminated (M K ,V-K) CMD has been constructed and used to derive the bulge metallicity distribution, by comparison with empirical RGB templates. The bulge metallicity is found to peak at near solar value, with a sharp cutoff just above solar, and a tail towards lower metallicity that does not appreciably extend below [M/H] ∼ −1.5.

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The aim of this paper is to provide a handy tool to compute the impact of type Ia SN (SNIa) events on the evolution of stellar systems. An effective formalism to couple the rate of SNIa explosions from a single burst of star formation and the star formation history is presented, which rests upon the definition of the realization probability of the SNIa event (A Ia ) and the distribution function of the delay times ( f Ia (τ)).It is shown that the current SNIa rate in late type galaxies constrains A Ia to be on the order of 10 −3 (i.e. 1 SNIa every 1000 M of gas turned into stars), while the comparison of the current rates in early and late type galaxies implies that f Ia ought to be more populated at short delays. The paper presents analytical formulations for the description of the f Ia function for the most popular models of SNIa progenitors, namely Single Degenerates (Chandrasekhar and Sub-Chandrasekhar exploders), and Double Degenerates. These formulations follow entirely from general considerations on the evolutionary behavior of stars in binary systems, modulo a schematization of the outcome of the phases of mass exchange, and compare well with the results of population synthesis codes, for the same choice of parameters. The derivation presented here offers an immediate astrophysical interpretation of the shape of the f Ia functions, and have a built in parametrization of the key properties of the alternative candidates. The important parameters appear to be the minimum and maximum masses of the components of the binary systems giving rise to a SNIa explosions, the distribution of the primary mass and of the mass ratios in these systems, the distribution of the separations of the DD systems at their birth. The various models for the progenitors correspond to markedly different impact on the large scales; correspondingly, the model for the progenitor can be constrained by examining the relevant observations. Among these, the paper concentrates on the trend of the current SNIa rate with parent galaxy type. The recent data by Mannucci et al. (2005, A&A, 433, 807) favor the DD channel over the SD one, which tends to predict a too steep distribution function of the delay times. The SD scenario can be reconciled with the observations only if the distribution of the mass ratios in the primordial binaries is flat and the accretion efficiency onto the WD is close to 100%. The various models are characterized by different timescales for the Fe release from a single burst stellar population. In particular the delay time within which half of the SNIa events from such a population have occurred, ranges between 0.3 and 3 Gyr, for a wide variety of hypothesis on the progenitors.

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We explore the formation of α‐enhanced and metal‐rich stellar populations in the nuclei of luminous ellipticals under the assumption of two extreme galaxy formation scenarios based on hierarchical clustering, namely a fast clumpy collapse and the merger of two spirals. We investigate the parameter space of the star formation time‐scale, the slope of the initial mass function (IMF) and stellar yields. In particular, the latter add a huge uncertainty in constraining time‐scales and IMF slopes. We find that ‐‐ for Thielemann, Nomoto & Hashimoto nucleosynthesis ‐‐ in a fast clumpy collapse scenario an [α/Fe] overabundance of ∼ 0.2 dex in the high‐metallicity stars can be achieved with a Salpeter IMF and star formation time‐scales of the order 109 yr. The scenario of two merging spirals that are similar to our Galaxy, instead, fails to reproduce the α‐enhanced abundance ratios in the metal‐rich stars, unless the IMF is flattened during the burst ignited by the merger. This result is independent of the burst time‐scale. We suggest that abundance gradients give hints to distinguish between the two extreme formation scenarios considered in this paper.