Abstract. We present a set of low and intermediate mass star yields based on a modeling of the TP-AGB phase which affects the production of nitrogen and carbon. These yields are evaluated by using them in a Galaxy Chemical Evolution model, with which we analyze the evolution of carbon abundances. By comparing the results with those obtained with other yield sets, and with a large amount of observational data, we conclude that the model using these yields combined with those from Woosley & Weaver (1995) for massive stars properly reproduce all the data. The model reproduces well the increase of C/O with increasing O/H abundances. Since these massive star yields do not include winds, it implies that these stellar winds might have a smoother dependence on metallicity than usually assumed and that a significant quantity of carbon proceeds from LIM stars.

Aims. We analyze the impact on the Galactic nitrogen abundances with a new set of low and intermediate mass star yields. These yields contain a significant yield of primary nitrogen from intermediate mass stars. Methods. We use these yields as an input for a galactic chemical evolution model and study the nitrogen abundances in the halo and in the disc, and compare them with models obtained using other yield sets and with a large amount of observational data. Results. We find that with these new yields our model reproduces the observed trends adequately. In particular, these yields solve the historical problem of nitrogen evolution, giving the right level of relative abundance N/O by producing a primary component in intermediate mass stars. Moreover, using different evolutionary rates for each radial region of the Galaxy, we can explain the observed N dispersion.

Thermally pulsing asymptotic giant branch (TP-AGB) models of bulge stars are calculated using a synthetic model. The goal is to infer typical progenitor masses and compositions by reproducing the typical chemical composition and central star masses of planetary nebulae (PNe) in the Galactic bulge. The AGB tip luminosity and the observation that the observed lack of bright carbon stars in the bulge are matched by the models.Five sets of galactic bulge PNe were analyzed to find typical abundances and central star of planetary nebulae (CSPN) masses. These global parameters were matched by the AGB models. These sets are shown to be consistent with the most massive CSPN having the largest abundances of helium and heavy elements. The CSPN masses of the most helium rich (He/H 0.130 or Y 0.34) PNe are estimated to be between 0.58 and 0.62 M ⊙ . The oxygen abundance in form log (O/H) + 12 of these highest mass CSPN is estimated to be ≈8.85.TP-AGB models with ZAMS masses between 1.2 and 1.8 M ⊙ with Y ZAMS ≈ 0.31− 0.33 and Z ZAMS ≈ 0.19 − 0.22 fit the typical global parameters, mass, and abundances of the highest mass CSPN. The inferred ZAMS helium abundance of the most metal enriched stars implies dY /dZ ∼ 4 for the Galactic bulge. These models produce no bright carbon stars in agreement with observations of the bulge. These models produce an AGB tip luminosity for the bulge in agreement with the observations. These models suggest the youngest main sequence stars in the Galactic bulge have enhanced helium abundance (Y ≈ 0.32) on the main sequence and their ages are between 2 and 4 Gyrs.The chemical evolution of nitrogen in the Galactic bulge inferred from the models is consistent with the cosmic evolution inferred from HII regions and unevolved stars. The inferred ZAMS N/O ratio (log N/O ≈ −0.35) of bulge PNe with the largest CSPN masses are shown to be above the solar ratio. The inferred ZAMS N/O ratios of the entire range of PNe metallicities is consistent with both primary and secondary production of nitrogen contributing to the chemical evolution of nitrogen in the Galactic bulge.The inferred ZAMS value of C/O is less than 1. This indicates the mass of the PNe progenitors are low enough (M 1.8 M ⊙ ) to not produce carbon stars via the third dredge-up.

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We examine two scenarios for formation of the planetary nebula K648: a prompt scenario where the planetary nebula is ejected and formed immediately after a helium shell flash and a delayed scenario where a third dredge up occurs and the envelope is ejected during the following interpulse phase. We present models of both scenarios and find that each can produce K648-like systems. We suggest that the prompt scenario is more favorable but cannot rule out the delayed scenario.Comment: 19 pages, 2 figures, latex, 2 files containing postscript figures. Paper to appear in Astrophysical Journal, 483, 837 (1997