Discussion

We investigated variations of the parameters which influence the chemical evolution, namely of the IMF index, from 0.33 to the Scalo (1998) value; of the starformation efficiency v (from 2 to 200 Gyr-1); and of the infall timescale t (from 0.01 to 0.7 Gyr) and matched the results with the most recent data for the metallicity distribution and the evolution of [a/Fe] abundance ratios with metallicity (see Figure 48.1). We found out that in order to reproduce all the constraints it is necessary to adopt a short formation timescale (0.01-0.1 Gyr) and an intense efficiency of star formation (10-20 Gyr-1), combined with an IMF flatter (x = 0.95 for M > M0) than that suitable for the Solar neighbourhood. The reference model we plotted is also able to explain the different trends of [O/Fe] and [Mg/Fe] seen in the observations. Moreover, if we compare (Figure 48.2) the [N/O] versus [O/H] plot with the data from planetary nebulae of Gorny et al. (2004), we see that in order to reproduce the average trend of observations it is necessary to assume a

Figure 48.2. The evolution of [N/O] versus [O/H] in the Galactic bulge in our reference model (solid line) compared with a model (Matteucci 1986) in which primary production of N from massive stars is assumed (dashed line). If the average trend of these observations is representative of the pristine [N/O] values, the fit is achieved with the latter model. Data for N and O in bulge PNe are from Gorny et al. (2004).

Figure 48.2. The evolution of [N/O] versus [O/H] in the Galactic bulge in our reference model (solid line) compared with a model (Matteucci 1986) in which primary production of N from massive stars is assumed (dashed line). If the average trend of these observations is representative of the pristine [N/O] values, the fit is achieved with the latter model. Data for N and O in bulge PNe are from Gorny et al. (2004).

primary production of nitrogen from massive stars of all masses at every metal-licity, in analogy with the Solar neighbourhood (Ballero et al. 2006). The most promising way of doing this seems to be constituted by use of the rotational yields of Meynet etal. (2006), as was shown in fact by Chiappini etal. (2006). In any case, no firm conclusions regarding nitrogen can be drawn due to the fact that nitrogen is self-enriched to some extent in planetary nebulae.

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Astronomy Centre, University of Sussex, Brighton, UK

Topics figuring in this conference include limits to high metallicity, metal-licity characteristics of stellar populations, [M/Fe] in bulges and discs, effects of metallicity on star formation and the initial mass function, its relation to planet formation, effects of high metallicity on stellar evolution, yields and galactic chemical evolution, metal-rich H11 regions and metallicities at high redshift.

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