Abundances of O Na Mg and Al

These four elements were chosen because they are produced mainly by massive stars that explode as Type-II supernovae. Therefore their ratios to iron (which is produced mainly by Type-I supernovae) reflect the formation timescale of the parent population. As shown in Figure 14.1, wherever the thin and thick discs are well separated, the bulge manifests a third distinct trend. For Al and Mg, the separation is clearly visible for [Fe/H] < 0, whereas for O the distinction among the three populations was confirmed by statistical tests. Concerning Na, at subsolar metallicities a distinction can be made neither between bulge and disc stars nor

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Figure 14.1. Plots of [O/Fe], [Mg/Fe], [Na/Fe] and [Al/Fe] ratios against [Fe/H] for our sample of bulge giants (filled grey circles) compared with the thin-disc (open triangles) and thick-disc samples (black squares) from Reddy et al (2006) and Bensby et al. (2004, 2005). The black upward- and downward-pointing triangles are for Arcturus and ^-Leo, respectively. In the [O/Fe] panel, only the Bensby et al. (2004) [O i] measurements are shown. Notice the clear separations between the thin-disc, thick-disc and bulge stars.

Figure 14.1. Plots of [O/Fe], [Mg/Fe], [Na/Fe] and [Al/Fe] ratios against [Fe/H] for our sample of bulge giants (filled grey circles) compared with the thin-disc (open triangles) and thick-disc samples (black squares) from Reddy et al (2006) and Bensby et al. (2004, 2005). The black upward- and downward-pointing triangles are for Arcturus and ^-Leo, respectively. In the [O/Fe] panel, only the Bensby et al. (2004) [O i] measurements are shown. Notice the clear separations between the thin-disc, thick-disc and bulge stars.

between the two discs. At high metallicities, the bulge stars have higher sodium abundances than than those of the two discs. The [Al/Fe], [Mg/Fe] and [O/Fe] ratios for the bulge stars are higher than those for the thick-disc ones, which are themselves more enhanced than those for the thin-disc stars. These results support a scenario in which the formation timescale of the bulge was shorter than that of the thick disc, which was in its turn shorter than that of the thin disc and the three populations have experienced different chemical trajectories.

4.3 Nucleosynthesis of the bulge

To investigate the enrichment by massive stars we compared our measurements with the predicted yields of Chieffi and Limongi (2004), using Mg instead of iron as an indicator of metallicity. Figure 14.2 shows the following.

Figure 14.2. The run of the [O/Mg] (left) and [Na/Fe] (right) ratios with metallicity for the bulge stars of our sample (circles). In the top panels, the predicted yields for SNII of 15M0 (full line), 2OM0 (dashed line), 25M0 (dotted line) and 35M0 (dash-dotted line) from Chieffi & Limongi (2004) are overlain. In the bottom panels, the bulge is compared with the Galactic thin (squares) and thick (triangles) discs, as in Bensby et al. (2004), using [Mg/H] as a proxy for metallicity.

Figure 14.2. The run of the [O/Mg] (left) and [Na/Fe] (right) ratios with metallicity for the bulge stars of our sample (circles). In the top panels, the predicted yields for SNII of 15M0 (full line), 2OM0 (dashed line), 25M0 (dotted line) and 35M0 (dash-dotted line) from Chieffi & Limongi (2004) are overlain. In the bottom panels, the bulge is compared with the Galactic thin (squares) and thick (triangles) discs, as in Bensby et al. (2004), using [Mg/H] as a proxy for metallicity.

• The [O/Mg] ratio decreases with [Fe/H], whereas the yields are essentially metallicity-independent. In the [O/Mg] versus [Mg/H] plot, thin disc, thick disc and bulge lie on the same sequence: the bulge merely extends the sequence to higher [Mg/H] values. The possibility of a decrease in production of O arising from an increase in winds with increasing metallicity has been invoked to explain the decrease in [O/Mg], which is not predicted by the yields. This point has still to be investigated from a theoretical point of view.

• The [Na/Mg] ratio increases with [Fe/H] with values lower than those predicted by Chieffi and Limongi, especially at low [Fe/H]. In the [Na/Mg] versus [Mg/H] plot, for [Mg/H] < 0 the three populations exhibit different trends according to their different formation timescales: the thin disc is above the thick disc, which itself is above the bulge. This tendency suggests an additional contribution of Na by longer-lived progenitors (such as SN I or intermediate-mass AGB). This additional source starts contributing first in the thin disc, at lower metallicities then in the thick disc and the bulge.

The [Al/Mg] ratio, which is plotted in Lecureur et al. (2006), undergoes an increase with metallicity that is well predicted by the metallicity dependence of the yields. In contrary to the case with Na, the three populations seem to be merged, despite the occurrence of a large dispersion of Al at high metallicity. This suggests that the contribution by AGB stars may be important for Na but not for Al, possibly because of the higher temperature required to sustain the Mg-Al cycle than for the Ne-Na cycle.

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