Annu. Rev. Astron. Astrophys. 1997. 35: 503-556
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6.3. Bulge Alpha Elements

To date, the only extant detailed abundance analyses of alpha elements for Galactic bulge stars are by McWilliam & Rich (1994) and A McWilliam, A Tomaney & RM Rich in preparation). McWilliam & Rich (1994) found that Mg and Ti are enhanced by ~ +0.4 dex in almost all bulge stars, even at solar [Fe/H]; however, the abundances of Ca and Si appear to follow the normal trend of alpha / Fe ratio with [Fe/H] (see Figure 5).

Figure 5

Figure 5. Trends of alpha-element abundances in the Galactic bulge, from McWilliam & Rich (1994). Filled triangles indicate the average [(Mg+Ti)/Fe] and open boxes indicate the average [(Si+Ca)/Fe]. For Si and Ca the trends follow the solar neighborhood relation (solid line), whereas the Mg and Ti abundances are enhanced by ~ 0.4 dex for most stars, similar to the halo values.

Some overlap exists between the chemical properties of the McWilliam & Rich (1994) bulge giant sample and the disk F dwarfs of Edvardsson et al (1993): In general, the disk results (Edvardsson et al 1993) show that Mg and Ti are slightly enhanced relative to Si and Ca, which is similar to, but less extreme than, the +0.4-dex enhancements of Mg and Ti in the bulge. Edvardsson et al (1993) identified a subgroup of stars with 0.1-dex enhancements of Na, Mg, and Al; these are conceivably related to the bulge giants, which have large Mg and Al enhancements. The bulge [O/Fe] ratio is not well constrained: The extant data are insufficient to determine whether oxygen behaves like Mg and Ti or Si and Ca. However, any oxygen enhancement in the bulge must be less than +0.5 dex (A McWilliam, A Tomaney & RM Rich, in preparation).

The unusual mixture of alpha-element abundances in the bulge is evidence that alpha elements are made in different proportions by different SN; i.e. there are different flavors of SN with different alpha-element yields. This conclusion is borne out by predicted alpha-element yields (e.g. Woosley & Weaver 1995), as shown in Figure 6. Figure 6 illustrates that enhanced Mg could occur with relatively more 35-Modot SN progenitors than in the disk. The enhanced Ti is not explained by any SN nucleosynthesis predictions.

Figure 6

Figure 6. Production factors from models of SN II by Woosley & Weaver (1995). Ejected element abundances for various progenitor masses are indicated by connected symbols; O and Mg are produced in large quantitiesat high mass (~ 35 Modot) but not in the lower mass (15-25 Modot) SN, which are responsible for most of the Si and Ca production. None of the models give significant enhancements of Ti relative to Fe, contrary to observations of stars in the Galactic bulge and halo. Note that production factor is defined as the ratio of the mass fraction of an isotope in the SN ejecta, divided by its corresponding mass fraction in the Sun. The mass of the progenitor making the indicated elements is given in the key in the upper right.

The Ti enhancements seen in bulge stars present a nice qualitative explanation for the well-known phenomenon that the spectral type of bulge M giants is later than disk M giants with the same temperature. Frogel & Whitford (1987) suggested that the later spectral types were due to overall super-metallicity of the bulge stars; McWilliam & Rich (1994) argued that the Ti enhancements are sufficient to create the stronger bulge M giant TiO bands, without affecting overall metallicity. The enhanced Mg abundances may also explain Rich (1988) high [Fe/H] results, which were based on measurements of the Mg b lines and assumed that the bulge giants have the solar [Mg/Fe] ratio.

Unfortunately, the unusual mixture of alpha-element abundances for the bulge makes it difficult to use these elements to estimate the bulge formation time scale; the simple picture of SN Ia and SN II implies a different time scale depending on which elements are considered. However, the observed Mg overabundances agree with the predictions of (Matteucci & Brocato 1990) and a rapid formation time scale for the bulge.

Terndrup et al (1995), Sadler et al (1996) analyzed low-resolution spectra of 400 bulge giants and found the average [Fe/H] ~ -0.11 dex, consistent with the result of McWilliam & Rich (1994). The [Mg/Fe] ratios +0.3 dex and +0.11 dex respectively.

Multi-population synthesis analysis of low-resolution integrated light spectra of the Galactic bulge by Idiart et al (1996a) indicated a mean bulge abundance ratio of [Mg/Fe] = +0.45 dex. Using the same technique for elliptical galaxies and bulges of external spirals, Idiart et al (1996b) showed a general Mg enhancement of ~ +0.5 dex. Worthey et al (1992), using single-population models, analyzed spectra of giant elliptical galaxies and found Mg enhancements relative to Fe between +0.2 to +0.3 dex. These results provide supporting evidence in favor of enhanced Mg in the bulge, as claimed by McWilliam & Rich (1994). An obvious question arising from the population synthesis results is whether Ti is enhanced in external bulges and elliptical galaxies.

The abundance results for alpha elements in the bulge show that chemical abundance ratios are a function of environmental parameters. In this regard, further study of the detailed chemical composition of Galactic components will lead to an understanding of how environment affects chemical evolution, which can be used to interpret low-resolution low-S/N spectra of distant galaxies. In particular, it is necessary to check the McWilliam & Rich (1994) results for O, Ca, and Si because results for these three elements are less reliable than for Mg and Ti.

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