Background

In many respects, bulges of spiral galaxies are very similar to elliptical galaxies. Both adhere to many common scaling relations, including the fundamental plane, possess high stellar densities, have little in the way of gas and dust, and appear essentially old, with enhanced abundance ratios of a-elements with respect to iron. In our own Milky Way, the bulge accounts for ~20% of the Galaxy's baryons - a factor of ten greater than the stellar halo. Despite this significance, relatively few detailed chemical-evolution models of the bulge exist,1 due in part to a dearth of high-resolution spectroscopic studies of its individual stars. Having said that, in lieu of such data, notable exceptions have appeared in the literature, drawing upon extant metallicity-distribution functions (MDFs) and inferred abundance ratios derived from lower-resolution data.

1 In contrast to the case of the halo, which, despite its trace baryonic contribution to the Milky Way, has had at least ten times the number of models published to explain its origin.

Koppen & Arimoto (1989, 1990) assumed infall of primordial gas on a free-fall timescale (0.1 Gyr) and power-law initial mass functions (IMFs) of slope x = 1.05 (Koppen & Arimoto 1989) and x = 1.30 (Koppen & Arimoto 1990), over the mass range 0.05 < m/MQ < 60. Rapid and efficient star formation (10 Gyr-1) was halted by a supernova-driven wind after 1 Gyr, the metal-enriched outflowing gas providing fuel to the Galactic disc for future star formation. The Koppen & Arimoto models successfully recovered the bulge's MDF, present-day gas mass fraction and enhanced [a/Fe], despite the (i) neglect of Type-Ia supernovae and (ii) use of the instantaneous-recycling approximation.

Matteucci & Brocato (1990) and, later, Matteucci et al. (1999) relaxed these two limitations of Koppen & Arimoto, also concluding that flatter-than-Salpeter IMFs (1.1 < x < 1.3, over the mass range 0.1 < m/MQ < 100) in conjunction with (i) a Schmidt-like star-formation law, (ii) rapid infall of primordial gas on timescales of 0.01 Gyr (Matteucci & Brocato 1990) and 0.1 Gyr (Matteucci et al. 1999) and (iii) efficient star formation (20 Gyr-1) applied. Both models were successful in terms of recovering the bulge's MDF and enhanced [a/Fe].

Samland et al. (1997) suggested that the bulge's MDF is consistent with the use of a more traditional Salpeter IMF (x = 1.35, over the mass range 0.1 < m/MQ < 100) and a more prolonged star-formation phase (with the bulge being 3-5 Gyr younger than the halo, a conclusion that is perhaps less secure), with 'breathing' phases of infall and outflow throughout the bulge's history. Molla et al. (2000) also adopted the Salpeter IMF and assumed two infall phases ('bulge' and 'core', with a longer infall timescale for the dominant 'bulge' phase of 0.7 Gyr). As with Samland et al. (1997), infall and outflow lead to exchange of matter among halo, bulge and core, and ultimately to a predicted bulge MDF matching that observed.

All of the above models have their merits and detriments, but space precludes a detailed intercomparison. The prediction of a-enhanced abundance patterns across the full range of bulge metallicities (-1 < [Fe/H] < +0.5) is somewhat unique to the 'Matteucci' models, for obvious reasons (IMF + star-formation efficiencies + timescales).

The recent appearance of spectacular high-resolution spectroscopic data for the bulge (e.g. Lecureur et al. 2007 and references therein), makes it timely not only to revisit the traditional [a/Fe] patterns predicted by chemical-evolution models, but also to begin to inspect individual a-to-a-element predictions, to seek further insights into bulge formation (and, as we will suggest, stellar evolution). Such a preliminary analysis was undertaken by Gibson (1995), but the data quality at the time made the conclusions speculative, at best. In this short contribution, we revisit the issue of bulge abundance patterns, concentrating instead on a previously (somewhat) ignored component of the models - specifically, the sensitivity to the compilation of the yield from Type-II supernovae adopted.

Figure 45.1. Nucleosynthetic abundance-ratio (oxygen-to-magnesium: [O/Mg]) patterns at z = 0.02 predicted by the Solar-metallicity Type-II-supernova models of Woosley & Weaver (1995, WW95), Thielemann et al. (1996, TNH96) and Arnett (1991, A91). The shaded region is representative of the subsolar [O/Mg] range encountered in the Galactic bulge: -0.4 < [O/Mg] < -0.2 (McWilliam & Rich 1994; Lecureur et al. 2007).

Figure 45.1. Nucleosynthetic abundance-ratio (oxygen-to-magnesium: [O/Mg]) patterns at z = 0.02 predicted by the Solar-metallicity Type-II-supernova models of Woosley & Weaver (1995, WW95), Thielemann et al. (1996, TNH96) and Arnett (1991, A91). The shaded region is representative of the subsolar [O/Mg] range encountered in the Galactic bulge: -0.4 < [O/Mg] < -0.2 (McWilliam & Rich 1994; Lecureur et al. 2007).

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