High-quality abundances for thick- and thin-disk stars are now available together with kinematic information, allowing the abundance patterns in the thick and thin disks to be studied separately (see Chapter 5). These abundance patterns reflect the different star-formation histories of these two components. Moreover, whereas for metallicities [Fe/H] < -2.5 the interstellar medium was enriched essentially by massive stars, for [Fe/H] > — 1.5upto ~0.5 (the metallicity range of the thick/thin disks) the chemical enrichment is complex, involving not only the contribution of stars of all masses and of Type-Ia supernovae, but also metallicity effects that can modify the stellar yields. Clearly, the study of this problem requires the use of detailed chemical-evolution models in which the approximation of instantaneous recycling is not assumed.

In this work we assume that the two components form by gas accretion on different timescales. For the thin disk we adopt a model similar to that of Chiappini et al. (1997), but in this case we model the thin disk as an independent quantity,

cc 2

cc 2

Time (Gyr)

Figure 47.1. Variation with time of the star-formation rate (SFR) (top panels) and the predicted metallicity distribution (bottom panels) in the thick disk (left panels) and thin disk (right panels), from several models. The dashed line in the bottom-right panel shows, for comparison, the best model of Chiappini et al. (1997).

as did e.g. Pagel & Tautvaisiene (1995). In our models the thin disk forms by exponential infall of primordial gas with an e-folding time of ~7 Gyr in the Solar vicinity. For the thick disk we have fewer constraints, namely (a) its stars are older than ~ 10 Gyr, (b) the metallicity distribution has a peak around [Fe/H] = -0.5 and extends from about -1.5 to Solar or above, and (c) the thin disk is 4%-15% of the mass of the thin disk (Juric et al. 2006). The above constraints can be satisfied if we assume a shorter timescale of gas accretion for the thick disk.

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