Annu. Rev. Astron. Astrophys. 1997. 35: 503-556 Copyright © 1997 by Annual Reviews. All rights reserved

6.2. Disk Alpha Elements

Studies of disk dwarf stars by several workers (e.g. Clegg et al 1981, Tomkin et al 1985, François 1986, Gratton & Sneden 1987, Edvardsson et al 1993) confirmed the trend of increasing [ / Fe] with decreasing [Fe/H] in the Galactic disk, as established by the analysis of G dwarfs by Wallerstein (1962); typically [ / Fe] ~ +0.4 at [Fe/H] ~ -1.0. The data of Tomkin et al (1986), Edvardsson et al (1993) show that for Mg, Ca, and Si, there is a plateau at [ / Fe] = 0.0 above [Fe/H] ~ -0.2 dex (see Figure 3a). This plateau suggests a transition from one kind of chemical evolution environment to another, which is consistent with the idea that above [Fe/H] = -0.2, the ratio of SN Ia/SN II had reached a constant value.

Edvardsson et al (1993) found that when the disk stars are separated into bins of mean galactocentric radius, R_m, the -element enhancements are seen to be maintained to higher [Fe/H] at small R_m (see Figure 4). In Tinsley's picture of SN Ia and SN II this suggests that enrichment by SN II occurred to higher [Fe/H] in the inner disk than in the outer disk, before the first SN Ia occurred, in agreement with models of the disk that predict higher SFR in the inner disk than in the outer regions (e.g. Larson 1976, Matteucci & François 1989). Edvardsson et al's results also indicate that at the solar circle, old stars seem to show a distinctly different [ / Fe] trend than young stars, and this suggests that the SFR increased with time in the disk. There is a hint that the inner disk stars of the Edvardsson et al sample show a bimodal [ / Fe] ratio, rather than a slope with [Fe/H].

Figure 4. The run of [ / Fe], computed from [Mg+Si+Ca+Ti)/Fe], versus iron abundance for four ranges in galactocentric radius, from the data of Edvardsson et al (1993). In the inner disk, Rm 7 kpc, [ / Fe] is higher than the mean trend (indicated by the solid line), while the outer disk shows an [ / Fe] deficiency. The trends at large and small Rm seem to show a bimodal appearance, rather than the shift indicated in Figure 1 for different SFR.

In order to reduce scatter in the trend with metallicity, -element abundances have often been averaged; Lambert (1987) review popularized the mean relation between [ / Fe] and [Fe/H]. The work of Edvardsson et al (1993) indicated that the trends are not the same for all elements: Ca and Si abundances correlate very well, but both Mg and Ti are systematically over-enhanced relative to Ca and Si. These observations of subtle -element trends in the disk stars are similar to, but less extreme than, the enhanced Mg and Ti abundances found for Galactic bulge stars by McWilliam & Rich (1994). Nissen & Edvardsson (1992) found a somewhat steeper decline in [O/Fe] with [Fe/H] than other elements from Edvardsson et al (1993). If these differences within the element family withstand further scrutiny it shows that the elements are not made in a single process but are produced in different amounts by different SN.

(Cunha & Lambert 1992, 1994) studied the chemical composition of B stars in various subgroups of the Orion association ([Fe/H] ~ -0.05) and found evidence for self-contamination of the association by nucleosynthesis products from SN II. In particular the subgroups show an abundance spread of ~ 0.3 dex for O, correlated with Si abundance, but no dispersion larger than the measurement uncertainties could be found for Fe, C, and N. This pattern of abundance enhancement is consistent with self-enrichment of the gas by SN II only. Additional support for this idea includes the spatial correlation of the O-Si-rich stars and the fact that the most O-Si-rich stars are found only in the youngest subgroup of the association. The time lag between the oldest and youngest subgroups is ~ 11 × 10⁶ years (Blaauw 1991), which is comparable to the lifetime of the massive stars. Thus, the massive stars had enough time to explode as SN and enrich the molecular cloud, but the time scale was too short to permit any pollution by SN Ia. If the same enrichment observed by Cunha & Lambert (1994) occurred in a similar cloud of zero-metal gas, the metallicity of the final generation would be approximately [Fe/H] = -0.8 dex.

As demonstrated by (Cunha & Lambert 1992, 1994), chemical abundance studies of star-forming regions are a particularly useful way to study basic processes in chemical evolution and SN nucleosynthesis.