Annu. Rev. Astron. Astrophys. 1997. 35: 503-556
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6. ABUNDANCE TRENDS WITH METALLICITY

6.1. Alpha Elements

Enhancements of alpha elements in metal-poor stars were first identified by Aller & Greenstein (1960) and more firmly established by Wallerstein (1962), who found excesses of Mg, Si, Ca, and Ti relative to Fe. A corresponding enhancement for oxygen was first discovered by Conti et al (1967). The work of Clegg et al (1981), François (1987, 1988) showed that S is also overabundant in metal-poor stars. These enhancements increase linearly with decreasing metallicity, reaching a factor of two above the solar [alpha / Fe] ratios at [Fe/H] near -1; below [Fe/H] = -1 the enhancements are approximately constant. Figure 3a shows the general trend of [O/Fe] with [Fe/H]. It is important to emphasize that "alpha element" is simply a convenient phrase used to signify the observation that some even-Z elements (O, Mg, Si, S, Ca, and Ti) are overabundant relative to iron at low metallicity, and it does not signify that these are all products of a single nuclear reaction chain that occurs in the same astrophysical environment.

Figure 3

Figure 3. The trend of oxygen abundance with metallicity. The favored trend is shown in (a), a compilation of [O I] results: crosses from disk data of Edvardsson et al (1993), filled squares from Spite & Spite (1991), filled circles from Barbuy (1988), open triangles from Kraft et al (1992), Sneden et al (1991), open squares from Shetrone (1996a). (b) shows results from the O I triplet: crosses (Abia & Rebolo 1989) and filled triangles (Tomkin et al 1992); low S/N results from ultraviolet OH lines are indicated by open squares (Nissen et al 1994) and open triangles (Bessell et al 1991). Note the difference in the scale of the ordinate between (a) and (b).

As mentioned in the introductio Tinsley (1979) suggested that the [alpha / Fe] trend with [Fe/H] is due to the time delay between SN II, which produce alpha elements and iron-peak elements (e.g. Arnett 1978, Woosley & Weaver 1995), and SN Ia, which yield mostly iron-peak with little alpha element production (e.g. Nomoto et al 1984 Thielmann et al 1986). Thus, after the delay for the onset of SN Ia, the [alpha / Fe] ratio declines from the SN II value. The SN Ia time scale is an important consideration for this model. Iben & Tutukov (1984) favor a mechanism with mass transfer during the merging of a CO+CO white dwarf binary system; time scales for SN Ia from this model range from 108 to 1010 years, depending on progenitor masses and mass transfer parameters. Smecker-Hane & Wyse (1992) obtained estimates for the first SN Ia of 108 years.

Other explanations for the alpha-element trend have been put forward: Maeder (1991) suggested that exploding Wolf-Rayet stars (type Ib supernovae, SN Ib) might be responsible for the observed alpha-element abundance trend. Wolf-Rayet stars are the bare cores of massive stars that have lost their outer envelopes through copious stellar winds. The radiatively driven winds are metallicity-dependent, producing significant numbers of Wolf-Rayet stars above [Fe/H] ~ -1. The chemical yields depend on the mass-loss rates: At high metallicity the strong winds remove much of the helium before it is further transformed into heavy elements.

Edmunds et al (1991) suggested that metallicity-dependent element yields could be the source of the alpha-element abundance trend and predicted that SMR stars should possess subsolar [alpha / Fe] ratios. The theoretical element yields from SN II (e.g. Woosley & Weaver 1995) do not show such a metallicity dependence; however, some star formation theories have predicted a metallicity-dependent IMF (e.g. Kahn 1974, Yoshii & Saio 1986), which might conceivably result in a steady increase of the SN Ia/SN II ratio with increasing metallicity and thereby account for the observed alpha-element trend.

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