![]() | 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, Rm, the
-element enhancements
are seen to be maintained to higher [Fe/H] at small Rm
(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].
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Figure 4. The run of
[ |
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.