1. ABUNDANCE RATIOS AND GALACTIC CHEMICAL EVOLUTION
Detailed abundance analysis reveals that the Galactic disk, halo, and bulge exhibit unique abundance patterns of O, Mg, Si, Ca, and Ti and neutron-capture elements. These signatures show that environment plays an important role in chemical evolution and that supernovae come in many flavors with a range of element yields.
The 300-fold dispersion in heavy element abundances of the most metal-poor stars suggests incomplete mixing of ejecta from individual supernova, with vastly different yields, in clouds of 106 M.
The composition of Orion association stars indicates that star-forming regions are significantly self-enriched on time scales of 80 million years. The rapid self-enrichment and inhomogeneous chemical evolution models are required to match observed abundance trends and the dispersion in the age-metallicity relation.
2. CHEMICAL EVOLUTION OF STAR-FORMING REGIONS
3. ELEMENTAL ABUNDANCES IN QUASISTELLAR OBJECTS: Star Formation and Galactic Nuclear Evolution at High Redshifts
(QSO) elemental abundances provide unique probes of high-redshift star
formation and galaxy evolution. There is growing evidence from both the
emission and intrinsic absorption lines that QSO environments have
roughly solar or higher metallicities out to redshifts >4. The range
is not well known, but solar to a few times solar metallicity appears
to be typical. There is also evidence for higher metallicities in more
luminous objects and for generally enhanced N/C and Fe/ abundances compared with solar ratios.
These results identify QSOs with vigorous,
high-redshift star formationconsistent
with the early evolution of massive galactic nuclei or dense
protogalactic clumps. However, the QSOs offer new constraints. For
example, (a) most of the enrichment and star formation must
occur before the QSOs "turn on" or become observable, on time scales of
1 Gyr at least at the highest redshifts. (b)
The tentative result for enhanced Fe/ suggests that the first local star
formation began at least 1 Gyr before the QSO epoch. (c)
The star formation must ultimately be extensive to reach high
metallicities; that is, a substantial fraction of the local gas must be
converted into stars and stellar remnants. The exact fraction depends
on the shape of the initial mass function (IMF). (d) The highest
derived metallicities require IMFs that are weighted slightly more
toward massive stars than in the solar neighborhood. (e) High
metallicities also require deep gravitational potentials. By analogy
with the well-known massmetallicity
relation among low-redshift galaxies, metal-rich QSOs should reside in
galaxies (or protogalaxies) that are minimally as massive (or as
tightly bound) as our own Milky Way.