1. ABUNDANCE RATIOS AND GALACTIC CHEMICAL EVOLUTION
4 to +0.5
dex, and the solar iron abundance is
(Fe) =
7.51 ± 0.01 dex. The average values of [Fe/H] in the solar
neighborhood, the halo, and Galactic bulge are
0.2,
1.6, and
0.2 dex
respectively.
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
10,000
AU of the young star with (sub)millimeter single-dish telescopes,
millimeter interferometers, and ground-based as well as space-borne
infrared observatories have only become possible within the past few
years. Results are compared with detailed chemical models that
emphasize the coupling of gas-phase and grain-surface chemistry.
Molecules that are particularly sensitive to different routes of
formation and that may be useful in distinguishing between a variety of
environments and histories are outlined. In the cold, low-density
prestellar cores, radicals and long unsaturated carbon chains are
enhanced. During the cold collapse phase, most species freeze out onto
the grains in the high-density inner region. Once young stars ignite,
their surroundings are heated through radiation and/or shocks,
whereupon new chemical characteristics appear. Evaporation of ices
drives a "hot core" chemistry rich in organic molecules, whereas shocks
propagating through the dense envelope release both refractory and
volatile grain material, resulting in prominent SiO, OH, and H2O
emission. The role of future instrumentation in further developing
these chemical and temporal diagnostics is discussed.
3. ELEMENTAL ABUNDANCES IN QUASISTELLAR OBJECTS: Star Formation and Galactic Nuclear Evolution at High Redshifts
Compiled by
G.T.Petrov, 2004