Copyright © 1997 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 1997. 35:
503-556 Copyright © 1997 by Annual Reviews. All rights reserved |
In this section, I discuss some implications and uses of the most basic chemical composition information, namely metallicity. The word metallicity has more than one meaning: The precise definition is that metallicity is the mass fraction of all elements heavier than helium, denoted by the symbol Z; this is not always practical for observers because information usually does not exist for all elements. For observational stellar astronomy, metallicity is more often used to refer to the iron abundance. Unless explicitly stated the word metallicity used here refers to [Fe/H], the logarithmic iron abundance relative to the solar value.
Because the main-sequence lifetimes
of G and F dwarfs are comparable to the age of the Galaxy, all the G dwarfs
ever born are assumed to still exist (although see discussion of
metallicity-dependent lifetimes by
Bazan & Mathews
1990),
and so these stars can provide a complete picture of Galactic chemical
evolution. Early studies of the metallicity distribution of G dwarfs,
within about 25 pc of the sun
(van den Bergh
1962,
Schmidt 1963,
Pagel & Patchett
1975),
showed that there is a deficit of metal-poor stars relative to the
prediction of the Simple model; this is the well-known G-dwarf
problem. The metallicities
of these early studies were based on UV excesses (see
Wallerstein &
Carlson 1960,
Sandage 1969),
which are accurate to approximately
1
= 0.25 dex
(Pagel & Patchett
1975); although
Norris & Ryan
(1989)
claim uncertainties of ± 0.45 dex. The observed metallicity
distributions contain biases that must be taken into account in order to
obtain the true metallicity function (e.g. see
Sommer-Larsen
1991,
Pagel 1989).
Many possible explanations were presented to account for the G-dwarf problem (e.g. Audouze & Tinsley 1976), but infall of metal-poor gas onto the disk was the most favored solution. To fit the observed metallicity function by this scheme, the original disk was at most 5% of the present disk mass (Pagel 1989), with mass infall occurring over several billion years. Variants of the Simple model exist that include gas infall in various ways (e.g. Larson 1974, 1976, Lynden-Bell 1975, Clayton 1985, 1988, Pagel 1989). All of these models predict a strict age-metallicity relation (AMR) with no abundance dispersion.
In these models, the halo could not have been responsible
for the bulk of the gas infall because the present-day luminous halo mass
is only a few percent of the disk
(Sandage 1987,
Pagel 1989);
a metallicity function of the disk+halo still suffers a paucity of
metal-poor stars relative to the simple model (e.g.
Worthey 1996).
(Tosi 1988)
showed that infall of gas with metallicity 0.1
Z
provides
as effective an explanation of the observed disk metallicity distribution
function as infall of zero metallicity gas; however, infalling gas with
Z = 0.4
Z is
excluded by observations.
A number of studies over the last decade
and a half have combined star count and kinematic information with
metallicities estimated from UV excesses (e.g.
Sandage & Fouts
1987),
ubvy
photometry (e.g.
Nissen & Schuster
1991),
and low S/N spectra (e.g.
Carney et al 1987,
Jones et al 1995).
The assembled databases have been used to imply the existence of various
Galactic populations. For example, the thick disk of
Gilmore & Reid
(1983)
is characterized by scale height of ~ 1.3 pc, mean [Fe/H] ~
-0.6 dex, and dispersion 0.3 dex
(Gilmore & Reid
1983,
Gilmore 1984,
Gilmore & Wyse
1985,
Wyse & Gilmore
1986),
with no apparent metallicity gradient.
(Wyse & Gilmore
1995)
conclude that the data are best fit by overlapping thick and thin disks;
the thick disk has a mean metallicity of [Fe/H] ~
-0.7 dex, ranging from -0.2 to -1.4 dex. A low metallicity tail,
extending down to [Fe/H] ~ -2 to -3, was claimed by
(Norris & Ryan
(1991),
Beers &
Sommer-Larsen (1995),
Pagel &
Tautvaisiene (1995).
Typical star count models yield thick disk to thin disk ratios of a few
percent (e.g.
Majewski 1993).
The thin disk metallicity peaks near [Fe/H] = -0.25 dex, ranging from
+0.2 to -0.8 dex
(Wyse & Gilmore
1995).