| Annu. Rev. Astron. Astrophys. 1997. 35:
503-556 Copyright © 1997 by . All rights reserved |
One of the themes of this article has been the idea that chemical composition is a function of Galactic environment. In order to learn how environmental parameters can affect chemical evolution, we must make accurate measurements of elemental abundances in all the accessible locations. The different components of our Galaxy are excellent places to make such detailed studies and will ultimately provide us with the means to interpret lower resolution, lower S/N spectra of external galaxies. Chemical analysis of stars in local group galaxies will also be of great use in this regard; in particular, the red giant branch stars permit us to sample the whole history of a stellar system.
The new large telescopes and efficient spectrographs will be of immense help for measuring the composition of the red giants at the distance of the local group galaxies. However, telescope aperture and spectrograph efficiency alone will not be enough to meet this task; we must also make routine the ability to measure chemical abundances from noisy spectra. For elements with many lines, one can derive abundances from noisy spectra by combining several line regions to produce an average line profile of high S/N. When abundances for elements with few lines are required, an average abundance can be measured by combining noisy spectra from many stars, which are acquired with a fiber spectrograph. Such methods will be commonplace in the future but have already been demonstrated in the works of Jones et al (1995), Carney et al (1987), Pilachowski & Armandroff (1996).
I cannot stress enough the importance of accurate abundance measurement and reliable estimates of measurement uncertainty: If you don't have accurate measurements, or if you don't know how accurate your measurements are, you cannot draw reliable conclusions. In this regard, large surveys, as exemplified by Edvardsson et al (1993), are particularly useful; for large samples of homogeneous, high quality data with identical analysis, the zero-point uncertainties are reduced.
In the near future, the study of the composition of local group dwarf spheroidal galaxies dSphs) will be particularly fruitful. These low-mass systems have experienced relatively low rates of chemical evolution, frequently with mean metallicities near [Fe/H] = -2; as such, these systems may contain a large fraction of stars from the first stellar generations. The low average metallicity of the dSphs and the large numbers of stars in a small area of sky offer the opportunity to make efficient searches for extremely low metallicity stars; we could learn whether a lower limit to metallicity really does exist, as predicted by (Audouze & Silk (1995), and how the IMF is affected by metallicity. With more extremely low metallicity stars, we could accumulate additional evidence for composition dispersion at low metallicity and perhaps measure SN element yields, and we also might find more stars with super-enhanced r-process abundances, which are useful for measuring the age of the Galaxy. Furthermore, the observed populations in dSphs, which are indicative of star formation bursts (e.g. Smecker-Hane et al 1994), can be used to measure an approximate time scale for SN Ia.
In the bulge, the oxygen and carbon abundances are desperately
needed and might best be measured with infrared spectra of OH and CO lines.
Confirmation is required for the low abundances of Si and Ca relative to
Mg and Ti in the bulge. The heavy element abundance pattern in the bulge
offers a probe of the importance of SN II; thus, measurements of Eu and Ba
abundance as a function of metallicity would be useful. The trend of carbon
abundance with metalicity must be resolved for the halo. For the extant
stars with metallicity in the range -4
[Fe/H]
-2,
improved limits on the dispersion of Co, Cr, and Mn abundances with
metallicity would test the primordial enrichment model suggested by
Searle & McWilliam.
Also in this metallicity range, heavy element abundances for a large sample
of stars would test the idea that discrete enrichment events, by individual
SN, are responsible for the observed dispersion. Abundances of C, N, O, Na,
Mg, and Al for main-sequence stars in globular clusters would provide
important evidence for or against the role of primordial abundance
dispersion in globular
clusters. In the Galactic disk, improved measurements of stellar metallicity
towards the Galactic center and anti-center would be useful for
understanding
the radial metallicity gradient, both present and past. An extensive study
of the composition of stars in star-forming regions, similar to the work
of Cunha & Lambert
(1994),
will provide direct information on chemical enrichment and SN yields. It
would also be nice to see the experts agree on whether there is a mean
age-metallicity
relation (AMR) in the solar neighborhood, what fraction of the halo heavy
elements were made in the s-process, and the value of the solar iron
abundance.
Acknowledgments
This work would not have been possible without the support of the Observatories of the Carnegie Institution of Washington. I would like to thank the many colleagues who have taught me a few things over the years and helped me on my way; they are Olin Eggen, David Lambert, George Preston, Mike Rich, Allan Sandage, and Leonard Searle. I'd also like to express my deep appreciation to my wife, Kim, who helped with figure preparations, kept me happy, and encouraged me to finish this manuscript.