|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by Annual Reviews. All rights reserved
Except for the lightest elements, the history of the chemical composition of the Galaxy is dominated by nucleosynthesis occurring in many generations of stars. Stars of low mass have long lifetimes, some comparable to the age of the Galaxy, and their envelopes have preserved much of their original chemical composition. These stars are useful because they are fossils containing information about the history of the evolution of chemical abundances in the Galaxy. After the Big Bang, the story of nucleogenesis is concerned mostly with the physics of stellar evolution and nucleosynthesis in stars, with how the environment dictated the kinds of stars that formed to enrich the Galactic gas, and with how the enriched gas mixed with the interstellar medium to form subsequent stellar generations (Hoyle 1954). We can try to understand these processes and chemical evolution from theoretical models, but the best way to learn about the history of the elements in the Galaxy is to look at the fossils.
Time and space do not permit me to discuss in sufficient detail the many exciting developments that have occurred in the area of chemical evolution. Therefore, I restrict myself to areas most closely aligned with my research, which is usually concerned with high-resolution abundance analysis of stars in our Galaxy; in particular I do not discuss all the elements, or families of elements, and some elements may be conspicuous by their absence.
Despite its obvious flaws, a good starting point for developing a mental picture of chemical evolution is the Simple one-zone model (e.g. Schmidt 1963, Searle & Sargent 1972, Pagel & Patchett 1975). The model assumes evolution in a closed system, with generations of stars born out of the interstellar gas (ISM). In each generation, a fraction of the gas is transformed into metals and returned to the ISM; the gas locked up in long-lived low-mass stars and stellar remnants no longer takes part in chemical evolution. Newly synthesized metals from each stellar generation are assumed to be instantaneously recycled back into the ISM and instantaneously mixed throughout the region; thus, in this model, metallicity always increases with time, and the region is perfectly homogeneous at all times.
The ratio of mass of metals ejected to mass locked up, y, is a quantity commonly called the yield. The term yield has another meaning: Supernova (SN) nucleosynthesis theorists use it to refer to the mass of a particular element ejected in a SN model. The yield depends on the mass of metals ejected by stars (usually a function of mass) and the relative frequency of different mass stars born in a stellar generation (this is the initial mass function, or IMF). The mean IMF has been measured empirically (e.g. Scalo 1987) and over Galactic time appears to have been approximately constant; however, for individual molecular clouds, large deviations from the mean IMF occur.
Another chemical evolution parameter is the star formation rate (SFR), which has been postulated to be proportional to some power of the gas density and the total mass density. In the Simple model, the SFR affects the time evolution of the metallicity but does not affect the final metallicity function of the system after the gas has been exhausted.
Given the yield, the metallicity function of long-lived stars for the Simple model is as follows:
If evolution continues to gas exhaustion, then the Simple model predicts that the average mass fraction of metals of long-lived stars is equal to the yield, <z> = y. In principle the mean metal content of a stellar system can tell us about the yield. Because the yield is the ratio of mass of metals produced to the mass in low-mass stars per generation, it is sensitive to the IMF: An IMF skewed to high-mass stars would have a higher yield because more stars are massive enough to produce metals as SN, and there are fewer low-mass stars to lock away the gas.
Abundance ratios can serve as a diagnostic of the IMF and SFR parameters and time scale for chemically evolving systems. Tinsley (1979) proposed that type Ia supernovae (SN Ia, resulting from mass accretion by a C-O white dwarf) are the major producers of iron in the Galaxy and that the SN Ia progenitors have longer lifetimes than the progenitors of type II supernovae (SN II, resulting from exploding massive stars), which are the source of Galactic oxygen; Tinsley argued that the time delay between SN II and SN Ia, of at least 108 years, is responsible for the enhanced [O/Fe] (1) ratios observed in halo stars. Theoretical predictions of SN II element yields show that [ / Fe] (where includes the elements O, Mg, Si, S, Ca, and Ti) increases with increasing progenitor mass (e.g. Woosley & Weaver 1995). In principle, the IMF of a stellar system could be inferred from the observed [ / Fe] ratios. Note that if a stellar system is found to have a high average metallicity, and an IMF skewed to high-mass stars is responsible for increasing the yield, then the composition should reflect an increased [ / Fe] ratio that is due to the increased [ / Fe] from high mass SN II. In fact, this idea was used by Matteucci & Brocato (1990) to explain the putative high metallicity of the Galactic bulge, with the prediction that [ / Fe] is enhanced in the bulge.
The [ / Fe] ratio is also sensitive to the SFR in Tinsley's model: If the SFR is high, then the gas will reach higher [Fe/H] before the first SN Ia occur, and the position of the knee in the [ / Fe] versus [Fe/H] diagram (Figure 1) will be at a higher [Fe/H]. Also, because the knee marks the time of the first SN Ia, then the formation time scale of a stellar system can be estimated by noting the fraction of stars with [Fe/H] below this point.
Figure 1. A schematic diagram of the trend of -element abundance with metallicity. Increased initial mass function and star formation rate affect the trend in the directions indicated. The knee in the diagram is thought to be due to the onset of type Ia supernovae (SN Ia).
Another potentially useful diagnostic of the [O/Fe] ratio was pointed out by Wyse & Gilmore (1991): In a star-burst system, the O/Fe ratio of the gas is initially above solar owing to nucleosynthesis by SN II, but as time continues after the burst (with no new star formation) the SN II diminish, only SN Ia enrich the gas; ultimately subsolar [O/Fe] ratios occur. Wyse & Gilmore (1991) claimed that the composition of the LMC is fit by this model.
Elements like C, O, and those in the iron-peak, thought to be produced in stars from the original hydrogen, are sometimes labeled as "primary." The label "secondary" is reserved for elements thought to be produced from preexisting seed nuclei, such as N and s-process heavy elements. The abundance of a primary element is expected to increase in proportion to the metallicity, thus [M/Fe] is approximately constant. For a secondary element, [M/Fe] is expected to increase linearly with [Fe/H] because the yield is proportional to the abundance of preexisting seed nuclei. One difficulty is that N and the s-process elements (both secondary) do not show the expected dependence on metallicity.
1 [A/B] refers to an abundance ratio in log10 solar units, where A and B represent the number densities of two elements: [A/B] = log10(A/B)* - log10(A/B). Note that (M) = log10 (M/H). Back.