Introduction processes governing galactic chemical evolution

After the Big Bang, stellar nucleosynthesis became by far the major element-producing factory. During the mass-loss episodes experienced by most stars, newly produced elements are (partially) transferred to the interstellar medium (ISM), which thereby evolves chemically. The element yields and the transfer mechanisms are both dependent on stellar masses and evolution (Chapters 5 and 6).

The interstellar medium initially consisted almost exclusively of primordial H and He, and this low metallicity favoured the accretion of very massive stars, ~ 30 M0 < M < ~ 300 M0. Even though adequate models of these stars have not yet been developed, they are considered to have a very short life. The total mass of the ejected heavy elements as well as their abundance pattern strongly depend on a stellar model, especially on the details of stellar death. In the case of a successful explosion, heavy r-process and Fe-peak elements are abundant in stellar ejecta whereas the inner core is converted into a neutron star. In the other case, only products of explosive burning in the outer shells are ejected; most material falls back onto the core to generate a massive black hole that could play an important role in the subsequent formation of a galaxy. In both cases the amount of material ejected into the interstellar medium could be enough to overcome the metallicity threshold in a stellar neighbourhood for the formation of low-mass stars, sAZ ~ -3; therefore small low-metallicity long-lived stars could have sampled an array of elements generated by one or very few progenitor(s) (Silk and Bouwens, 2001; Bromm and Larson, 2004; Schneider, 2006; Cowan and Sneden, 2006).

In parallel with low-mass stars, massive stars, ~ 10 to 100 M0, continue to be formed even as the metallicity increases. After a rather short life (~ 10Myr) these meet a dramatic death as SNe II, leaving behind a neutron star or black hole. There are ejecta, amounting (in the case of a successful explosion) to up to ~ 90% of the original stellar mass. Along with the initial constituents, the ejecta carry products of burning, explosive burning and r-, p-, y - and v-nucleosynthesis. This material, ejected at high speed, not only contributes to the planetary nebulae and interstellar medium but also to the intergalactic matter (see Fig. 2.1). Because of their short lifetime and the large quantity of newly produced nuclides in the ejecta, massive stars dominate the chemical evolution of a galaxy.

Medium-sized stars (from 3 to 10 M0) generate elements up to the iron peak and contribute most of the s-process elements and appreciable amounts of C and N to the interstellar medium. The degenerate core left behind develops into a white dwarf. In the case of a binary system, however, a white dwarf can accrete enough mass to initiate explosive burning as a SNe Ia. Strong a-nuclei up to the iron-peak elements are the major products of such events. It has also been proposed, however, that the accretion-induced collapse of a white dwarf into a neutron star may be associated with the production of heavy r-nuclei and may provide occasional coupling of high r-process and high s-process enrichments in the envelope of the surviving low-mass partner star (Qian and Wasserburg, 2003).

Small stars (~ M©) have such long lifetimes, up to the Hubble time, that they simply store the elements, both those initially available and those newly produced. If the ages of such stars are known, they can be used to trace the composition of the interstellar medium back to the time and place when and where they were formed. Even though the accumulation of dust in the interstellar medium favours the formation of small stars during late stages of galaxy evolution, in rare cases such stars were formed long ago: ultra-metal-poor stars present an example (Section 3.2).

The composition of the interstellar medium evolves through these varied stellar contributions, and new stars accreting from it inherit this evolving contribution and build further on it. In this chapter we examine this galactic evolution, mainly concentrating on the solar neighbourhood, for which most data are available. Spec-troscopic observations on stars of various metallicities, as well as analytical data on presolar grains, will be reviewed and will form the basis for the models outlined at the end of the chapter. Modern models of chemical evolution treat the Galaxy as an open system, envisaging mass exchange with the intergalactic medium.

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