Galactic system, metallicity evolution

This question can be examined further by studying metallicity as a function of gas fraction. In the context of the simple, closed box, chemical evolution, Edmunds (1990) derived a few simple theorems that show that outflows of gas and inflows of metal-poor gas cause galactic systems to deviate from the closed box model in similar ways. Specifically, defining the effective yield yef f... 

The site of the r-process is also not clear, but it seems that the conditions needed to reproduce Solar-System r-process abundances may hold in the hot bubble caused by neutrino winds in the immediate surroundings of a nascent neutron star in the early stages of a supernova explosion (see Fig. 6.10). Circumstantial evidence from Galactic chemical evolution supports an origin in low-mass Type II supernovae, maybe around 10 M (Mathews, Bazan Cowan 1992 Pagel Tautvaisiene 1995). Another possibility is the neutrino-driven wind from a neutron star formed by the accretion-induced collapse of a white dwarf in a binary system (Woosley Baron 1992) leading to a silent supernova (Nomoto 1986). In stars with extreme metal-deficiency, the heavy elements sometimes display an abundance pattern characteristic of the r-process with little or no contribution from the s-process, and the... 

The model assumes that evolution takes place in a closed system, with successive generations of stars being bom into the interstellar medium. At each generation, a fraction of the gas is transformed into metals and returned to the interstellar medium. Gases imprisoned in stars of low mass and compact residues play no further role in galactic evolution. In this model, metaUicity is bound to increase as time goes by. And so the arrow of galactic time is defined. Evolution will continue until no further gas is available to form new stars. 

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