Chemical evolution of the Milky Way and its Satellites

1 How to model galactic chemical evolution Before going into the detailed chemical evolution history of the Milky Way and its satellites, it is necessary to understand how to model, in general, galactic chemical evolution. The basic ingredients to build a model of galactic chemical evolution can be summarized as : • Initial conditions; • Stellar birthrate function (the rate at which stars are formed from the gas and their mass spectrum); • Stellar yields (how elements are produced in stars and restored into the interstellar medium); • Gas flows (infall, outflow, radial flow). When all these ingredients are ready, we need to write a set of equations describing the evolution of the gas and its chemical abundances which include all of them. These equations will describe the temporal variation of the gas content and its abundances by mass (see next sections). The chemical abundance of a generic chemical species i is defined as: According to this definition it holds: i=1,n X i = 1, (2) where n represents the total number of chemical species. Generally, in theoretical studies of stellar evolution it is common to adopt X, Y and Z as indicative of the abundances by mass of hydrogen (H), helium (He) and metals (Z), respectively. The baryonic universe is madeup mainly of H and some He while only a very small fraction resides in metals (all the elements heavier than