Gas flows and the chemical evolution of galaxies - III. Graphical analysisand secondary elements

The general structure of the solutions of the chemical-evolution equations of a galaxy undergoing an arbitrary time-dependent accretion of metal-free gas is studied with graphical analysis. The behaviour is determined by the time evolution of the ratio a of accretion to star formation rate. This giv es rise to two distinct regimes. (a) If a decreases monotonically with time , the properties are always between those predicted from the Simple Model a nd a Linear Accretion Model; the stellar mean metallicity and the abundance ratio of secondary and primary elements are thus restricted to a narrow ra nge of a factor of 2. (b) Lower gas metallicities, larger secondary/primary ratios and stellar metallicities than expected from the Linear Accretion M odels can only be reached if the accretion rate increases rapidly to a valu e larger than the star formation rate, e.g. by sudden massive accretion of gas into highly evolved gas-poor galaxies.