Chemical evolution models

People involved at OAB: Romano, Tosi.

Models of Galactic chemical evolution are nowadays able to reproduce the vast majority of the observed characteristics of our Galaxy. There are, however, a number of open questions on the evolution of the Galaxy, which still require further studies (e.g. Tosi 2003c). Some of these issues are being examined in detail at the Bologna Observatory. In 2003, we have proceeded in the effort of accurately analysing the feedback between stellar and chemical evolution, the evolution of the abundance gradients and the impact of Galactic chemical evolution models on cosmology. To this aim, new models for $D$, $^3He$, $^4He$, $^{12}C$, $^{13}C$, $^{14}N$, $^{16}O$, $^{17}O$, $^{18}O$, $^{20}Ne$, $^{22}Ne$ and heavier species up to Fe have been computed and compared with the available data, adopting all the most recent and reliable stellar yields, IMF and stellar lifetimes. The role of novae has also been studied in detail. These binary systems have long been recognized to be able to produce peculiar elements with large overproduction factors during outbursts (José and Hernanz 1998 and refs. therein). Accounting for that in the chemical evolution code allows us to explain fairly well the evolution of $^7Li$ and some of the CNO isotopes (Romano and Matteucci 2003).

A collaboration exists with the International Space Science Institute in Berne (Switzerland) to study all the aspects of stellar and galactic evolution affecting the abundances of the light elements. All the Galactic chemical evolution models able to reproduce the largest set of observational constraints have shown that the primordial abundance of $D$ and $^3He$ must have been fairly low. This implies that the baryon/photon ratio was fairly high during the Big Bang, a result emphasized by the MAXIMA and BOOMERANG, and most recently WMAP, experiments on the cosmic microwave background. Our group has shown (Romano et al. 2003) that the primordial abundances of the light elements resulting from the WMAP data are in excellent agreement with the predictions of those among our chemical evolution models which best reproduce the galactic properties. This result is interesting in many respects. Indeed, it shows that the predictions of the standard theory of Big Bang nucleosynthesis, updated theories of galactic and stellar formation and evolution as well as the most recent observational inferences on the primordial element abundances can be all gathered together in a single, common, coherent evolutionary scenario.