CHEMICAL EVOLUTION OF NEUTRON CAPTURE ELEMENTS IN OUR GALAXY AND IN THE DWARFS SPHEROIDAL GALAXIES OF THE LOCAL GROUP

CESCUTTI, GABRIELE

CHEMICAL EVOLUTION OF NEUTRON CAPTURE ELEMENTS IN OUR GALAXY AND IN THE DWARFS SPHEROIDAL GALAXIES OF THE LOCAL GROUP

CESCUTTI, GABRIELE

2007-04-02

http://thesis2.sba.units.it/store/handle/item/12287

http://hdl.handle.net/10077/11562

Doctoral Thesis

Contributor(s)

SENATORE, GAETANO

Abstract

We model the evolution of the abundances of several neutron capture elements (Ba, Eu, La, Sr, Y and Zr) in the Milky Way and then we extend our predictions to some dwarf spheroidal galaxies of the Local Group. Two major neutron capture mechanisms on iron seeds are generally invoked: the slow process (s-process) and the rapid process (r-process), where the slow and the rapid are defined relative to the timescale of the β-decay. Nucleosynthesis calculations for r-process are very few, owing to the difficulties in modelling the physics the r-process and the lack of knowledge about the sites of productions of these elements. For s-process elements instead some calculations are available but the sites of production are also uncertain. By adopting a chemical evolution model for the Milky Way already reproducing the evolution of several chemical elements (H, He, C, N, O, α-elements and iron peak elements), we compare our theoretical results with accurate and new stellar data of neutron capture elements and we are able to impose strong constraints on the nucleosynthesis of the studied elements. We can suggest the stellar sites .of production for each element. In particular, the r-process component of each element (if any) is produced in the mass range from 10 to 30 Mʘ, whereas the s-process component arises from stars in the range from l to 3 Mʘ. Using the same chemical evolution model, extended to different galactocentric distances, we obtain results on the radial gradients of the Milky Way. We compare the results of the model not only for the neutron capture elements but also for α-elements and iron peak elements with new data of Cepheids stars. For the first time with these data, it is possible to verify the predictions for the gradients of very heavy elements. We conclude that the model, with an inside-out scenario for the building up of the disc and a constant density distribution of the gas for the halo phase, can be considered successful; in fact, for almost all the considered elements with our nucleosynthesis prescriptions, the model well reproduces the observed abundance gradients. We give a possible explanation to the considerable scatter of neutron capture elements observed in low metallicity stars in the solar vicinity, compared to the small star tostar scatter observed for the α-elements. In fact, we have developed a stochastic chemical evolution model, in which the main assumption is a random formation of new stars, subject to the condition that the cumulative mass distribution follows a given initial mass function. With our model we are able to reproduce the different features of neutron capture elements and α-elements. The reason for this resides in the random birth of stars coupled with different stellar mass ranges from where α-elements and neutron capture elements originate. In particular, the site of production of α-elements is the whole range of the massive stars, whereas the mass range of production for neutron capture elements has an upper limit of 30 Mʘ . Finally, we test the prescriptions for neutron capture elements also for the dwarf spheroidal galaxies of the Local Group. We use a chemical evolution model already able to reproduce the abundances for α-elements in these systems. We conclude that the same prescriptions used for the Milky Way well reproduce the main features of neutron capture elements also in the dwarf spheroidal galaxies for which we have observational data. In dwarf spheroidal galaxies for which we do not have observational data we only give predictions. We predict that the chemical evolution of these elements in dwarf spheroidal galaxies is different from the evolution in the solar vicinity. This is due to their different histories of star formation relative to our Galaxy and indicates that dwarf spheroidal galaxies (we see nowadays) cannot be the building blocks of our Galaxy.