Aperitivo Scientifico: Origine degli elementi pesanti nell’Universo: prime evidenze nell’evento di neutron star merger GW170817

by Alberto Mengoni (BO), Dr. Sergio Cristallo (PG)

Friday, 2 February 2018 from to (Europe/Rome)
at Bologna ( Sala Riunioni, 1st floor )
via Irnerio, 46
The detection of signals from the gravitational waves generated by neutron star mergers, published at the beginning of October 2017, carried and showed some further information of paramount importance for nuclear physics studies. Firstly, a gamma-ray burst has been observed, only a few seconds after the detection of the gravitational wave signal. In addition, and more importantly, it has been observed that the light curves (“color evolution”) emitted after the gravitational wave and the subsequent gamma-ray burst, is consisted with the opacity induced by the presence of heavy elements in the event [2]. This observation can be considered a first confirmation that the astrophysical site for the rapid neutron capture process, the “r process”, postulated by the pioneering work of Burbidge, Burbidge, Fowler and Hoyle (B2FH) exactly 60 years ago [3], could be indeed a neutron star merger event.

In order to study the energetics that powers the explosive nucleosynthesis of neutron star mergers and the resulting elemental abundance pattern of the produced material, detailed nuclear physics information are needed, the most relevant of which are investigated in two non-LHC CERN experiments: ISOLDE and n_TOF  At ISOLDE, nuclear structure and decay properties of unstable nuclei, such as masses and half-lives, are studied as the properties of these nuclei are the key ingredients for modeling the r-process. The neutron time-of-flight facility n_TOF, on the other hand, owns the ideal setups for measuring neutron induced reactions cross sections, the basic ingredients for the modeling the “slow neutron capture process” or s-process, taking place during the quiescent phases of stellar evolution. About half of the chemical elements heavier than iron are formed by each one of these two processes and only a full understanding of these two mechanisms, postulated in the pioneering work of B2FH, allows for a direct comparison of the expected and observed abundances of the chemical elements present in the universe. 

In the two lectures, an overview of the present understanding of the processes which are responsible for the synthesis of the chemical elements in the primordial as well as in the evolved universe will be presented. Particular attention will be devoted to the big-bang nucleosynthesis (including the cosmic lithium abundance problem) and to the processes related to the production of neutron capture elements, i.e. all the elements heavier than iron.

[1] J. S. Bloom and S. Sigurdsson, "A cosmic multimessenger gold rush," Science (16 October 2017)
[2] M. R. Drout et al., "Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis," Science (16 October 2017)
[3] E. Margaret Burbidge, G. R. Burbidge, William A. Fowler, and F. Hoyle, “Synthesis of the Elements in Stars”, Rev. Mod. Phys. 29, 547 (1 October 1957)