Speaker
Description
Binary neutron star (BNS) mergers are among the most intriguing events known in the universe, with impressive scientific potential spanning many different research fields in physics and astrophysics. On August 2017, the detection of the gravitational-wave GW170817 signal with the corresponding electromagnetic counterpart confirmed that heavy elements such as lanthanides and actinides can be generated in the aftermath of BNS mergers through r-process nucleosynthesis. Such elements are not the unique nucleosynthetic outcome in BNS mergers though, since the mass range of the yields strongly depends on the neutron richness of the matter expelled during the coalescence (ejecta), parametrized by the electron fraction (Ye). Specifically, the production of elements with mass number A < 80-90 is enhanced in the region of the ejecta with Ye >~0.25, at the expense of the heavier ones. Recent works featuring state-of-the-art BNS simulations showed that the Ye-range in the ejecta can extend up to ~ 0.4, complementing the distribution of heavy r-process elements with the presence of lighter nuclides among the final yields.
At the moment of writing, the production of elements with A < 80-90 has not been widely examined in the context of BNS mergers. Yet, understanding the details
behind their formation could help in better constraining the physics of compact binary mergers for different reasons, e.g.: I) the identification of individual
absoprtion features in the spectra of electromagnetic counterparts is in principle easier for light elements, II) the nucleosynthesis pattern at small atomic numbers exhibits a larger variability with respect to the binary properties.
In this work, we quantitatively investigate the nucleosynthesis of elements with atomic number Z <~ 38 in the ejecta of BNS mergers, combining an extensive set of nuclear reaction network calculations performed with SkyNet with datasets extracted from numerical BNS simulations modelling the ejecta of GW170817-like binaries. Among the various results, we find that a non-neglibible amount of iron-group elements is synthesized in the high-Ye tail of the ejecta, sometimes at a comparable level with respect to the production of some of the most abundant r-process nuclides. We also investigate how the nucleosynthesis of light elements correlates with some binary properties, like the equation of state employed for the description of neutron-star (NS) matter or the ratio between the two NSs masses.
Our study also leads to astrophysically relavant implications regarding the chemical evolution of our Solar system. Contrarily to previous findings, we show that the recently observed signature of 60Fe and 244Pu isotopes in deep ocean sediments, dating back to the past 3-4 million of years, is compatible with a single nearby BNS event, occurred ~ 100 pc away from the Earth.
