Periodic Reporting for period 4 - RADIOSTAR (Radioactivities from Stars to Solar Systems)
Periodo di rendicontazione: 2022-03-01 al 2023-08-31
The understanding of our own place in the Universe is one of the most fundamental quests of humankind. A related question is if the Solar System was born under common or special circumstances because the circumstances of a stellar birth, from its location within the Galaxy to the specific nature of the birth environment, significantly determine the formation and early evolution of the associated planetary system. This is due to the effect of radiation from massive stars, of dynamical encounters with other stars, and of the heat released by radioactive decay. The latter range from the decay of aluminium-26 (within roughly one million year) impacting on the evolution of the first planetesimals, to the decay of the longer-lived U and Th (billion of years) impacting on the evolution of the Earth. The main problem is that we still do not know in which kind of environment the Sun was born. While we know that it was most likely born in a stellar cluster, we do not know if this cluster was located within a small or a giant stellar nursery - also known as molecular cloud. To investigate the origin of our Solar System we have used the abundances of radioactive nuclei produced in stars by nuclear reactions as clocks to determine the time of occurrence of different astrophysical events before the birth of the Sun, and as fingerprints of the local stellar sources present in the environment of such birth. The immediate aim of our investigation is the interpretation of measurements provided by analyses of meteoritic rocks, which demonstrate that radioactive nuclei with half-lives or the order of millions of years were present at the birth of the Sun. We have found that radioactive nuclei made by rapid neutron-captures originated most likely from a last neutron-star merger event that happened 100-200 million years before the formation of the Sun. The radioactive nuclei made by the slow neutron-captures, instead, most likely originated from giant stars. Their abundances require a relatively long time interval, 9 to 26 million years, to have elapsed from the time of the formation of the molecular cloud to the time of the formation of the first solids in the Solar System. These values are robust, as they include uncertainties from stellar and galactic physics. They confirm that the Sun was born in a long-living giant molecular cloud, such as the Orion molecular cloud complex. The very short-lived nuclei (such as the heating source aluminium-26) most likely originated from massive star sources within this cloud. We found that massive stars winds provide a possible site of origin, while analysis of all the (15) radioactive nuclei made in core-collapse supernovae confirm that the yields from these sources are not consistent with the data.
To predict the abundances of radioactive nuclei in the Milky Way galaxy at the time of the Suns' birth required for the comparison to the meteoritic data we need to understand: first, how these nuclei are produced by nuclear interactions in astronomical objects such as stars, supernovae, and the mergers of compact objects like neutron stars, and second how their abundances evolve in the galactic interstellar medium once they are ejected by their stellar sources. To address the first question we have calculated the production of radioactive nuclei in a variety of possible sources. We have predicted that the winds of a star of mass roughly above 30 times than the Sun, located nearby the Sun's birth, would have carried enough radioactive aluminium-26, chlorine-36, and calcium-41 to match the observed abundances. Core-collapse supernovae, the final phases of the lives of massive star, also produce 15 of the radioactive nuclei of interest. By computing their production and exploring some of the many related uncertainties, we confirmed that a nearby supernova cannot match the observations. To address the second question, we developed the first open-source computational code to follow the evolution of the abundances of radioactive nuclei in the galactic interstellar medium and evaluated the effect of galactic uncertainties. We also produced new statistical methods to consider quantitatively the fact that radioactive abundances are not perfectly mixed in the interstellar medium (see figure cote2019). We exploited these advancements to calculate the abundances of palladium-107, cesium-135, and hafnium-182 in the Galaxy. We established that the early Solar System origin of these nuclei can be fully and self-consistently ascribed to the slow neutron-capture process in low-mass giant stars, with a relatively long time interval of 9 - 26 million years from the formation of the molecular could to the first solids in the Solar System. We have found that iodine-129, plutonium-244, and curium-247 are produced in the correct proportions to match the meteoritic data by a neutron-star merger event (see figure cote21). Our results were disseminated to the scientific community via more than 60 publications in journals with impact factors and almost 100 presentations at international conferences and workshops. We have organized an international conference on Astrophysics with Radioactive Isotopes (https://indico.cern.ch/event/820113/(si apre in una nuova finestra)) and delivered more than 10 outreach actions to the general public, including articles, a student pamphlet translated in 7 languages (https://futurumcareers.com/how-were-the-chemical-elements-born(si apre in una nuova finestra)) videos (e.g. https://erc.europa.eu/projects-figures/stories/discovering-origin-our-sun(si apre in una nuova finestra)) and ratio interviews.
With our development of the modelling of the evolution of radioactive nuclei in the Galaxy, coupled with the detailed analysis of their production in several types of stellar sources, we have provided the most complete framework to date to investigate the origin of radioactive nuclei in the Solar System. For the first time, we have also quantitatively evaluated the impact of stellar and galactic uncertainties on the results. Thanks to this approach we have found the first observational evidence of a neutron-star merger event occurring in the Galaxy 100-200 million years before the time of the formation of the Sun. Such an event produces the best match to the meteoritic iodine-129/curium-247 ratio (see figure cote21). We have also found the most robust proof to date that the Sun was born in a long-lived molecular cloud and confirmed that the winds of a massive star could have been responsible for the shortest lived radioactive isotopes, including aluminium-26. This is true even in the likely event that such a massive star was located in a binary system. Our newly developed tools and methodologies include the first exploitation of the ratios of radioactive nuclei also to each other and not only relative to a stable isotope, as exclusively done prior to our work. Our results will have far-reaching implications as they can be applied to infer the impact of new experiments in nuclear physics and meteoric analysis. For example, we are currently translating the impact of the first experiment of the decay of tallium-205 and its effect on the production of lead-205, onto the interpretation of the meteorite data and the origin of the Solar System.