Galactic 'gold mine' explains the origin of nature's heaviest elements May 19, 2016 The origin of many of the most precious elements on the periodic table, such as gold, silver and platinum, has perplexed scientists for more than six decades. Now a recent study has an answer, evocatively conveyed in the faint starlight from a distant dwarf galaxy. In a roundtable discussion, published today, The Kavli Foundation spoke to two of the researchers behind the discovery about why the source of these heavy elements, collectively called "r-process" elements, has been so hard to crack. "Understanding how heavy, r-process elements are formed is one of hardest problems in nuclear physics," said Anna Frebel, assistant professor in the Department of Physics at the Massachusetts Institute of Technology (MIT) and also a member of the MIT Kavli Institute for Astrophysics and Space Research (MKI). "The production of these really heavy elements takes so much energy that it's nearly impossible to make them experimentally," Frebel continued. "The process for making them just doesn't work on Earth. So we have had to use the stars and the objects in the cosmos as our lab." Read more at: http://phys.org/news/2016-05-galactic-gold-nature-heaviest-elements.html#jCp
http://www.nature.com/nature/journal/v531/n7596/full/nature17425.html R-process enrichment from a single event in an ancient dwarf galaxy Elements heavier than zinc are synthesized through the rapid (r) and slow (s) neutron-capture processes1, 2. The main site of production of the r-process elements (such as europium) has been debated for nearly 60 years2. Initial studies of trends in chemical abundances in old Milky Way halo stars suggested that these elements are produced continually, in sites such as core-collapse supernovae3, 4. But evidence from the local Universe favours the idea that r-process production occurs mainly during rare events, such as neutron star mergers5, 6. The appearance of a plateau of europium abundance in some dwarf spheroidal galaxies has been suggested as evidence for rare r-process enrichment in the early Universe7, but only under the assumption that no gas accretes into those dwarf galaxies; gas accretion8 favours continual r-process enrichment in these systems. Furthermore, the universal r-process pattern1, 9 has not been cleanly identified in dwarf spheroidals. The smaller, chemically simpler, and more ancient ultrafaint dwarf galaxies assembled shortly after the first stars formed, and are ideal systems with which to study nucleosynthesis events such as the r-process10, 11. Reticulum II is one such galaxy12, 13, 14. The abundances of non-neutron-capture elements in this galaxy (and others like it) are similar to those in other old stars15. Here, we report that seven of the nine brightest stars in Reticulum II, observed with high-resolution spectroscopy, show strong enhancements in heavy neutron-capture elements, with abundances that follow the universal r-process pattern beyond barium. The enhancement seen in this ‘r-process galaxy’ is two to three orders of magnitude higher than that detected in any other ultrafaint dwarf galaxy11, 16,17. This implies that a single, rare event produced the r-process material in Reticulum II. The r-process yield and event rate are incompatible with the source being ordinary core-collapse supernovae18, but consistent with other possible sources, such as neutron star mergers.
