Observing the signature of a single prolific r-process event in an  ultra-faint dwarf galaxy

Anna Frebel (MIT) 

The heaviest chemical elements in the periodic table are synthesized                                                         
through the rapid neutron-capture (r-) process but the astrophysical
site where r-process element nucleosynthesis occurs is still unknown.
The best  candidate sites are ordinary core-collapse supernovae (deaths of massive stars)
and mergers of two orbiting exotic neutron stars.     

13 billion year old small dwarf galaxies preserve a "fossil"  record of early
chemical enrichment that provides the means to isolate  and study clean
signatures of individual nucleosynthesis events. Based
on new spectroscopic data from the 6.5m Magellan Telescope, we found 
seven stars in the recently discovered dwarf galaxy Reticulum II  that show
extreme overabundances of these heavy r-process elements. 

This "r-process" enhancement implies that the r-process material in Reticulum II was
synthesized in a single prolific event. Our results   are clearly incompatible
with yield predictions from an ordinary\ core-collapse supernova but
instead consistent with that of a neutron star merger. This first signature
of a neutron star merger in the early universe holds the key to finally,
after 60 years, identifying   the cosmic production site of the r-process.