Isotope astrophysics - Two cradles for the heavy elements

Where do the Solar System's heavy elements come from? We have know for decades that many of the elements heavier than iron must have been formed in Supernovae. But it now appears impossible to produce all the so called ‘r-process’ isotopes in a single type of supernova. Instead, those r-process isotopes with a mass number less than about 134 probably come from rare events involving accreting white dwarfs; and the more common core-collapse supernovae take over for heavier isotopes.