Sites of rapid neutron-capture nucleosynthesis

5 problems

• Why do some r-process-enhanced stars have extra thorium and uranium?

  Open

  In old stars enhanced in r-process elements the medium-heavy elements follow the Sun's pattern closely, yet a sizable minority show extra thorium and uranium, hinting that the conditions or sites differ.

• Do collapsar accretion disks eject neutron-rich r-process matter?

  Contested

  The disk around a black hole born inside a collapsing star could blow off neutron-rich gas that makes gold, but whether the gas stays neutron-rich enough is disputed.

• Which site dominates the r-process inventory of the Milky Way?

  Contested

  Neutron-star mergers, rare collapsing stars and flares from magnetars can all make r-process elements; their shares are unknown.

• Can neutron-star mergers explain r-process elements in the oldest stars?

  Contested

  Some very old stars already contain r-process elements, yet merging neutron stars need time to spiral together.

• How much lanthanide mass did the GW170817 kilonova eject?

  Partially resolved

  The glow of the 2017 neutron-star merger depends on how much of the rare-earth elements (lanthanides) it made, which is uncertain because their atomic data are poor.