The R-Process Alliance: Actinide Mass Fractions of R-Process Events through Cosmic Time

The discovery of the AT 2017gfo kilonova event established that binary neutron star mergers (BNSMs) are capable of creating at least some r-process elements, up to the lanthanides. Whether BNSMs can also create the heaviest group of elements, the actinides, is highly anticipated to be answered soon as well with the near- and mid-IR sensitivity of JWST. In order to establish BNSMs as one of the dominant sites of r-process nucleosynthesis, it is imperative that they not only form actinides, but in similar quantities as observed in metal-poor stars. However, the signature of the most easily probed actinide element, thorium (Th), has been identified in only ~30 metal-poor stars, over the last 3 decades. In this talk, I will present homogeneously determined Th abundances of over 50 new metal-poor stars, more than doubling the current sample. Each of these stars have chemical signatures preserved from unique r-process enrichment events that occurred in the past. Leveraging this, I will present the actinide mass fractions of a large sample of past r-process events, obtained using state-of-the-art r-process nucleosynthesis simulations and constrained with the newly observed Th abundances. These results provide a first representative picture of actinide signatures in metal-poor stars to compare with future JWST observations of actinide signatures in kilonovae and establish whether BNSMs can create the heaviest r-process elements.