Producing nature's heaviest elements in compact object mergers
Nicole Vassh
University of Notre Dame, USA

Determining the astrophysical site(s) which are responsible for the heaviest elements observed in nature has been a long outstanding question and compact object mergers have become a leading candidate. In the era of LIGO/VIRGO, gravitational wave detection enables identification and pointed electromagnetic follow-up of merger events, permitting new insights into these rare and interesting systems. The observations of the GW170817 neutron star merger electromagnetic counterpart suggested the production of heavy elements via the rapid neutron capture process (r process) due to the light curve compatibility with the presence of high opacity elements such as the lanthanides. What can be gleaned regarding heavy element production from this merger event is however subject to large uncertainties from the nuclear physics and astrophysics unknowns affecting nucleosynthesis calculations. For instance the rare-earth abundance peak, a feature of enhanced lanthanide production at A~164 seen in the solar r-process abundances, is not robustly produced in r-process calculations when nuclear physics inputs are varied. We will discuss a method which employs Markov Chain Monte Carlo to find the nuclear masses capable of producing a peak compatible with the observed solar r-process residuals. The aim of this method is to the learn which astrophysical conditions can be consistent with both observation and the latest nuclear data. Nuclear and astrophysics uncertainties also make it difficult to know if merger events populate the heaviest observed nuclei, the actinides. An r process which reaches the actinides is also likely to host fission, which is largely experimentally uncharted for neutron-rich nuclei, further compounding the difficulty in predicting the nucleosynthetic outcome. We will discuss a possible direct signature of actinide production in merger environments as well as the potential for future experimental and theoretical efforts to refine our knowledge of fission in the r process. The question of where nature primarily produces the heavy elements can only be answered through such collaborative efforts between experiment, theory, and observation.