The recent detection of strontium  in the kilonova spectra from the neutron star – neutron star merger (GW170817) has confirmed that these mergers are a key site of r-process nucleosynthesis. To explain r-process abundances in the Galaxy, these mergers must therefore occur on shorter timescales than originally assumed. In order for two neutron stars to merge it is likely that the progenitor will involve an evolutionary stage where one neutron star lies within the envelope of the companion star. The companion star is the less massive of the two original stars, going through its evolution more slowly and so has not yet undergone core collapse.
During this phase, the neutron star will accrete hydrogen-rich material from the envelope, leading to rp-process-like nucleosynthesis, similar to that predicted in an X-ray burst. However, unlike in an X-ray burst, these conditions are not transient and due to the continuing source of energy, rp-process material can be effectively ejected into the envelope, enhancing the companion in proton-rich material. Once the companion undergoes core collapse, this material will be ejected and therefore provides a source of proton-rich material, which cannot be produced by the r-process, to the interstellar medium.
Preliminary models of this scenario have been developed  by our MSc.R. student but further work is needed both on the models and the nuclear physics reaction network. The astrophysical models are being developed by our colleagues, in the NuGRID collaboration. We will develop a state-of-the-art reaction network, and use these models to perform sensitivity studies of the nucleosynthesis, under the range of possible conditions, highlighting key reactions. We will then undertake measurements targeting these reactions. The PhD student will develop the network and perform the sensitivity study, as well as plan and run one experiment.
We have a well-established track record in studying such proton- and alpha-induced reactions, both directly and indirectly. Of particular note is the recent successful indirect study, at GANIL using VAMOS, AGATA and MuGAST, of the 15O()19Ne reaction. This reaction is assumed to trigger X-ray bursts and has been identified as the highest priority for measurement in all recent X-ray burst studies. In addition, our Opportunities grant will develop an active target ideally suited for targeting (,p) reactions, which have been highlighted as influential in X-ray bursts and so may also play a role in this scenario.
 D. Watson et al., Nature 574 (2019)  J. Keegans et al., Monthly Notices of the Royal Astron. Soc. 485 (2019)