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Neutron star in common envelope phase – progenitor to NS-NS merger

Department of Physics

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Dr A Laird , Dr C Diget No more applications being accepted Awaiting Funding Decision/Possible External Funding

About the Project

The recent detection of strontium [1] 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 studies of this scenario have been performed [2] and the astrophysical models are being developed by our colleagues in the U.S. We are developing a state-of-the-art reaction network, and will use these models to perform sensitivity studies of the nucleosynthesis, under the range of possible conditions, highlighting key reactions. We will then perform measurements targeting these reactions. The PhD student will be involved in these sensitivity studies under various conditions, as well as the planning and running of experiments and subsequent data analysis.

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(a,g)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, we are developing an active target ideally suited for targeting (a,p) reactions, which have been highlighted as influential in X-ray bursts and so may also play a role in this scenario.

[1] D. Watson et al., Nature 574 (2019)
[2] J. Keegans et al., Monthly Notices of the Royal Astron. Soc. 485 (2019)
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