# Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event

## Abstract

The cosmic origin of elements heavier than iron has long been uncertain. Theoretical modelling1,2,3,4,5,6,7 shows that the matter that is expelled in the violent merger of two neutron stars can assemble into heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted8,9,10,11,12 to power a distinctive thermal glow (a ‘kilonova’). The discovery of an electromagnetic counterpart to the gravitational-wave source13 GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements14,15,16,17,18. Here we report models that predict the electromagnetic emission of kilonovae in detail and enable the mass, velocity and composition of ejecta to be derived from observations. We compare the models to the optical and infrared radiation associated with the GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. The ejected mass and a merger rate inferred from GW170817 imply that such mergers are a dominant mode of r-process production in the Universe.

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## Acknowledgements

D.K. is supported in part by a Department of Energy (DOE) Office early career award DE-SC0008067, a DOE Office of Nuclear Physics award DE-SC0017616, and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the US DOE under contract number DE-AC02-05CH11231. This work was supported in part by the DOE SciDAC award DE-SC0018297. E.R.-R. acknowledges support from a Niels Bohr Professorship funded by DNRF, and support from UCMEXUS, the David and Lucile Packard Foundation. This research is funded in part by the Gordon and Betty Moore Foundation through grant GBMF5076. E.Q. was funded in part by the Simons Foundation through a Simons Investigator Award. J.B. is supported by the National Aeronautics and Space Administration (NASA) through the Einstein Fellowship Program, grant number PF7-180162, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US DOE under contract number DE AC02-05CH11231. J.B. is an Einstein Fellow.

## Author information

Authors

### Contributions

D.K. carried out the model calculations and analysis and led the writing of the manuscript. B.M. helped with the text, aided in the theoretical interpretation, and contributed to the schematic figure of mass ejection. J.B. carried out multi-dimensional radiation transport calculations to estimate the effects of asymmetry on the light curves. E.Q. provided theoretical interpretations and aided in the writing of the manuscript. E.R.-R. provided theoretical input and estimates of the contribution of mergers to the r-process in the Galaxy.

### Corresponding author

Correspondence to Daniel Kasen.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

Reviewer Information Nature thanks R. Chevalier and C. Miller for their contribution to the peer review of this work.

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## Extended data figures and tables

### Extended Data Figure 1 Dependence of model light curves on the ejecta density profile and compositional stratification.

The models all have mass M = 0.025M and velocity vk = 0.25c. a, Comparison of models with a homogenous composition to one where the lanthanide mass fraction varies from Xlan = 10−6 at the outer ejecta edge to Xlan = 10−4 in the interior (see equation (7)). b, Comparison of models with different density gradient in the outer layers. A shallower exponent (n < 10) leads to a cooler photosphere and suppresses the early ultraviolet and blue emission. The light curves at times t ≥ 1 d and in redder bands are essentially independent of the outer density profile.

### Extended Data Figure 2 Multi-dimensional models demonstrating the orientation dependence of asymmetric kilonova light curves.

a, Bolometric light curves of light r-process ejecta (with M = 0.025M, vk = 0.15c and Xlan = 10−5) distributed in a conical polar region of opening half angle 45°. b, Bolometric light curves of an oblate ellipsoidal distribution of heavy r-process ejecta (with M = 0.04M, vk = 0.1c and Xlan = 10−2) with an axis ratio of a = 4. The orientation effects lead to modest variations in the peak brightness. However, these models do not account for both a polar and an ellipsoidal component being present and influencing each other.

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Kasen, D., Metzger, B., Barnes, J. et al. Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event. Nature 551, 80–84 (2017). https://doi.org/10.1038/nature24453

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