Neutron Star Mergers Create Heavy Elements

Gravitational wave observations combined with optical and gamma-ray data confirm earlier predictions, offer insights into how the galaxy produces lead, mercury, and other elements.

Image courtesy of Dan Kasen, Lawrence Berkeley National Laboratory

Theoretical calculation of the evolution of the spectrum of light from a kilonova such as that associated with gravitational wave signal GW170817. The right panel shows an illustration of the expanding radioactive debris cloud ejected from a neutron star merger that gives rise to light over 2.97 days (left panel, t=time).

The Science

Physicists have long discussed how elements heavier than iron could be produced in the cosmos. Possibilities included supernova explosions or a merger of two neutron stars.  A key prediction for the time evolution of the optical signal following a neutron star merger has now been confirmed: a short-lived blue component consistent with light element formation, and a long-lived red component lasting for weeks, consistent with heavy element formation. The data are coming from observations of light following the August 17, 2017, detection of a neutron star merger gravitational wave signal. The merger formed heavy elements, such as neodymium and lead. The light signals match the computer model predictions.

The Impact

The observation of gravitational wave signal GW170817 points to a new source for the heavy elements in the universe—the material ejected as two neutron stars merge to form a black hole. The high neutron content of the ejecta facilitates production of the heavy elements. The ejecta masses suggest that the neutron star mergers may be the dominant source of heavy element production in the galaxy.

Summary

GW170817 was the first gravitational wave detection involving objects with masses typical of neutron stars. Gamma-rays from it were observed by the Fermi telescope, confirming its nature as a gamma-ray burst. Follow-up studies revealed an optical signal that faded after a few days plus an infrared signal that persisted for nearly two weeks. These signals are consistent with computer model predictions for a kilonova that produces significant quantities of heavy elements via the r-process (rapid neutron capture). The shorter lived, spectrally featureless optical emission is compatible with an initial ejecta component composed of lighter elements, while the long-lived infrared signal is from a secondary component that is powered by the radioactive decay of heavy elements which heat the plasma. Heavy elements with 58 < atomic number < 90 scatter the light strongly, leading to a long-lived emission.

Contact

Prof. Dan Kasen
Lawrence Berkeley National Laboratory and the University of California, Berkeley
[email protected]

Funding

  • D.K. was funded by a U.S. Department of Energy (DOE) early career award DE-SC0008067, a DOE Office of Science, Office of Nuclear Physics award DE-SC0017616, and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Division of Nuclear Physics, DOE under contract DE-AC02-05CH11231. This work was supported in part by the DOE Scientific Discovery through Advanced Computing (SciDAC) award DE-SC0018297.
  • E.R.R. was funded by a Niels Bohr Professorship funded by the Danish National Research Foundation, and support from University of California Institute for Mexico and the United States, 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.
  • Einstein Fellow J.B. is supported by the National Aeronautics and Space Administration (NASA) through the Einstein Fellowship Program, grant PF7-180162, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060.
  • This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science user facility under contract DE AC02-05CH11231.

Publications

D. Kasen, B. Metzger, J. Barnes, E. Quataert, and E. Ramierz-Ruiz, “Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event.” Nature 551, 80-84 (2017). [DOI: 10.1038/nature24453]

Related Links

Lawrence Berkeley National Laboratory news release: Scientists Decode the Origin of Universe’s Heavy Elements in the Light from a Neutron Star Merger

University of California, Santa Cruz Scientific Papers from the 1M2H Collaboration

Highlight Categories

Program: ASCR , NP

Performer: University , DOE Laboratory , SC User Facilities , ASCR User Facilities , NERSC

Additional: Collaborations , Non-DOE Interagency Collaboration