The first spectroscopic evidence that heavy elements are created by the merger of two neutron stars has been found by an international team of scientists. By analysing light captured by the Very Large Telescope in Chile, the team has shown that strontium was produced in a huge “kilonova” explosion that followed such a merger. As well as confirming that neutron-star mergers are a significant source of heavy elements, the study also provides the first spectroscopic evidence that neutron stars comprise neutron-rich matter.
Physicists already know that hydrogen and helium were created shortly after the big bang and that heavier elements up to and including iron are formed by nuclear fusion within stars. But the origins of the 64 naturally occurring elements heavier than iron have been difficult to pin down. One potential source is asymptotic giant branch (AGB) stars, which are cool and luminous objects. The slow neutron capture process (s-process) of nucleosynthesis is believed to occur in these stars, creating about half of the heavy elements in the universe.
The other half is believed to be created by the rapid neutron capture process (r-process) of nucleosynthesis. This requires an enormous flux of neutrons and is thought to be responsible for creating the heaviest naturally-occurring elements such as gold, platinum and uranium.
Prime candidate
Although the r-process was first proposed about 60 years ago, it had not been clear exactly where such neutron fluxes could occur. In August 2017, astronomers witnessed the collision of two neutron stars that resulted in a kilonova explosion and the creation of a black hole. This involved the violent acceleration of vast quantities of neutrons, making it a prime candidate for r-process nucleosynthesis.
Initial analysis of light from this event – dubbed GW170817 – revealed a glow that was attributed to the radioactive decay of heavy elements. This was interpreted as strong evidence that neutron-star mergers are responsible for the other half of the universe’s heavy elements.
Now, an international team led by Darach Watson at the Niels Bohr Institute in Copenhagen is the first to spot the spectroscopic signature of one of those heavy elements in light from the kilonova. That element is strontium, which has a strong spectroscopic feature at an observed wavelength of about 810 nm (on the boundary between visible and infrared light). While most strontium is expected to be created by the s-process, about 30% is attributed to r-process nucleosynthesis.
“Unequivocal evidence”
“Before this we were unable to identify any specific element created in a neutron star merger,” says Watson, describing the observation as “unequivocal evidence” that heavy elements are created in kilonovas.
Seeing relatively light-weight strontium came as a bit of a surprise because it had been thought that only the heaviest elements would be made in neutron-star collisions. Team member Jonatan Selsing comments, “Now we know that the lighter of the heavy elements are also created in these mergers…it tells us that neutron star collisions produce a broad range of the heavy elements, from the lightest to the very heaviest”.
Spectacular collision of two neutron stars observed for first time
Watson and colleagues also point out that their detection of strontium is the first spectroscopic evidence that neutron stars are made mostly of neutrons. This comes almost exactly 85 years after the existence of neutron stars was first postulated by Walter Baade and Fritz Zwicky.
The team is now working towards finding spectroscopic evidence for heavier elements in light from GW170817. Potential candidates including barium and the lanthanides – which have strong spectroscopic features at wavelengths shorter than about 650 nm.
The observation is reported in Nature.
