Astronomers using ESO’s Very Large Telescope (VLT) have detected a freshly made heavy element, strontium, in the aftermath of GW170817, a merger of two neutron stars. The detection confirms that the heavier elements in the Universe can form in neutron star mergers, providing a missing piece of the puzzle of chemical element formation.
Watson et al found signatures of strontium formed in a neutron-star merger. Image credit: ESO / L. Calçada / M. Kornmesser.
On August 17, 2017, LIGO and Virgo detectors detected gravitational waves passing the Earth. The event, the fifth ever detected, was named GW170817.
The ripples in space and time that the detectors measured suggested a neutron star merger, since each star of the binary weighed between 1 and 2 times the mass of our Sun.
The event was localized to NGC 4993, a lenticular galaxy about 130 million light years from Earth in the constellation of Hydra.
Following the GW170817 merger, a fleet of space- and ground-based telescopes began monitoring the emerging kilonova — the visible counterpart of the merging of two extremely dense objects — over a wide range of wavelengths.
The X-shooter instrument on the VLT telescope took a series of spectra from the ultraviolet to the near infrared.
Initial analysis of these spectra suggested the presence of heavy elements in the source, but astronomers could not pinpoint individual elements until now.
“By reanalyzing the 2017 data from the merger, we identified the signature of one heavy element in this fireball, strontium, proving that the collision of neutron stars creates this element in the Universe,” said Dr. Darach Watson, an astronomer with the University of Copenhagen.
Astronomers have known the physical processes that create the elements since the 1950s. Over the following decades they have uncovered the cosmic sites of each of these major nuclear forges, except one.
“This is the final stage of a decades-long chase to pin down the origin of the elements,” Dr. Watson said.
“We know now that the processes that created the elements happened mostly in ordinary stars, in supernova explosions, or in the outer layers of old stars.”
“But, until now, we did not know the location of the final, undiscovered process — known as r-process , or rapid neutron capture — that created the heavier elements in the periodic table.”
In r-process, an atomic nucleus captures neutrons quickly enough to allow very heavy elements to be created.
Although many elements are produced in the cores of stars, creating elements heavier than iron, such as strontium, requires even hotter environments with lots of free neutrons.
Rapid neutron capture only occurs naturally in extreme environments where atoms are bombarded by vast numbers of neutrons.
“This is the first time that we can directly associate newly created material formed via neutron capture with a neutron star merger, confirming that neutron stars are made of neutrons and tying the long-debated rapid neutron capture process to such mergers,” said Dr. Camilla Juul Hansen, a researcher in the Max Planck Institute for Astronomy.
“We actually came up with the idea that we might be seeing strontium quite quickly after the event,” said Dr. Jonatan Selsing, scientist at the University of Copenhagen.
“However, showing that this was demonstrably the case turned out to be very difficult. This difficulty was due to our highly incomplete knowledge of the spectral appearance of the heavier elements in the periodic table.”
The results appear today in the journal Nature.
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Darach Watson et al. Identification of strontium in the merger of two neutron stars. Nature, published online October 23, 2019; doi: 10.1038/s41586-019-1676-3
