The first detection of heavy elements forged in a neutron star collision

The first-ever identification of a heavy element forged in a neutron star collision completes the puzzle of element formation.

Whilst scientists have long known that many of the natural elements such as helium, carbon and aluminium are forged in stars, there has remained a puzzle as to how some heavier elements are created. The creation of these elements requires hotter environments than even the cores of stars can provide. There is also a need for an abundance of free neutrons to feed a process called rapid neutron capture.

Now, a freshly forged heavy element — strontium — as been detected in the aftermath of a neutron star collision — a type of kilonova — for the first time. The observation was made by astronomers using the ESO’s X-shooter spectrograph on the Very Large Telescope (VLT). The detection provides the first evidence of the theory that heavier elements are formed in neutron star collisions.

The findings are published in the journal Nature.

Newly created strontium, an element used in fireworks, has been detected in space for the first time following observations with ESO’s Very Large Telescope. 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. (Credit: ESO/ ESO/L. Calçada/ ESO/E. Pian et al./S. Smartt & ePESSTO/L. Calçada)

Since the first detection of gravitational waves in 2017 the ESO has had its telescopes based in Chile trained on one source of such gravitational radiation — the neutron star merger GW170817 in the galaxy NGC 4993. The hope of ESO astronomers was that the kilonova explosion would reveal the presence of freshly created heavy elements.

This wide-field image generated from the Digitized Sky Survey 2 shows the sky around the galaxy NGC 4993. This galaxy was the host to a merger between two neutron stars, which led to a gravitational wave detection, a short gamma-ray burst and an optical identification of a kilonova event. ( ESO and Digitized Sky Survey 2)

These telescopes monitored GW170817 over a range of wavelengths in the electromagnetic spectrum, with the VLT covering a series of spectra from ultraviolet to near-infrared. It was in the analysis of these spectra that the team of European scientists discovered the suggestion of the presence of heavy elements. Despite this, they were unable to pinpoint individual elements. That was, until now.

This chart shows the sprawling constellation of Hydra (The Female Sea Serpent), the largest and longest constellation in the sky. Most stars visible to the naked eye on a clear dark night are shown. The red circle marks the position of the galaxy NGC 4993, which became famous in August 2017 as the site of the first gravitational wave source that was also identified in light visible light as the kilonova GW170817. NGC 4993 can be seen as a very faint patch with a larger amateur telescope. (ESO, IAU and Sky & Telescope)

“By reanalysing the 2017 data from the merger, we have now identified the signature of one heavy element in this fireball, strontium, proving that the collision of neutron stars creates this element in the Universe,” says the study’s lead author Darach Watson from the University of Copenhagen in Denmark.

Strontium is found naturally on Earth, mostly in the soil and concentrated in certain minerals. The brilliant red colour is some fireworks are a result of the burning of strontium salts.

Since research into nucleosynthesis began, scientists have discovered most of the major nuclear forges responsible for the creation of the elements, except for one, rapid neutron capture or the r-process.

The brilliant red colour in some fireworks is a result of strontium. Now detect in the aftermath of a neutron star collision.

“This is the final stage of a decades-long chase to pin down the origin of the elements,” explains Watson. “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 rapid neutron capture, that created the heavier elements in the periodic table.”

The r-process is the reaction in which an atomic nucleus captures neutrons rapidly enough to facilitate the creation of the heavier elements of the periodic table. It requires an extreme environment with immensely high temperatures where atoms can be 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,” adds Camilla Juul Hansen from the Max Planck Institute for Astronomy in Heidelberg, who played a major role in the study.

This montage of spectra taken using the X-shooter instrument on ESO’s Very Large Telescope shows the changing behaviour of the kilonova in the galaxy NGC 4993 over a period of 12 days after the explosion was detected on 17 August 2017. Each spectrum covers a range of wavelengths from the near-ultraviolet to the near-infrared and reveals how the object became dramatically redder as it faded. (ESO/E. Pian et al./S. Smartt & ePESSTO)

Though this is the first detection of heavy elements in the aftermath of a neutron star collision, scientists are only now gaining a better understanding of such kilonova events.

“We actually came up with the idea that we might be seeing strontium quite quickly after the event,” says University of Copenhagen researcher Jonatan Selsing, who was a key author on the paper. “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.”

Even though LIGO’s observation of the GW170817 merger was the fifth detection of gravitational waves, the event located in the galaxy NGC 4993 was the first to visibly confirmed by telescopes on Earth.

As the combined efforts of LIGO, VIRGO and the VLT and other telescopes continues, our understanding of the characteristics of neutron stars and the results of their mergers only promises to improve.