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Thursday, 24 October 2019

Finally, we have a clearer idea of ​​the formation process of the heavy elements of the Universe


Shortly after the Big Bang, the first light atomic nuclei formed during primordial nucleosynthesis. Later, in the heart of the stars, will be forged heavier atoms until iron. But to form even heavier atoms, a process of fast neutron capture , called the r process, is necessary. Such a process takes place only under extreme physical conditions. Recently, astrophysicists have identified the formation of strontium during the fusion of neutron stars , revealing that the formation of such heavy elements takes place during this type of cosmic event.

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Astrophysicists have detected the formation of a heavy element in space, forged as a result of a collision between two neutron stars. The results also confirmed that " neutron stars contain many neutrons, " said Darach Watson, an astrophysicist at the Niels Bohr Institute at the University of Copenhagen. " It sounds really stupid, but it's something we do not know for sure. Now all we have found is pointing to elements that have only formed in the presence of many neutrons . "


The three lightest elements of the Universe - hydrogen, helium and lithium - were created in the very first moments of the cosmos, just after the Big Bang, during an event called primordial nucleosynthesis. Most elements heavier than lithium, up to the iron, were forged billions of years later, in the heart of the stars.

R process and fusion of neutron stars
But the way in which elements heavier than iron, such as gold and uranium, have been created, has long been uncertain. Previous research suggested a key clue: for atoms to reach large sizes, they needed to absorb neutrons quickly. Such fast neutron capture, known as the r-process, occurs only in nature, in extreme environments where atoms are bombarded by a large number of neutrons.

Diagram explaining the phenomenon of rapid capture of free neutrons by an atomic nucleus; still called process r. Credits: Rachel Freed
Previous work has suggested that a probable source of r-shaped elements could be the cosmic cataclysm caused by the fusion of neutron stars. In such stars, the internal pressure is so high that the electrons penetrate the protons to form neutrons.

In 2017, astronomers witnessed the fusion of two neutron stars. Astrophysicists made this discovery by detecting gravitational waves emitted from a collision about 130 million light years from Earth. Following the discovery of this fusion, dubbed GW170817, the researchers continued to make observations from Earth. " This explosion was moving at 30% of the speed of light, so it went from about 100 kilometers to the size of the Solar System in a day, " says Watson.

Kilonovas: cosmic sources of heavy elements

Watson and his colleagues suspected that if heavier elements were formed during GW170817, signatures of these elements could be detected in the explosive suites of fusion, known as kilonovas. They focused on the wavelengths of light (spectrum) that astrophysicists have linked by spectroscopy to specific elements.

Previous studies suggested the presence of heavy elements in kilonovas, but so far astronomers have not been able to locate individual elements. This is because " heavier elements can produce mixtures of tens of millions of spectral lines. We could never distinguish one element from another, "says Watson.

However, by re-analyzing the data from the 2017 merger, Watson and his colleagues identified the signature of the heavy strontium element. The results of the study were published in the journal Nature . On Earth, strontium occurs naturally in the soil and is concentrated in certain minerals. Strontium compounds even help give the fireworks a bright red color.

Detection facilitated by strontium structure

The key to this discovery lies in the atomic structure of strontium, relatively simple for such a heavy element. Due to its structure, the electrically charged version of strontium produces two powerful spectral lines in blue and infrared light. This discovery was surprising because, if strontium is a heavy element, it is also one of the lightest elements from process r.

The simple atomic structure of strontium facilitated its detection. Credits: SciencePhotoLibrary

In previous research, astrophysicists expected to find " heavier heavy elements, or heavier elements from the r process, looking at a kilonova, " says Watson. The discovery may be related to neutrinos, which normally cross the material but may occasionally collide with protons or neutrons.

" In order to create a relatively light heavy element like strontium, you must first destroy some neutrons. You have to bombard them with neutrinos, enough to disintegrate faster into protons and electrons . This tells us a little more about what happens in neutron stars and during such mergers.

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