The Origin Mystery Of Heavy Aromatic Nuclei

The universe's lightest elements, including hydrogen, helium, and a small amount of lithium, were created during the Big Bang. Heavier elements, up to iron, were formed in the cores of stars and when stars explode as supernovae. However, the origin of elements heavier than iron, such as gold and uranium, has long been a mystery. Recent research has provided evidence that these elements are created during neutron star mergers. Neutron stars are the superdense cores of stars left behind after cataclysmic, explosive star deaths known as supernovae. When neutron stars merge, they release a fraction of their combined mass, triggering a phenomenon called a kilonova, which spreads heavy elements into the universe.

Neutron capture

The process of neutron capture is one of the ways in which elements heavier than iron are produced. Neutron capture can occur either rapidly (r-process) or slowly (s-process). In the r-process, the neutron capture timescale is short compared to the decay timescale of the products. This process occurs inside supernovae explosions and during the merger of neutron stars. During a supernova explosion, free neutrons are created by electron capture in the final moments of core collapse. This can lead to the build-up of neutron-rich nuclei, which then decay into heavier elements once they are ejected into the interstellar medium. Neutron star mergers also provide an abundant source of free neutrons, which can combine with seed nuclei to form heavier elements. The r-process is responsible for the formation of elements heavier than lead and contributes to the abundances of many elements between iron and lead.

On the other hand, the s-process occurs when the neutron capture rate timescale is longer than the product decay timescales. The free neutrons for these low-flux neutron-capture events come from alpha particle reactions with carbon-13 or neon-22 in giant stars. After neutron capture, a neutron in the new nucleus may beta decay, creating a nucleus with a higher mass and proton number. This process can produce a range of heavy nuclei, with iron-peak nuclei as seeds. Examples of elements produced mainly by the s-process include strontium, yttrium, rubidium, and lead.

While the r-process occurs primarily during neutron star mergers, the s-process occurs mostly during the late stages of evolution in stars with 1-10 solar masses. The r-process is responsible for creating elements that are typically very heavy and near the end of the periodic table, such as gold, platinum, and uranium.

Neutron-star mergers

The process by which heavy elements are created in these mergers is called rapid neutron capture, or the "r-process." This process occurs when atomic nuclei absorb multiple neutrons in rapid succession, requiring an extremely high density of free neutrons. Such conditions are met during neutron-star mergers, where the gravitational pull is strong enough to crush protons and electrons together to form neutrons.

In 2017, astronomers witnessed, for the first time, a pair of neutron stars merging. This discovery, known as GW170817, was made by detecting gravitational waves, or ripples in spacetime, that radiated from a collision about 130 million light-years from Earth. Following this discovery, scientists continued to make telescopic observations and found signatures of heavy elements in the aftermath of the merger, known as a kilonova.

The presence of heavy elements within the kilonova had been suggested by prior research, but the 2017 event marked the first time astronomers could pinpoint individual elements. By reanalysing the data, scientists identified the signature of the heavy element strontium within the fireball. This discovery was surprising because strontium is one of the lightest r-process elements, and scientists had expected to find heavier elements in a kilonova.

The creation of strontium in a kilonova may be linked to ghostly particles known as neutrinos, which can occasionally collide with protons or neutrons. To create strontium, some neutrons must first be destroyed by bombarding them with neutrinos, causing them to decay more quickly into protons and electrons. This finding provides valuable insights into the processes occurring during neutron star mergers and the formation of heavy elements.

Further research is ongoing to detect other heavy elements resulting from neutron star collisions. The complex nature of these elements presents challenges, as there is limited data on the atomic structures of these heavy elements. However, within the next few years, scientists hope to collect more data that will enable them to identify additional heavy elements within kilonovas.

Supernovae explosions

The elements heavier than iron are formed in the extreme conditions of supernovae explosions. A supernova is a violent explosion of a star that occurs when a white dwarf star, the remnant of a low-mass star that has exhausted its nuclear fuel, undergoes a thermonuclear explosion. This happens when its mass is increased beyond its limit by accreting nuclear fuel from a companion star. The resulting runaway nucleosynthesis completely destroys the star and ejects its mass into space.

During a supernova explosion, a shock wave is sent out from the star's former surface at a speed of 10,000 km/s. This shock wave compresses the material it passes through and is the only place where many elements such as zinc, silver, tin, gold, mercury, lead, and uranium are produced. The explosion also blows the outer layers of the star into space, enriching the interstellar medium with heavy elements. These heavy elements are then incorporated into new stars, millions or billions of years later.

The process of creating elements heavier than iron in supernovae explosions is called rapid neutron capture, or the "r-process". This occurs when atoms are bombarded by a very large number of neutrons. The free neutrons in a supernova are created by electron capture in the final moments of core collapse. This leads to the build-up of neutron-rich nuclei, and the decay products of these lead to many of the chemical elements heavier than iron.

The r-process is responsible for elements heavier than lead and contributes to the abundances of many elements between iron and lead. It occurs in high neutron density with high-temperature conditions, where any heavy nuclei are bombarded with a large neutron flux to form highly unstable neutron-rich nuclei. These nuclei then undergo beta decay to form more stable nuclei with higher atomic numbers.

The r-process has been observed in the aftermath of a neutron star merger, which is the superdense core of a star left behind after a cataclysmic, explosive star death known as a supernova. In 2017, astronomers witnessed a pair of neutron stars merging and detected the presence of heavy elements within the explosive aftermath, known as a kilonova. By reanalyzing the data, astronomers identified the signature of the heavy element strontium within the fireball. This discovery provided a missing piece of the puzzle of chemical element formation.

Slow neutron capture

The slow neutron capture process, or s-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The s-process is responsible for the creation of approximately half of the atomic nuclei heavier than iron.

In the s-process, a seed nucleus undergoes neutron capture to form an isotope with one higher atomic mass. If the new isotope is stable, a series of increases in mass can occur. If it is unstable, then beta decay will occur, producing an element of the next higher atomic number. The process is slow, in the sense that there is sufficient time for this radioactive decay to occur before another neutron is captured. A series of these reactions produces stable isotopes by moving along the valley of beta-decay stable isobars in the table of nuclides.

The s-process is believed to occur mostly in asymptotic giant branch stars, seeded by iron nuclei left by a supernova during a previous generation of stars. In contrast to the rapid neutron capture process (r-process), which occurs over timescales of seconds in explosive environments, the s-process occurs over timescales of thousands of years, with decades passing between neutron captures. The extent to which the s-process moves up the elements in the chart of isotopes to higher mass numbers is determined by the degree to which the star in question is able to produce neutrons. The quantitative yield is also proportional to the amount of iron in the star's initial abundance distribution.

The main neutron source reactions for the s-process are:

• 13C(α,n)16O
• 22Ne(α,n)25Mg

The s-process can be approximated over a small mass region using the "local approximation", where the ratio of abundances is inversely proportional to the ratio of neutron-capture cross-sections for nearby isotopes on the s-process path. However, this approximation is only valid locally, for isotopes of nearby mass numbers, and does not hold at magic numbers where the ledge-precipice structure dominates.

The s-process was first identified as necessary from the relative abundances of isotopes of heavy elements and from a table of abundances published in 1956 by Hans Suess and Harold Urey. This data showed abundance peaks for strontium, barium, and lead, which are particularly stable nuclei. This implied that some abundant nuclei must be created by slow neutron capture. A table apportioning the heavy isotopes between the s-process and the r-process was published in the famous B2FH review paper in 1957.

Rapid neutron capture

The rapid neutron capture process, also known as the "r-process", is a vital mechanism responsible for the formation of elements heavier than iron. This process occurs in extreme environments where atoms are bombarded by a vast number of neutrons. The rapid succession of neutron capture, coupled with beta decay, leads to the creation of heavier atoms.

The r-process typically takes place during cataclysmic events in the universe, such as mergers between neutron stars or supernovae explosions. Neutron stars, being the superdense remnants of stars that underwent explosive deaths, possess strong gravitational forces capable of crushing protons and electrons together to form neutrons. When these neutron stars collide, the resulting explosion creates the perfect conditions for the r-process to occur.

The r-process is believed to contribute to about half of the production of elements heavier than iron. However, there are still uncertainties and ongoing debates regarding the specific astrophysical conditions and nuclear properties involved. Recent studies have suggested that neutron star mergers, along with other rare events like hypernovae and collapsars, might play a more significant role in the r-process than previously thought.

The detection of gravitational waves from colliding neutron stars has provided valuable insights. By studying the aftermath of these mergers, astronomers have identified spectral lines associated with specific elements, such as strontium, confirming the presence of heavy elements formed during these events.

While the r-process is rapid, there is another process known as the slow neutron capture process or the "s-process". This process occurs over much longer timescales, ranging from hundreds to thousands of years, and contributes to the formation of various heavy elements as well.

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