Where does gold come from? New insights into element synthesis in the
universe

Date:
November 15, 2021
Source:
GSI Helmholtzzentrum fu"r Schwerionenforschung GmbH
Summary:
How are chemical elements produced in our Universe? Where do
heavy elements like gold and uranium come from? Using computer
simulations, a research team shows that the synthesis of heavy
elements is typical for certain black holes with orbiting matter
accumulations, so-called accretion disks. The predicted abundance
of the formed elements provides insight into which heavy elements
need to be studied in future laboratories to unravel the origin
of heavy elements.



FULL STORY ==========================================================================
How are chemical elements produced in our Universe? Where do heavy
elements like gold and uranium come from? Using computer simulations,
a research team from the GSI Helmholtzzentrum fu"r Schwerionenforschung
in Darmstadt, together with colleagues from Belgium and Japan, shows
that the synthesis of heavy elements is typical for certain black holes
with orbiting matter accumulations, so-called accretion disks. The
predicted abundance of the formed elements provides insight into which
heavy elements need to be studied in future laboratories -- such as the Facility for Antiproton and Ion Research (FAIR), which is currently under construction -- to unravel the origin of heavy elements. The results
are published in the journal Monthly Notices of the Royal Astronomical
Society.


==========================================================================
All heavy elements on Earth today were formed under extreme conditions
in astrophysical environments: inside stars, in stellar explosions,
and during the collision of neutron stars. Researchers are intrigued
with the question in which of these astrophysical events the appropriate conditions for the formation of the heaviest elements, such as gold or
uranium, exist. The spectacular first observation of gravitational waves
and electromagnetic radiation originating from a neutron star merger in
2017 suggested that many heavy elements can be produced and released in
these cosmic collisions.

However, the question remains open as to when and why the material is
ejected and whether there may be other scenarios in which heavy elements
can be produced.

Promising candidates for heavy element production are black holes
orbited by an accretion disk of dense and hot matter. Such a system is
formed both after the merger of two massive neutron stars and during a so-called collapsar, the collapse and subsequent explosion of a rotating
star. The internal composition of such accretion disks has so far not
been well understood, particularly with respect to the conditions under
which an excess of neutrons forms. A high number of neutrons is a basic requirement for the synthesis of heavy elements, as it enables the rapid neutron-capture process or r-process. Nearly massless neutrinos play
a key role in this process, as they enable conversion between protons
and neutrons.

"In our study, we systematically investigated for the first time the
conversion rates of neutrons and protons for a large number of disk configurations by means of elaborate computer simulations, and we found
that the disks are very rich in neutrons as long as certain conditions are met," explains Dr. Oliver Just from the Relativistic Astrophysics group
of GSI's research division Theory. "The decisive factor is the total
mass of the disk. The more massive the disk, the more often neutrons
are formed from protons through capture of electrons under emission
of neutrinos, and are available for the synthesis of heavy elements by
means of the r-process. However, if the mass of the disk is too high,
the inverse reaction plays an increased role so that more neutrinos
are recaptured by neutrons before they leave the disk. These neutrons
are then converted back to protons, which hinders the r-process." As
the study shows, the optimal disk mass for prolific production of heavy elements is about 0.01 to 0.1 solar masses. The result provides strong
evidence that neutron star mergers producing accretion disks with these
exact masses could be the point of origin for a large fraction of the
heavy elements. However, whether and how frequently such accretion disks
occur in collapsar systems is currently unclear.

In addition to the possible processes of mass ejection, the research
group led by Dr. Andreas Bauswein is also investigating the light
signals generated by the ejected matter, which will be used to infer
the mass and composition of the ejected matter in future observations
of colliding neutron stars. An important building block for correctly
reading these light signals is accurate knowledge of the masses and
other properties of the newly formed elements. "These data are currently insufficient. But with the next generation of accelerators, such as
FAIR, it will be possible to measure them with unprecedented accuracy
in the future. The well-coordinated interplay of theoretical models, experiments, and astronomical observations will enable us researchers
in the coming years to test neutron star mergers as the origin of the
r-process elements," predicts Bauswein.

========================================================================== Story Source: Materials provided by GSI_Helmholtzzentrum_fu"r_Schwerionenforschung_GmbH.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. O Just, S Goriely, H-Th Janka, S Nagataki, A Bauswein. Neutrino
absorption and other physics dependencies in neutrino-cooled black
hole accretion disks. Monthly Notices of the Royal Astronomical
Society, 2021; DOI: 10.1093/mnras/stab2861 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211115123459.htm

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