Colliding neutron stars were touted as the main source of some of the heaviest elements on the periodic table. Now, not so much …
Collisions between neutron stars do not create the amount of chemical elements previously hypothesized, finds a new analysis of the evolution of the galaxy.
The research also reveals that current models cannot explain the amount of gold in the cosmos, creating an astronomical mystery.
The work produced a new-look periodic table, showing the stellar origins of natural elements from carbon to uranium.
All hydrogen in the Universe, including every molecule on Earth, was created by big Bang, which also produced a lot of helium and lithium, but not much else.
The rest of the elements found in nature are made up of different nuclear processes that take place inside the stars. Mass governs exactly which elements are forged, but they are all released into galaxies in the final moments of each star – explosively in the case of really large ones, or as dense, solar wind-like streams for those of the same class as the Sun.
“We can think of stars as gigantic pressure cookers in which new elements are created,” explained co-author Associate Professor Karakas, of Australia’s ARC Center of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D).
“The reactions that these elements make also provide the energy that makes stars shine for billions of years. As stars age, they produce heavier and heavier elements as their internal parts heat up. “
Half of all elements heavier than iron, such as thorium and uranium, were thought to have been produced when neutron stars, the superdense remnants of burnt suns, crashed into each other. Long theorized, neutron star collisions were not confirmed until 2017.
Now, however, a new analysis by Karakas and fellow astronomers Chiaki Kobayashi and Maria Lugaro reveals that the role of neutron stars may have been greatly overestimated and that another stellar process is responsible for producing most of the heavy elements.
“Neutron star mergers didn’t produce enough heavy elements early in the life of the Universe, and they still don’t do it now, 14 billion years later,” Karakas said.
“The universe hasn’t made them fast enough to justify their presence in very ancient stars and, overall, there simply aren’t enough collisions to explain the abundance of these elements around today.”
Instead, the researchers found that the heavy elements had to be created by a completely different type of stellar phenomenon: unusual supernovae that collapse while spinning very fast and generate strong magnetic fields.
The discovery is one of many that emerged from their research, just published in Astrophysical Journal. Their study is the first time that the stellar origins of all elements found in nature, from carbon to uranium, have been calculated based on first principles.
The new modeling, the researchers say, will fundamentally change the currently accepted model of how the universe has evolved. “For example, we built this new model to explain all the elements at once, and we found enough silver but not enough gold,” said co-author Associate Professor Kobayashi, of the University of Hertfordshire in the UK.
“Silver is produced in excess but gold is under-produced in the model compared to observations. This means that we may need to identify a new type of stellar explosion or nuclear reaction. “
The study refines previous studies that calculate the relative roles of stellar mass, age and disposition in the production of the elements.
For example, the researchers determined that stars smaller than about eight times the mass of the Sun produce carbon, nitrogen and fluorine, as well as half of all elements heavier than iron.
Massive stars over about eight times the mass of the Sun that also explode as supernovae at the end of their life, produce many of the elements from carbon to iron, including most of the oxygen and calcium needed for life.
“Aside from hydrogen, there is no single element that can be formed from just one type of star,” Kobayashi explained.
“Half of the carbon is produced by low-mass dying stars, but the other half comes from supernovae.
“And half of the iron comes from normal supernovae of massive stars, but the other half needs another form, known as a Type Ia supernova. These are produced in low-mass binary systems of stars. “
Gravity-bound pairs of massive stars, on the other hand, can turn into neutron stars. When these collide, the impact produces some of the heaviest elements found in nature, including gold.
In the new modeling, however, the numbers simply don’t add up.
“Even the most optimistic estimates of the collision frequency of neutron stars simply cannot explain the enormous abundance of these elements in the universe,” Karakas said. “This was a surprise. Spinning supernovae with strong magnetic fields appear to be the true source of most of these elements. “
Co-author Dr Maria Lugaro, who holds positions at Hungary’s Konkoly Observatory and Monash University in Australia, thinks the missing gold mystery may be solved soon enough.
“New discoveries are expected from nuclear facilities around the world, including Europe, the United States and Japan, which currently target rare nuclei associated with neutron star mergers,” he said.
“The properties of these nuclei are unknown, but they heavily control the production of the abundances of heavy elements. The astrophysical problem of missing gold can actually be solved by a nuclear physics experiment. “
The researchers admit that future research may find that neutron star collisions are more frequent than evidence so far suggests, in which case their contribution to the elements that make up everything from cell phone screens to fuel for nuclear reactors. it could be revised upwards again.
For the time being, however, they seem to be offering a lot less money for their bangs.
Reference: September 15, 2020, Astrophysical Journal.
DOI: 10.3847 / 1538-4357 / abae65