An international team of astronomers including Stella Offner of the University of Texas at Austin has proposed a new method for forming aluminum-26 in star systems that are forming planets. Because its radioactive decay is thought to provide a heat source for the building blocks of planets, called planetesimals, it is important for astronomers to know where aluminum-26 comes from. Their research is published in the current issue of The Astrophysical Journal.
“Atoms like aluminum and its radioactive isotopes of aluminum-26 allow us to carry out the archeology of the solar system,” Offner said. “It’s exciting that the abundances of different atoms today can provide clues about the formation of our solar system billions of years ago. “
Since its discovery in the Allende meteorite in 1976, astronomers have discussed the origin of the considerable amount of aluminum-26 in our early solar system. Some have suggested it was blown here by supernova explosions and winds from massive stars. However, these scenarios require a great deal of chance: Our sun and planets would have to form exactly the right distance from the massive stars, which are quite rare.
Offner’s team proposed an explanation that does not require an external source. They propose that aluminum-26 formed near the small sun in the inner part of its disk that forms a planet. As the material fell from the inner edge of the disk onto the sun, it created shockwaves that produced high-energy protons known as cosmic rays.
When it leaves the sun at almost the speed of light, cosmic rays fall into the surrounding disk, collide with the isotopes of aluminum-27 and silicon-28, and change into luminum-26.
Due to the very short half-life of about 770,000 years, aluminum-26 must be formed or mixed in the surrounding disk that forms a solar planet shortly before the condensation of the first solid matter into Our solar system. It plays an important part in the formation of planets like Earth, as it can provide enough heat through radioactive decay to produce planetary bodies with layered interiors (such as the solid core of Earth topped by a rock mantle and above it, a thin shell). The radioactive decay of aluminum-26 also helps to dry early planetesimals to produce rocky planets that are poor in water.
Aluminum-26 appears to have a fairly constant ratio to the isotopes of aluminum-27 in the oldest bodies of our solar system, comets and asteroids. Since the discovery of aluminum-26 in meteorites (which are chips outside asteroids), a significant amount of effort has been directed toward finding a plausible explanation for both its introduction into our early solar system and for the fixed ratio between aluminum-26 and aluminum -27.
Offner’s team focused their studies on a transition period during the formation of the sun: when the gas surrounding the small star runs out and the amount of gas falling on the sun decreases. significantly. Almost all small stars go through this transition over the last few tens to hundreds of thousands of years of formation.
As our sun was forming, gas infalling followed magnetic field lines to its surface. This produced a violent shockwave, the “accretion shock,” which accelerated the cosmic rays. These cosmic rays pass out until they hit the gas in the disk that forms a planet and caused chemical reactions. Scientists have calculated different models for this process.
“We found that low accretion rates are able to produce the amounts of aluminum-26, and the ratio of aluminum-26 to aluminum-27 that is present in the solar system,” said the paper’s lead author. , Brandt Gaches of the University of Germany. Cologne.
The proposed mechanism is generally valid for a wide range of low-mass stars, including sun stars. It is in these systems that astronomers have discovered the majority of exoplanets that are now known.
“Cosmic rays that have been accelerated by overhaul on a small forming star can provide a general pathway for aluminum-26 enrichment in many systems on the planet,” Gaches concluded, “and is one of the big questions whether the proposed mechanism of acceleration by shock waves is observed in star formation. ”
Examine stars that explode through the atomic nucleus
Brandt AL Gaches et al. Aluminum-26 Enrichment on the Surface of Protostellar Discs Due to Protostellar Cosmic Rays, The Astrophysical Journal (2020). DOI: 10.3847 / 1538-4357 / ab9a38
Provided by the University of Texas at Austin
Citation: Study of radioactive aluminum in stellar systems unblocks the secrets of formation (2020, 29 July) obtained on 29 July 2020 from https://phys.org/news/2020-07- radioactive-aluminum-stellar-formation-secrets.html
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