This artist’s concept available from NASA illustrates a star system that is a much younger version of ours. Dusty disks, like the one shown here surrounding the star, are believed to be the breeding ground for planets, including rocky ones like Earth. Credit: NASA / JPL-Caltech
An international team of astronomers, including Stella Offner of the University of Texas at Austin, has proposed a new method for the formation of aluminum-26 in star systems that are forming planets. Because their radioactive decay is believed to provide a heat source for the planet’s building blocks, 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 their radioactive aluminum-26 isotope allow us to perform the ‘archeology’ of the solar system,” Offner said. “It is exciting that the abundance of different atoms today can provide clues to the formation of our solar system billions of years ago.”
Since their discovery in the Allende meteorite in 1976, astronomers have debated the origin of the considerable amount of aluminum-26 in our early solar system. Some have suggested that it was blown up here by supernova explosions and massive star winds. However, these scenarios require a lot of possibilities: our sun and our planets would have to form at exactly the correct distance from the massive stars, which is quite rare.
Offner’s team has proposed an explanation that does not require an external source. They propose that aluminum-26 formed near the young sun on the inner side of the planet’s surrounding disk. As the material fell from the inner edge of the disk into the sun, it created shock waves that produced high-energy protons known as cosmic rays.
As the sun left almost at the speed of light, the cosmic rays crashed into the surrounding disk, colliding with the aluminum-27 and silicon-28 isotopes, transforming them into aluminum-26.
Due to its very short half-life of approximately 770,000 years, aluminum-26 must have formed or mixed in the planet-forming disk surrounding the young Sun shortly before the condensation of the first solid matter in our solar system. It plays an important role in the formation of planets like Earth, as it can provide enough heat through radioactive decay to produce planetary bodies with inner layers (like the Earth’s solid core topped by a rocky mantle and, above that, a thin crust). The radioactive decay of aluminum-26 also helps dry the first planetesimals to produce water-poor rocky planets.
This schematic of the proposed mechanism shows a sectional view of a young star and the surrounding gas disk, in which planets can form. Offner’s equipment model of the gas package is represented as a group of red dots. The ‘inner disk’ is the region from the star to the distance from Earth to the Sun (1 Astronomical Unit, or approximately 93 million miles). Some fraction of the enriched exhaust gas can fall onto the disk where the cosmic ray irradiation is weak. Regions I and II denote different regions of cosmic ray transport. Credit: Brandt Gaches et al./Univ. Cologne
Aluminum-26 appears to have a fairly constant relationship with the aluminum-27 isotope in the oldest bodies in our solar system, comets and asteroids. Since the discovery of aluminum-26 in meteorites (which are asteroid chips), a significant effort has been directed to find a plausible explanation for both its introduction into our early solar system and the fixed relationship between aluminum-26 and aluminum. -27.
Offner’s team focused their studies on a period of transition during the formation of the sun: when the gas that surrounds the young star is exhausted and the amount of gas that falls on the sun decreases significantly. Almost all young stars experience this transition during the past tens to hundreds of thousands of years of formation.
As our sun was forming, the descending gas followed magnetic field lines to its surface. This produced a violent shock wave, the “accretion shock,” which accelerated the cosmic rays. These cosmic rays flowed outward until they hit the gas in the disk that makes up the planet and caused chemical reactions. Scientists calculated different models for this process.
“We found that low accretion rates can produce the amounts of aluminum-26, and the ratio of aluminum-26 to aluminum-27 that is present in the solar system,” said lead article author Brandt Gaches of the University from Germany from Germany. Suburb.
The proposed mechanism is generally valid for a wide range of low-mass stars, including sun-like stars. It is in these systems that astronomers have discovered most of the known exoplanets now.
“Cosmic rays that were accelerated by accumulation in the formation of young stars may provide a general pathway for aluminum-26 enrichment in many planetary systems,” Gaches concluded, “and it is one of the big questions whether the proposed mechanism of acceleration through shock waves will be observed in the formation of stars. ”
Examining exploding stars through the atomic nucleus
Brandt AL Gaches et al. Aluminum-26 enrichment on the surface of protostellar disks 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 star systems unlocks formation secrets (2020, July 29) retrieved on July 29, 2020 from https://phys.org/news/2020-07-radioactive-aluminum-stellar-formation- secrets.html
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