Born through shocks and cosmic rays: The origin of Aluminium-26 in protostellar disks

Born through shocks and cosmic rays: The origin of Aluminium-26 in protostellar disks
The radioactive isotope Aluminium-26 found in planets and asteroids was locally formed in the gas disk around our young Sun / The study provides an answer to a key question about the chemical formation of the early solar system and the existence of habitable planets.

The decay of the radioactive isotope Aluminium-26 is thought to be the important heating source in planetesimals, which are the building blocks of planets and help to set the initial conditions for the formation of the solar system. Furthermore, Aluminium-26 is one of the most important “clocks” used to date solar system bodies.

Since its discovery in the Allende meteorite in 1976, it has been widely debated where the considerable amount of this isotope in the early Solar System came from. Transport from outside sources such as supernovae and winds from massive stars has been suggested. However, this scenario basically requires a lot of „good luck” in order to form Solar System-like planets.

An international team from scientists from the University of Cologne and the University of Texas at Austin have now proposed an explanation that does not require an outside source: the isotope was formed in the inner planet-forming disk close to the young Sun by cosmic ray protons that were accelerated in shockwaves produced by material accreting onto the star.

These shocks occur through the impact of gas flowing from the edge of the disk onto the forming star.

The article „Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays“ by Brandt A. L. Gaches, Stefanie Walch, Stella S. R. Offner and Carsten Münker is published in the current issue of the Astrophysical Journal.

“This article spawns from a spontaneous idea triggered by a talk within the colloquium series of the Collaborative Research Center 956 here at the University of Cologne. I particularly find that it is in line with the spirit of my ERC Starting Grant RADFEEDBACK, within which this research was conducted. This way science is really fun!”, narrates Stefanie Walch.

„There has been a long-standing debate among astrophysicists about the origin of the aluminium-26 in the solar system, particularly in calcium-aluminium-rich inclusions (pebble sized materials trapped inside minears) like those found in meteorites“, explains Dr. Brandt Gaches from the University of Cologne. „Supernovae or early solar activity had been suggested, but the solutions that had been proposed up to now had some issues“, says Gaches. „So we wanted to establish a general mechanism for it.“

While the aluminium-26 in meteorites can no longer be directly measured, having long since decayed, it’s presence can be inferred by excess amounts of magnesium-26, aluminium-26’s daughter isotope. Aluminium-26 appears to have a fairly constant ratio to the isotope of aluminium-27 in the oldest solar system bodies.

Since the discovery of aluminium-26 in meteorites a significant amount of effort has been directed towards finding a plausible enrichment mechanism. Such a mechanism should not only be able to explain the existence of Aluminum-26 but also the fixed ratio between aluminium-26 and aluminium-27.

Due to its very short half-life of about 770.000 years aluminium-26 must have been formed or mixed into the young Sun's protoplanetary disk shortly before the condensation of the first solid matter in the solar system.

It plays an important part in the formation of planets, like the Earth, since it can provide enough heat through radioactive decay to produce differentiated bodies and help to dry out early planetesimals to produce the water-poor rocky planets like those in the Solar System.

Enrichment of aluminium-26 acts through highly energetic collisions of parent nuclei, such as aluminium-27 and silicon-28, with protons or more massive cosmic rays. Conclusively Gaches and his colleagues calculated the conditions cosmic rays could interact with the material in the protoplanetary disk to produce the radioactive isotope.

Cosmic rays are high-energy protons that move through space at nearly the speed of light. They originate from the sun or sources outside the solar system. „We assume the cosmic rays are accelerated via crossing the shock front numerous times getting impulsive momentum gains at every crossing. They interact the disk and thus, the gas in the inner disk is enriched with 26Al.“

„In our work, we propose a new mechanism: enrichment via proton irradiation of the surface of protostellar disks during the Class I/II phase by cosmic rays accelerated in the accretion shocks of young protostars“, explains Gaches.
Gaches and his colleagues focused on a transition period during star formation: the phase during which the gas in the surrounding envelope becomes depleted and accretion onto the forming star decreases significantly, leading to the inner disk cooling down.

Nearly all young stars undergo this transition during the last few tens- to hundreds of thousands of years of formation.. During the formation process, gas from the disk falls onto the central young star probably following the magnetic field lines of the protostar. This causes a shockwave, the accretion shock, when hitting the dense material close to the nascent star. Cosmic rays are accelerated in the accretion shock. The scientists calculated different models of the process.

„We found that low accretion rates are able to produce the amounts of 26Al, and the ratio of 26Al/27Al that is present in the Solar System can be recovered.“

The proposed mechanism is generally valid for a wide range of low-mass stars, including solar type stars, where the majority of exoplanetary systems have been discovered. „Cosmic rays that were accelerated by accretion onto forming young stars may provide a general pathway for 26Al enrichment in many planetary systems“, concludes Gaches. „and it is one of the great questions if the proposed mechanism of acceleration through shockwaves will be observed in forming stars.“

The scientists proved their assumption using the model of a static disk, says Gaches. „Now that we have shown that this mechanism works, it could be included in more sophisticated disk models to get into the details of the process forming aluminium-26.“

Media Contact:
Dr. Brandt Gaches
gaches@ph1.uni-koeln.de
+49 221 470-8293

Press and Communications Team:
Robert Hahn
+49 221 470-2396
r.hahn@verw.uni-koeln.de

Publication:
https://iopscience.iop.org/article/10.3847/1538-4357/ab9a38