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Aluminum-26 Production in Protostellar Disks

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  • Figure 1: This figure shows a schematic of the proposed mechanism. We denote a gas parcel on the surface of the disk, which is free-falling across the disk, as the red 'cluster'. The gas is then accreted onto the protostar, then recycled through the outflow and splash back. The regions indicated show different regimes of energetic particle transport.
  • Figure 2: 26Al/27Al as a function of protostellar mass for different constant accretion rates for our Fiducial model. The dotted lines show the canonical 26Al/27Al ratio measured in CAIs. The hatched region shows the range of values on Solar system chondrites.

Brandt Gaches

Meteorites measurements show that at some point during the star and planet formation, our solar system was contaminated with a substantial amount of aluminium-26 (26Al). Within the oldest objects, calcium-aluminium-rich inclusions (CAIs), the  enrichment ratio is canonically 26Al/27Al = 5E-5. How the solar system was seeded with 26Al has been an open question since it's initial discovery in 1976. It has long since been proposed and argued that the solar system must have been seeded with 26Al by an external source, such as a supernova or the winds of high-mass stars. However, this would suggest that the radioactive element abundances we infer in our solar system may not be typical of other planetary systems. There is also debate on whether the hot gas from these external sources could mix with the much colder, denser gas that the solar system initially formed from. As such, an general internal mechanism would be more enticing!

We have proposed a new internal mechanism, wherein 26Al is produced in the surface of protostellar disks just before the onset of planet formation by energetic particles accelerated at the surface of protostars. Figure 1 shows a schematic of the proposed mechanism. Figure 2 shows the ratio, 26Al/27Al, as a function of protostellar mass for different accretion rates. We find that our proposed mechanism can produce amounts of 26Al consistent with Solar system measurements for low accretion rates. These accretion rates are thought to be experienced by nearly all protostars just before the formation CAIs, indicating our proposed mechanism will be generic to all protostellar systems.

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[Sky and Telescope]