Aluminum-26 Production in Protostellar Disks
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.