Aluminum-26 Enrichment in the Surface of Protostellar Disks Due to Protostellar Cosmic Rays.

The radioactive decay of aluminum-26 ($^{26}$Al) is an important heating source in early planet formation. Since its discovery, there have been several mechanisms proposed to introduce $^{26}$Al into protoplanetary disks, primarily through contamination by external sources. We propose a local mechanism to enrich protostellar disks with $^{26}$Al through irradiation of the protostellar disk surface by cosmic rays accelerated in the protostellar accretion shock. We calculate the $^{26}$Al enrichment, [$^{26}$Al/$^{27}$Al], at the surface of the protostellar disk in the inner AU throughout the evolution of low-mass stars, from M-dwarfs to proto-Suns. Assuming constant mass accretion rates, $\dot{m}$, we find that irradiation by MeV cosmic rays can provide significant enrichment on the disk surface if the cosmic rays are not completely coupled to the gas in the accretion flow. Importantly, we find that low accretion rates, $\dot{m} < 10^{-7}$ M$_{\odot}$ yr$^{-1}$, are able to produce canonical amounts of $^{26}$Al, $[^{26}{\rm Al}/^{27}{\rm Al}] \approx 5\times10^{-5}$. These accretion rates are experienced at the transition from Class I- to Class II-type protostars, when it is assumed that calcium-aluminum-rich inclusions condense in the inner disk. We conclude that irradiation of the inner disk surface by cosmic ray protons accelerated in accretion shocks at the protostellar surface may be an important mechanism to produce $^{26}$Al. Our models show protostellar cosmic rays may be a viable model to explain the enrichment of $^{26}$Al found in the Solar System.