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The radioactive decay of aluminum-26 ($^{26}$Al) is an important heatingsource in early planet formation. Since its discovery, there have been severalmechanisms proposed to introduce $^{26}$Al into protoplanetary disks, primarilythrough contamination by external sources. We propose a local mechanism toenrich protostellar disks with $^{26}$Al through irradiation of theprotostellar disk surface by cosmic rays accelerated in the protostellaraccretion shock. We calculate the $^{26}$Al enrichment, [$^{26}$Al/$^{27}$Al],at the surface of the protostellar disk in the inner AU throughout theevolution of low-mass stars, from M-dwarfs to proto-Suns. Assuming constantmass accretion rates, $\dot{m}$, we find that irradiation by MeV cosmic rayscan provide significant enrichment on the disk surface if the cosmic rays arenot completely coupled to the gas in the accretion flow. Importantly, we findthat low accretion rates, $\dot{m} < 10^{-7}$ M$_{\odot}$ yr$^{-1}$, are ableto produce canonical amounts of $^{26}$Al, $[^{26}{\rm Al}/^{27}{\rm Al}]\approx 5\times10^{-5}$. These accretion rates are experienced at thetransition from Class I- to Class II-type protostars, when it is assumed thatcalcium-aluminum-rich inclusions condense in the inner disk. We conclude thatirradiation of the inner disk surface by cosmic ray protons accelerated inaccretion shocks at the protostellar surface may be an important mechanism toproduce $^{26}$Al. Our models show protostellar cosmic rays may be a viablemodel to explain the enrichment of $^{26}$Al found in the Solar System.

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