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Distributions of Short-Lived Radioactive Nuclei Produced by Young Embedded Stellar Clusters

Most star formation in the Galaxy takes place in clusters, where the most massive members can affect the properties of other constituent solar systems. This paper considers how clusters influence star formation and forming planetary systems through nuclear enrichment from supernova explosions, where massive stars deliver short-lived radioactive nuclei (SLRs) to their local environment. The decay of these nuclei leads to both heating and ionization, and thereby affects disk evolution, disk chemistry, and the accompanying process of planet formation. Nuclear enrichment can take place on two spatial scales: [1] Within the cluster itself ($\ell\sim1$pc), the SLRs are delivered to the circumstellar disks associated with other cluster members. [2] On the next larger scale ($\ell\sim2-10$pc), SLRs are injected into the background molecular cloud; these nuclei provide heating and ionization to nearby star-forming regions, and to the next generation of disks. For the first scenario, we construct the expected distributions of radioactive enrichment levels provided by embedded clusters. Clusters can account for the SLR mass fractions inferred for the early Solar Nebula, but typical SLR abundances are lower by a factor of $\sim10$. For the second scenario, we find that distributed enrichment of SLRs in molecular clouds leads to comparable abundances. For both the direct and distributed enrichment processes, the masses of $^{26}$Al and $^{60}$Fe delivered to individual circumstellar disks typically fall in the range $10-100pM_\odot$ (where $1pM_\odot=10^{-12}M_\odot$). The corresponding ionization rate due to SLRs typically falls in the range $ζ_{SLR}\sim1-5\times10^{-19}$ sec$^{-1}$. This ionization rate is smaller than that due to cosmic rays, $ζ_{CR}\sim10^{-17}$ sec$^{-1}$, but will be important in regions where cosmic rays are attenuated (e.g., disk mid-planes).