  Young stellar objects are observed to have large X-ray fluxes and are thought
to produce commensurate luminosities in energetic particles (cosmic rays). This
particle radiation, in turn, can synthesize short-lived radioactive nuclei
through spallation. With a focus on $^{26}$Al, this paper estimates the
expected abundances of radioactive nulcei produced by spallation during the
epoch of planet formation. In this model, cosmic rays are accelerated near the
inner truncation radii of circumstellar disks, $r_{\scriptstyle X}\approx0.1$
AU, where intense magnetic activity takes place. For planets forming in this
region, radioactive abundances can be enhanced over the values inferred for the
early solar system (from meteoritic measurements) by factors of $\sim10-20$.
These short-lived radioactive nuclei influence the process of planet formation
and the properties of planets in several ways. The minimum size required for
planetesimals to become fully molten decreases with increasing levels of
radioactive enrichment, and such melting leads to loss of volatile components
including water. Planets produced with an enhanced radioactive inventory have
significant internal luminosity which can be comparable to that provided by the
host star; this additional heating affects both atmospheric mass loss and
chemical composition. Finally, the habitable zone of red dwarf stars is
coincident with the magnetic reconnection region, so that planets forming at
those locations will experience maximum exposure to particle radiation, and
subsequent depletion of volatiles.
