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We use the gas-grain chemistry code UCLCHEM to explore the impact ofcosmic-ray feedback on the chemistry of circumstellar disks. We model theattenuation and energy losses of the cosmic-rays as they propagate outwardsfrom the star and also consider ionization due to stellar radiation andradionuclides. For accretion rates typical of young stars, $\dot M_* \sim10^{-9}-10^{-6}$ M_\odot yr$^{-1}$, we show that cosmic rays accelerated by thestellar accretion shock produce a cosmic-ray ionization rate at the disksurface $\zeta \gtrsim 10^{-15}$ s$^{-1}$, at least an order of magnitudehigher than the ionization rate associated with the Galactic cosmic-raybackground. The incident cosmic-ray flux enhances the disk ionization atintermediate to high surface densities ($\Sigma > 10$ g cm$^{-2}$) particularlywithin 10 au of the star. We find the dominant ions are C$^+$, S$^+$ and Mg$^+$in the disk surface layers, while the H$_3^+$ ion dominates at surfacedensities above 1.0 g cm$^{-2}$. We predict the radii and column densities atwhich the magneto-rotational instability (MRI) is active in T Tauri disks andshow that ionization by cosmic-ray feedback extends the MRI-active regiontowards the disk mid-plane. However, the MRI is only active at the mid-plane ofa minimum mass solar nebula disk if cosmic-rays propagate diffusively ($\zeta\propto r^{-1}$) away from the star. The relationship between accretion, whichaccelerates cosmic rays, the dense accretion columns, which attenuate cosmicrays, and the MRI, which facilitates accretion, create a cosmic-ray feedbackloop that mediates accretion and may produce luminosity variability.

Impact of Cosmic-Ray Feedback on Accretion and Chemistry in Circumstellar Disks