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

Abstract

We use the gas-grain chemistry code UCLCHEM to explore the impact of cosmic-ray feedback on the chemistry of circumstellar disks. We model the attenuation and energy losses of the cosmic-rays as they propagate outwards from the star and also consider ionization due to stellar radiation and radionuclides. For accretion rates typical of young stars, ${\dot{M}}_{\ast }\sim 10^{-9}-10^{-6}$ M_\odot yr${}^{-1}$, we show that cosmic rays accelerated by the stellar accretion shock produce a cosmic-ray ionization rate at the disk surface $\zeta \gtrsim 10^{-15}$ s${}^{-1}$, at least an order of magnitude higher than the ionization rate associated with the Galactic cosmic-ray background. The incident cosmic-ray flux enhances the disk ionization at intermediate to high surface densities ($\Sigma >10$ g cm${}^{-2}$) particularly within 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 surface densities above 1.0 g cm${}^{-2}$. We predict the radii and column densities at which the magneto-rotational instability (MRI) is active in T Tauri disks and show that ionization by cosmic-ray feedback extends the MRI-active region towards the disk mid-plane. However, the MRI is only active at the mid-plane of a minimum mass solar nebula disk if cosmic-rays propagate diffusively ($\zeta \propto r^{-1}$) away from the star. The relationship between accretion, which accelerates cosmic rays, the dense accretion columns, which attenuate cosmic rays, and the MRI, which facilitates accretion, create a cosmic-ray feedback loop that mediates accretion and may produce luminosity variability.

19 pages, 9 figures, accepted to ApJ