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Exclusion of Cosmic Rays in Protoplanetary Disks. II. Chemical Gradients and Observational Signatures

The chemical properties of protoplanetary disks are especially sensitive to their ionization environment. Sources of molecular gas ionization include cosmic rays, stellar X-rays and short-lived radionuclides, each of which varies with location in the disk. This behavior leads to a significant amount of chemical structure, especially in molecular ion abundances, which is imprinted in their submillimeter rotational line emission. Using an observationally motivated disk model, we make predictions for the dependence of chemical abundances on the assumed properties of the ionizing field. We calculate the emergent line intensity for abundant molecular ions and simulate sensitive observations with the Atacama Large Millimeter/Sub-millimeter Array (ALMA) for a disk at D=100 pc. The models readily distinguish between high ionization rates ($ζ\gtrsim10^{-17}$ s$^{-1}$ per H$_2$) and below, but it becomes difficult to distinguish between low ionization models when $ζ\lesssim10^{-19}$ s$^{-1}$. We find that \htdp\ emission is not detectable for sub-interstellar CR rates with ALMA (6h integration), and that \ntdp\ emission may be a more sensitive tracer of midplane ionization. HCO$^+$ traces X-rays and high CR rates ($ζ_{\rm{CR}}\gtrsim10^{-17}$ s$^{-1}$), and provides a handle on the warm molecular ionization properties where CO is present in the gas. Furthermore, species like HCO$^+$, which emits from a wide radial region and samples a large gradient in temperature, can exhibit ring-like emission as a consequence of low-lying rotational level de-excitation near the star. This finding highlights a scenario where rings are not necessarily structural or chemical in nature, but simply a result of the underlying line excitation properties.