Based on the interstellar CO/H2 ratio, carbon monoxide-based censuses of protoplanetary disks in Lupus, sigma Orionis, and Chamaeleon I found no disks more massive than the minimum-mass solar nebula, which is inconsistent with the existence of exoplanets more massive than Jupiter. Observations and models are converging on the idea that ionization-driven chemistry depletes carbon monoxide in T-Tauri disks. Yet the extent of CO depletion depends on the incident flux of ionizing radiation, and some T-Tauri stars may have winds strong enough to shield their disks from cosmic rays. There is also a range of X-ray luminosities possible for a given stellar mass. Here we use a suite of chemical models, each with a different incident X-ray or cosmic-ray flux, to assess whether CO depletion is a typical outcome for T-Tauri disks. We find that CO dissociation in the outer disk is a robust result for realistic ionization rates, with abundance reductions between 70% and 99.99% over 2 Myr of evolution. Furthermore, after the initial dissociation epoch, the inner disk shows some recovery of the CO abundance from CO2 dissociation. In highly ionized disks, CO recovery in the inner disk combined with depletion in the outer disk creates a centrally peaked CO abundance distribution. The emitting area in rare CO isotopologues may be an indirect ionization indicator: in a cluster of disks with similar ages, those with the most compact CO isotopologue emission see the highest ionization rates.
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