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Based on the interstellar CO/H2 ratio, carbon monoxide-based censuses ofprotoplanetary disks in Lupus, sigma Orionis, and Chamaeleon I found no disksmore massive than the minimum-mass solar nebula, which is inconsistent with theexistence of exoplanets more massive than Jupiter. Observations and models areconverging on the idea that ionization-driven chemistry depletes carbonmonoxide in T-Tauri disks. Yet the extent of CO depletion depends on theincident flux of ionizing radiation, and some T-Tauri stars may have windsstrong enough to shield their disks from cosmic rays. There is also a range ofX-ray luminosities possible for a given stellar mass. Here we use a suite ofchemical models, each with a different incident X-ray or cosmic-ray flux, toassess whether CO depletion is a typical outcome for T-Tauri disks. We findthat CO dissociation in the outer disk is a robust result for realisticionization rates, with abundance reductions between 70% and 99.99% over 2 Myrof evolution. Furthermore, after the initial dissociation epoch, the inner diskshows some recovery of the CO abundance from CO2 dissociation. In highlyionized disks, CO recovery in the inner disk combined with depletion in theouter disk creates a centrally peaked CO abundance distribution. The emittingarea in rare CO isotopologues may be an indirect ionization indicator: in acluster of disks with similar ages, those with the most compact CO isotopologueemission see the highest ionization rates.

Ionization-Driven Depletion and Redistribution of CO in Protoplanetary Disks