  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.
