Top: CO gas-phase fraction vs. gas temperature T_mp calculated in the 500 < r < 1000 AU radial annulus during a time period covering a strong luminosity burst at t = 0.13 Myr. The dashed, solid, and dash-dotted lines show the pre-burst, burst, and post-burst phases with durations of 1.0 kyr, 0.2 kyr, and 5 kyr, respectively. The arrows indicate this evolution sequence. The horizontal and vertical dotted lines are the CO fraction of 0.5 and the gas temperature at which the CO desorbs. The grey-shaded area highlights the phase-space region with an abnormally high fraction of the gas-phase CO. The thick dash-dotted line highlights the –T_mp model track in this region. Bottom: CO₂ gas-phase fraction vs. T_mp calculated in the 100 < r < 300 AU radial annulus during a strong luminosity burst at t = 0.082 Myr. The dashed, solid, and dash-dotted lines show the pre-burst, burst, and post-burst phases with durations of 0.4 kyr, 0.2 kyr, and 0.2 kyr, respectively. The horizontal and vertical dotted lines mark the CO₂ fraction of 0.5 and the gas temperature at which the CO₂ desorbs. The grey-shaded area highlights the phase-space region with an abnormally high fraction of the gas-phase CO₂.