Mid-plane temperature within the model (active) accretion disk at orbital radii of r = 1 AU (left panel) and r = 5 AU (right panel). Contours are shown on the Ṁ − α plane, spanning the range of parameters relevant to young protoplanetary nebulae. Level curves corresponding to the assumed water ice sublimation temperature of T_ice = 170 K are shown with dashed lines. As disks evolve, their accretion rates diminish, meaning that Ṁ constitutes a proxy for time, as shown on the left panel. A small region of gravitationally unstable parameter space where Toomre Q < 1 is labeled on the top-left corner of the right panel.