Results from the chemical modeling for DM Tau, case L, Model 2 and ξ = 3 × 10^-17s^-1. Top: predicted o-H₂D⁺ column densities as a function of radius for several values of grain size: a = 0.1 μm (black); 1 (red) and 10 (blue); and CO abundance: x[CO] = 10^-6 (thin lines) 10^-5 (medium) and 10^-4 (thick). The green lines correspond to the best 3σ upper limits derived from observations with p = 0 and p = −1. Bottom: observed spectrum (green) and predicted line profiles (same color and thickness convention).