Peak radius of N₂H⁺ (left column), and HCO⁺ (right column) simulated emission versus central protostellar luminosity for the fiducial models (black hatch) without disk (top row), and with disk (bottom row). The different shaded areas show how the measured peak radius is altered by the spatial resolution used for observations. The solid curve shows the relation for i = 45°. For N₂H⁺, the shaded area indicates the range of peak radii for inclinations between 0° (face-on) and 90° (edge-on). For HCO⁺, the shaded area indicates the range of peak radii for inclinations between 15° and 90°, while the dotted line shows the peak radius versus luminosity for i = 0° (edge-on).