Effect of the cosmic ray ionization rate on ¹³CO and C¹⁸O J = 3− 2 line fluxes, shown in the top and bottom panel respectively. Models shown here have α_visc = 10⁻³ and M_* = [ 0.2, 0.32 ] M_⊙. Solid light blue lines show the model line fluxes where CO conversion through grain-surface chemistry is calculated with a cosmic ray ionization rate ζ_cr = 1 × 10⁻¹⁷ s⁻¹. For the black dashed lines ζ_cr is increased up to 5 × 10⁻¹⁶ s⁻¹. The dashed blue line shows the upper limit of the effect of grain-surface chemistry, where we have removed all CO in the region of the disk where CO conversion by grain-surface chemistry is effective. Observations in Lupus are shown in orange if detected and in gray if an upper limit (Ansdell et al. 2016; Yen et al. 2018). The stacked non-detections are shown in black (see also Sect. 3.3). The red shaded lane highlights the age range of the Lupus star-formingregion.