(a) Left: example of XR Case 1 fitting of P2 abundances (HCO⁺ from HC¹⁸O⁺) using astrochemical model Grid 2 (i.e. CO ice binding energy of 1100 K and T_crid = 100 K) showing the projected best χ² values in the ζ versus t plane. The location of minimum χ² is marked with a white cross. (b) Right: as (a), but now for [CO] reduced by a factor of two (allowing for potential CO envelope contamination).