Estimates of R_D = N(DCO⁺)/N(HCO⁺) and T_K across different studies: estimates in Orion, both single fields and median per bin (green and red circles, respectively; this study), in IRDCs (black diamonds; Miettinen et al. 2011), in high-mass star-forming regions (colour-coded squares; Gerner et al. 2014, 2015, see text for discussion on T_K). The black line represents the best fit to the median values of our estimates in Orion and it is exemplified by Eq. B.1.