Scatterplots of the nitrogen isotopic ratio as a function of T_dust (left panel) and CO depletion factor (right panel). Red stars represent protostellar cores from this work, whilst the blue dots represent the prestellar source analysed by Redaelli et al. (2018). T_dust and f_D values for YSOs are taken from Emprechtinger et al. (2009). For prestellar objects, the T_dust values are taken from Chacón-Tanarro et al. (2019) and Redaelli et al. (2018); the values of f_D are taken from Crapsi et al. (2005).