Overview of measurements of the nitrogen isotopic ratio in low- to intermediate-mass cores and Class0. The horizontal line indicates the value of the bulk isotopic ratio of nitrogen adopted in this study ( = 330). Black dots indicate indirect measurements based on the double-isotope method. Colored points refer to direct measurements, with N₂H⁺:N¹⁵NH⁺ outlined in blue. The N₂H⁺:¹⁵NNH⁺ ratios are indicated by an asterisk. See Appendix A.1 and Table A.1 for details.