2018ApJ...852..130K

We present a complete census of the ¹⁴N/¹⁵N isotopic ratio in the most abundant N-bearing molecules toward the cold envelope of the protocluster OMC-2 FIR4, the best known Sun progenitor. To this scope, we analyzed the unbiased spectral survey obtained with the IRAM 30 m telescope at 3, 2, and 1 mm. We detected several lines of CN, HCN, HNC, HC₃N, N₂H⁺, and their respective ¹³C and ¹⁵N isotopologues. The lines' relative fluxes are compatible with LTE conditions, and moderate line opacities have been corrected via a population diagram method or theoretical relative intensity ratios of the hyperfine structures. The five species lead to very similar ¹⁴N/¹⁵N isotopic ratios, without any systematic difference between amine- and nitrile-bearing species as previously found in other protostellar sources. The weighted average of the ¹⁴N/¹⁵N isotopic ratio is 270 ± 30. This ¹⁴N/¹⁵N value is remarkably consistent with the [250-350] range measured for the local galactic ratio but significantly differs from the ratio measured in comets (around 140). High-angular resolution observations are needed to examine whether this discrepancy is maintained at smaller scales. In addition, using the CN, HCN, and HC₃N lines, we derived a ¹²C/¹³C isotopic ratio of 50 ± 5.