2020MNRAS.493.3412F

We show the first interferometric maps of the ¹⁴N/¹⁵N ratio obtained with the Atacama Large Millimeter Array (ALMA) towards the Solar-like forming protocluster OMC-2 FIR4. We observed N₂H⁺, ¹⁵NNH⁺, N¹⁵NH⁺ (1-0), and N₂D⁺(2-1) from which we derive the isotopic ratios ¹⁴N/¹⁵N and D/H. The target, OMC-2 FIR4, is one of the closest analogues of the environment in which our Sun may have formed. The ALMA images, having synthesized beam of ∼1.5 arcsec x 1.8 arcsec, i.e. ∼600 au, show that the emission of the less abundant isotopologues is distributed in several cores of ∼10 arcsec (i.e. ∼0.02 pc or 4000 au) embedded in a more extended N₂H⁺emission. We have derived that the ¹⁴N/¹⁵N ratio does not vary from core to core, and our interferometric measurements are also consistent with single-dish observations. We also do not find significant differences between the ¹⁴N/¹⁵N ratios computed from the two ¹⁵N-bearing isotopologues, ¹⁵NNH⁺ and N¹⁵NH⁺. The D/H ratio derived by comparing the column densities of N₂D⁺and N₂H⁺changes by an order of magnitude from core to core, decreasing from the colder to the warmer cores. Overall, our results indicate that: (1) ¹⁴N/¹⁵N does not change across the region at core scales, and (2) ¹⁴N/¹⁵N does not depend on temperature variations. Our findings also suggest that the ¹⁴N/¹⁵N variations found in pristine Solar system objects are likely not inherited from the protocluster stage, and hence the reason has to be found elsewhere.