Nitrogen (N) fractionation is used as a tool to search for a link between the chemical history of the Solar System and star-forming regions. A large variation of 14 N/ 15 N is observed towards different astrophysical sources, and current chemical models cannot reproduce it. With the advent of high angular resolution radiotelescopes it is now possible to search for N-fractionation at core scales. We present IRAM NOEMA observations of the J=1-0 transition of N 2 H + , 15 NNH + and N 15 NH + towards the high-mass protocluster IRAS 05358+3543. We find 14 N/ 15 N ratios that span from ∼100 up to ∼220 and these values are lower or equal than those observed with single-dish observations towards the same source. Since N-fractionation changes across the studied region, this means that it is regulated by local environmental effects. We find also the possibility, for one of the four cores defined in the protocluster, to have a more abundant 15 NNH + with respect to N 15 NH + . This is another indication that current chemical models may be missing chemical reactions or may not take into account other mechanisms, like photodissociation or grain surface chemistry, that could be important.

We show the first interferometric maps of the 14 N/ 15 N ratio obtained with the Atacama Large Millimeter Array (ALMA) towards the Solar-like forming protocluster OMC-2 FIR4. We observed N 2 H + , 15 NNH + , N 15 NH + (1-0), and N 2 D + (2-1), from which we derive the isotopic ratios 14 N/ 15 N 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 synthesised beam of ∼ 1. ′′ 5 × 1. ′′ 8, i.e. ∼ 600 au, show that the emission of the less abundant isotopologues is distributed in several cores of ∼ 10 ′′ (i.e. ∼ 0.02 pc or 4000 au) embedded in a more extended N 2 H + emission. We have derived that the 14 N/ 15 N 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 14 N/ 15 N ratios computed from the two 15 N-bearing isotopologues, 15 NNH + and N 15 NH + . The D/H ratio derived by comparing the column densities of N 2 D + and N 2 H + changes by an order of magnitude from core to core, decreasing from the colder to the warmer cores. Overall, our results indicate that: (1) 14 N/ 15 N does not change across the region at core scales, and (2) 14 N/ 15 N does not depend on temperature variations. Our findings also suggest that the 14 N/ 15 N variations found in pristine Solar System objects are likely not inherited from the protocluster stage, and hence the reason has to be found elsewhere.