No nitrogen fractionation on 600 au scale in the Sun progenitor analogue OMC-2 FIR4
Résumé
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 × 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.
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