Seeds of Life in Space (SOLIS). XIII. Nitrogen fractionation towards the protocluster OMC-2 FIR4

TL;DR

This study investigates nitrogen isotopic ratios in the protocluster OMC-2 FIR4 to understand chemical enrichment processes relevant to our Solar System's origins, using high-resolution interferometric observations of HCN and N2H+ isotopologues.

Contribution

First high-resolution comparison of nitrogen fractionation in HCN and N2H+ in a Solar System precursor environment, revealing minimal regional variation and insensitivity to cosmic-ray ionization.

Findings

Nitrogen isotopic ratios in HCN range from ~250 to 500.

N2H+ ratios vary from ~200 to 400.

Ratios are largely unaffected by local physical conditions or cosmic-ray irradiation.

Abstract

Isotopic fractionation is an important tool to investigate the chemical history of our Solar System (SS). In particular, the isotopic fraction of nitrogen (14N/15N) is lower in comets and other pristine SS bodies with respect to the value measured for the protosolar nebula, suggesting a local chemical enrichment of 15N during the SS formation. Therefore, interferometric studies of nitrogen fractionation in SS precursors are imperative for us to obtain clues about our astrochemical origins. In this work, we investigated the variation of the 14N/15N ratio in one of the closest analogues of the environment in which the SS was born: the protocluster OMC-2 FIR4. We present the first comparison at high angular resolution between HCN and N2H+ using interferometric data. We analysed observations of the HCN isotopologues H13CN and HC15N in the OMC-2 FIR4 protocluster, specifically the transitions H13CN (1-0) and HC15N (1-0), from NOEMA within the context of the IRAM Seeds Of Life In Space Large Program. We combined our results with analysis of archival data obtained with ALMA of N2H+ and its 15N isotopologues. Our results show a small regional variation in the ratio for HCN from ~250 to 500. The ratios in the central regions of FIR4, where the candidate protostars are located, are largely consistent (~300). They also show little variation from the part of the protocluster known to harbour a high cosmic-ray ionisation rate, to the portion with lower rate. We also found a small variation in the ratio of N2H+ across different regions from ~200 to ~400. These results suggest that local changes in the physical parameters occurring on small linear scales probed by our observations do not seem to affect the 14N/15N ratio in either HCN or N2H+ and hence that this is independent of the molecule used. Moreover, the high level of irradiation due to cosmic rays does not affect the N-fractionation either.