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