15N Fractionation in Infrared-Dark Cloud Cores
  Nitrogen is one of the most abundant elements in the Universe and its 14N/15N
isotopic ratio has the potential to provide information about the initial
environment in which our Sun formed. Recent findings suggest that the Solar
System may have formed in a massive cluster since the presence of short-lived
radioisotopes in meteorites can only be explained by the influence of a
supernova. The aim of this project is to determine the 14N/15N ratio towards a
sample of cold, massive dense cores at the initial stages in their evolution.
We have observed the J=1-0 transitions of HCN, H13CN, HC15N, HN13C and H15NC
toward a sample of 22 cores in 4 Infrared-Dark Clouds (IRDCs). IRDCs are
believed to be the precursors of high-mass stars and star clusters. Assuming
LTE and a temperature of 15K, the column densities of HCN, H13CN, HC15N, HN13C
and H15NC are calculated and their 14N/15N ratio is determined for each core.
The 14N/15N ratio measured in our sample of IRDC cores range between ~70 and
>763 in HCN and between ~161 and ~541 in HNC. They are consistent with the
terrestrial atmosphere (TA) and protosolar nebula (PSN) values, and with the
ratios measured in low-mass pre-stellar cores. However, the 14N/15N ratios
measured in cores C1, C3, F1, F2 and G2 do not agree with the results from
similar studies toward the same massive cores using nitrogen bearing molecules
with nitrile functional group (-CN) and nitrogen hydrides (-NH) although the
ratio spread covers a similar range. Amongst the 4 IRDCs we measured relatively
low 14N/15N ratios towards IRDC G which are comparable to those measured in
small cosmomaterials and protoplanetary disks. The low average gas density of
this cloud suggests that the gas density, rather than the gas temperature, may
be the dominant parameter influencing the initial nitrogen isotopic composition
in young PSN.
