Nitrogen fractionation towards a pre-stellar core traces isotope-selective photodissociation

Abstract

Isotopologue abundance ratios are important to understand the evolution of astrophysical objects and ultimately the origins of a planetary system like our own. Being nitrogen a fundamental ingredient of pre-biotic material, understanding its chemistry and inheritance is of fundamental importance to understand the formation of the building blocks of life. We present here single-dish observations of the ground state rotational transitions of the ${}^{13}$C and ${}^{15}$N isotopologues of HCN, HNC and CN with the IRAM 30m telescope. We analyse their column densities and compute the ${}^{14}$N/${}^{15}$N ratio map for HCN. The ${}^{15}$N-fractionation of CN and HNC is computed towards different offsets across L1544. The ${}^{15}$N-fractionation map of HCN shows a clear decrease of the ${}^{14}$N/${}^{15}$N ratio towards the southern edge of L1544, where carbon chain molecules present a peak, strongly suggesting that isotope-selective photodissociation has a strong effect on the fractionation of nitrogen across pre-stellar cores. The ${}^{14}$N/${}^{15}$N ratio in CN measured towards four positions across the core also shows a decrease towards the South-East of the core, while HNC shows opposite behaviour. The uneven illumination of the pre-stellar core L1544 provides clear evidence that ${}^{15}$N-fractionation of HCN and CN is enhanced toward the region more exposed to the interstellar radiation field. Isotope-selective photodissociation of N${}_2$ is then a crucial process to understand ${}^{15}$N fractionation, as already found in protoplanetary disks. Therefore, the ${}^{15}$N-fractionation in pre-stellar material is expected to change depending on the environment within which pre-stellar cores are embedded. The ${}^{12}$CN/${}^{13}$CN ratio also varies across the core, but its variation does not affect our conclusions on the effect of the environment on the fractionation of nitrogen.

Accepted for publication in A&A Letters

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• Combined model for ${}^{15}\mathrm{N}$, ${}^{13}\mathrm{C}$, and spin-state chemistry in molecular clouds
• Nitrogen fractionation in ammonia and its insights on nitrogen chemistry