Letter to the Editor

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

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: spezzano@mpe.mpg.de
² University of Bologna, Dipartimento di Chimica “Giacomo Ciamician”, Bologna, Italy

Received: 19 June 2022
Accepted: 12 July 2022

Abstract

Context. Isotopologue abundance ratios are important to understand the evolution of astrophysical objects and ultimately the origins of a planetary system such as our own. With nitrogen being 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.

Aims. We aim to study the ¹⁴N/¹⁵N ratio in HCN, HNC, and CN across the prototypical pre-stellar core L1544. This study allows us to test the proposed fractionation mechanisms for nitrogen.

Methods. We present here single-dish observations of the ground state rotational transitions of the ¹³C and ¹⁵N isotopologues of HCN, HNC, and CN with the IRAM 30 m telescope. We analyse their column densities and compute the ¹⁴N/¹⁵N ratio map across the core for HCN. The ¹⁵N fractionation of CN and HNC is computed towards different offsets across L1544.

Results. The ¹⁵N-fractionation map of HCN towards a pre-stellar core is presented here for the first time. Our map shows a very clear decrease in the ¹⁴N/¹⁵N 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 ¹⁴N/¹⁵N ratio in CN measured towards four positions across the core also shows a decrease towards the south-east of the core, while HNC shows the opposite behaviour. We also measured the ¹²CN/¹³CN ratio towards four positions across the core.

Conclusions. The uneven illumination of the pre-stellar core L1544 provides clear evidence that ¹⁵N fractionation of HCN and CN is enhanced towards the region more exposed to the interstellar radiation field. Isotope-selective photodissociation of N₂ is then a crucial process to understand ¹⁵N fractionation, as already found in protoplanetary disks. Therefore, the ¹⁵N fractionation in pre-stellar material is expected to change depending on the environment within which pre-stellar cores are embedded. The ¹²CN/¹³CN ratio also varies across the core, but its variation does not affect our conclusions as to the effect of the environment on the fractionation of nitrogen. Nevertheless, the interplay between the carbon and nitrogen fractionation across the core warrants follow-up studies.