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

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.

Combined model for 15N, 13C, and spin-state chemistry in molecular clouds

“…We find that all three 14 N/ 15 N ratios increase when the external illumination grows in strength. HNC does present a slightly different response compared to CN and HCN in that its 14 N/ 15 N ratio increases less when G 0 is increased, but the overall trend is obviously different compared to what Spezzano et al (2022) observed in L1544. Also, even with the increased amount of external illumination, and hence a larger emphasis on the difference between self-shielding of N 2 and N 15 N, the simulation results still show only 1% level differences to the fiducial S3 simulation.…”

“…Table 3 collects the parameters of the simulations, including details on the chemical network used in each simulation, and on the adopted physical parameters. In particular, in one simulation we test the effect of isotope-selective photodissociation for N 2 , considering that this has been invoked as an explanation for observed N fractionation gradients across the prestellar core L1544 (Spezzano et al 2022). The simulations S1, S2, and S3 represent, respectively, the center, inner region, and the outer envelope of a prototypical prestellar core.…”

“…Recently, Spezzano et al (2022) presented observations of the 14 N/ 15 N ratios of CN, HCN, and HNC toward the prestellar core L1544 and found 14 N/ 15 N gradients across the core. For example for HNC, they find that the 14 N/ 15 N ratio increases toward the southeast where the core is more exposed to the interstellar radiation field (ISRF; Spezzano et al 2016Spezzano et al , 2022. However, CN and HCN present a reverse trend in their data set.…”

“…In this paper, we present the first model that includes this effect; our model connects carbon and nitrogen isotopic fractionation and also includes a description of spin-state chemistry for light hydrogen-containing species. We employed the new model to study (combined) carbon and nitrogen fractionation in physical conditions representing a region in and around a prestellar core, searching especially for signs of gradients in the spatial distributions of isotopic abundance ratios that have been recently inferred observationally toward the prestellar core L1544 (Spezzano et al 2022). We also included isotope-selective photodissociation of N 2 and studied its effect on the abundances of nitrogen-bearing species, and we paid special attention to deviations of the simulated isotopic abundance ratios from the corresponding elemental ratios.…”

Dust traps in protoplanetary disks efficiently form organic macromolecular matter

“…3.). In dense interstellar clouds, dust grains rst accumulate refractory carbon and PAHs before acquiring ice mantles of simple species enriched in minor isotopes 36,37 . Noble gases are e ciently trapped in these mantles 38 and moderate chemical processing result in minor production of SOM 39 .…”