The nitrogen isotopic ratio of HC3N towards the L1544 prestellar core

TL;DR

This study measures the nitrogen isotopic ratio in HC3N within the L1544 prestellar core, providing insights into nitrogen fractionation processes relevant to the origins of cometary material and the solar system.

Contribution

First detection of $^{15}$N isotopologue lines of HC3N in a prestellar core, enabling the first direct isotopic ratio measurement in such an environment.

Findings

HC3N has a $^{14}$N/$^{15}$N ratio of 400±20 in L1544.

HC3N is slightly depleted in $^{15}$N compared to the local interstellar medium.

Results suggest CN+C2H2 as the likely formation pathway for HC3N.

Abstract

The origin of the heavily fractionated reservoir of nitrogen in comets remains an issue in the theory of their formation and hence of the solar system. Whether the fractionated reservoir traced by comets is inherited from the interstellar cloud or is the product of processes taking place in the protostar, or in the protoplanetary disk, remains unclear. So far, observations of nitrogen isotopic ratios in protostars or prestellar cores have not securely identified such a fractionated reservoir owing to the intrinsic difficulty of direct isotopic ratios measurements. In this article, we report the detection of 5 rotational lines of \ce{HC3N}, {including the weaker components of the hyperfine multiplets}, and two rotational lines of its $^{15}$N isotopologue, towards the L1544 prestellar core. Based on a MCMC/non-LTE multi-line analysis at the hyperfine level, we derive the column densities of \ce{HC3N} ($8.0\pm0.4\tdix{13}$\cc) and \ce{HC3^{15}N} ($2.0\pm0.4\tdix{11}$\cc) and derive an isotopic ratio of 400$\pm$20(1$\sigma$). This value suggests that \ce{HC3N} is slightly depleted in $^{15}$N in L1544 with respect to the elemental $^{14}$N/$^{15}$N ratio {of $\approx$330} in the present-day local interstellar medium. Our study also stresses the need for radiative calculations at the hyperfine level. Finally, the comparison of the derived ratio with those obtained in CN and HCN in the same core seems to favor CN+C$_2$H$_2$ as the dominant formation route to HC$_3$N. However, uncertainties in the isotopic ratios preclude definitive conclusions.