2018MNRAS.480.1174H

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 disc, 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 paper, we report the detection of five rotational lines of HC_3N, including the weaker components of the hyperfine multiplets, and two rotational lines of its ¹⁵N isotopologue, towards the L1544 prestellar core. Based on a MCMC/non-LTE multiline analysis at the hyperfine level, we derive the column densities of HC_3N (8.0±0.4×10¹³ cm^–2) and HC_3¹⁵N (2.0±0.4×10¹¹ cm^–2) and derive an isotopic ratio of 400 ± 20(1σ). This value suggests that HC_3N is slightly depleted in ¹⁵N in L1544 with respect to the elemental ¹⁴N/¹⁵N ratio of ≃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 favour CN+C₂H₂ as the dominant formation route to HC₃N. However, uncertainties in the isotopic ratios preclude definitive conclusions.