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 \pm 0.4 { \times 10 ^ { 13 } } { cm ^ { -2 } } ) and \ce HC3^15N ( 2.0 \pm 0.4 { \times 10 ^ { 11 } } { cm ^ { -2 } } ) 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 .