Fractionation in young cores: Direct determinations of nitrogen and carbon fractionation in HCN

Abstract

Nitrogen fractionation is a powerful tracer of the chemical evolution during star and planet formation. It requires robust determinations of the nitrogen fractionation across different evolutionary stages. We aim to determine the $ $N/$ $N and $ $C/$ $C ratios for HCN in six starless and prestellar cores and to compare the results between the direct method using radiative transfer modeling and the indirect double isotope method, assuming a fixed $ $C/$ $C ratio. We present IRAM observations of the HCN $1$-$0$, HCN 3-2, HC15N 1-0 and H13CN $1$-$0$ transitions toward six embedded cores. The $ $N/$ $N ratio was derived using both the indirect double isotope method and directly through non-local thermodynamic equilibrium (NLTE) 1D radiative transfer modeling of the HCN emission. The latter also provides the $ $C/$ $C ratio, which we compared to the local interstellar value. The derived $ $N/$ $N ratios using the indirect method are generally in the range of 300-550. This result could suggest an evolutionary trend in the nitrogen fractionation of HCN between starless cores and later stages of the star formation process. However, the direct method reveals lower fractionation ratios of around sim 250, mainly resulting from a lower $ $C/$ $C ratio in the range sim 20--40, as compared to the local interstellar medium value of 68. This study reveals a significant difference between the nitrogen fractionation ratio in HCN derived using direct and indirect methods. This can influence the interpretation of the chemical evolution and reveal the pitfalls of the indirect double isotope method for fractionation studies. However, the direct method is challenging, as it requires well-constrained source models to produce accurate results. No trend in the nitrogen fractionation of HCN between earlier and later stages of the star formation process is evident when the results of the direct method are considered.