A study of the 13C and 15N fractionation in low-mass starless cores

The nitrogen isotopic ratio 14N/15N can provide crucial insights into the chemical evolution during star and planet formation (e.g., Ceccarelli+2014). The 14N/15N ratio is often determined assuming a constant 12C/13C ratio (double isotope method). However, this assumption has been challenged by recent astrochemical models of carbon fractionation which show a significant variation in carbon fractionation in starless cores at different chemical ages and physical conditions (Colzi+2020, Sipilä+2023). I will present IRAM 30m observations of HCN, H13CN, and HC15N toward 6 low-mass starless cores (Jensen+2024). The 12C/13C and 14N/15N ratios for HCN are derived using the double isotope method and directly through 1D line-radiative transfer modeling of the emission lines. The results imply that a fixed 12C/13C ratio is a poor assumption for starless cores and reveal a substantial difference between the nitrogen fractionation ratio in HCN derived using direct and indirect methods. This can strongly affect the interpretation of the chemical processing in fractionation studies and questions the reliability of the double isotope method.