Gas phase Elemental abundances in Molecular cloudS (GEMS). XI. The evolution of HCN, HNC, and N_2H^+ isotopic ratios in starless cores

Isotopic ratios have been used as chemical diagnostics to investigate the origin of the material in the Solar System. These isotopic ratios depend on the physical conditions at formation but can be altered during the star formation process through different physical and chemical processes. Our aim is to determine the HCN, HNC, and N_2H^+ isotopic ratios and the chemical age in a large sample of starless cores located in different environments. This work uses IRAM 30m data to constrain the D/H isotopic ratios of HCN, HNC, and N_2H^+ as well as the ^14N/^15N ratio of HCN and HNC. We also modeled the deuterium fractions with the chemical code DNAUTILUS 2.0 Deuterated compounds are detected in all of our sample cores, with average DNC/HNC, DCN/HCN, and N_2D^+/N_2H^+ values of 0.054±0.019, 0.036±0.033, and 0.15±0.11, respectively. The deuterium fractions (D_frac) show a weak correlation with temperature and a large scatter that reflects that other factors such as core evolution could also play a significant role. Our chemical model is able to reproduce all the observed values with 0.2-0.3 Myr in Taurus and 0.3-0.5 Myr in Perseus and Orion. The ^14N/^15N isotopic ratio is found to be different between HCN/HC^15N (430±120) and HNC/H^15NC (296±64). We find no correlation between these ratios and the deuterium fractions, but we report a weak correlation with temperature was found. The deuterium fractions of HCN, HNC, and N_2H^+ can be used as evolutionary tracers of starless cores as long as the physical parameters are well constrained. The HCN/HC^15N and HNC/H^15NC ratios are not correlated with D_frac, suggesting that the detected variations are not correlated with the core evolutionary stage. The average value of the HCN/HC^15N ratio in our sample is significantly higher than the values measured in protostars and protoplanetary disks, possibly indicating that nitrogen fractionation processes are taking place during the protostellar phase.