Gas phase Elemental abundances in Molecular cloudS (GEMS)

XI. The evolution of HCN, HNC, and N₂H⁺ isotopic ratios in starless cores

¹ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
² Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain
³ Exoplanets and Planetary Formation Group, School of Earth and Planetary Sciences, National Institute of Science Education and Research, Jatni 752050, Odisha, India
⁴ Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India

^★ Corresponding authors: atasa@cab.inta-csic.es; afuente@cab.inta-csic.es

Received: 13 February 2025
Accepted: 3 July 2025

Abstract

Context. 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.

Aims. Our aim is to determine the HCN, HNC, and N₂H⁺ isotopic ratios and the chemical age in a large sample of starless cores located in different environments.

Methods. This work uses IRAM 30 m data to constrain the D/H isotopic ratios of HCN, HNC, and N₂H⁺ as well as the ¹⁴N/¹⁵N ratio of HCN and HNC. We also modeled the deuterium fractions with the chemical code DNAUTILUS 2.0.

Results. Deuterated compounds are detected in all of our sample cores, with average DNC/HNC, DCN/HCN, and N₂D⁺/N₂H⁺ 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∕HC¹⁵N (430±120) and HNC∕H¹⁵NC (296±64). We find no correlation between these ratios and the deuterium fractions, but we report a weak correlation with temperature was found.

Conclusions. The deuterium fractions of HCN, HNC, and N₂H⁺ can be used as evolutionary tracers of starless cores as long as the physical parameters are well constrained. The HCN/HC¹⁵N and HNC/H¹⁵NC 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/HC¹⁵ N 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.