H2CN/H2NC abundance ratio: a new potential temperature tracer for the interstellar medium.

The H₂NC radical is the high-energy metastable isomer of H₂CN radical, which has been recently detected for the first time in the interstellar medium towards a handful of cold galactic sources, besides a warm galaxy in front of the PKS 1830−211 quasar. These detections have shown that the H₂CN/H₂NC isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature (T _kin), but the shortage of them in warm sources still prevents us from confirming this hypothesis and shedding light on their chemistry. In this work, we present the first detection of H₂CN and H₂NC towards a warm galactic source, the G+0.693−0.027 molecular cloud (with T _kin > 70 K), using IRAM 30-m telescope observations. We have detected multiple hyperfine components of the |$N_{K_\text{a}K_\text{c}} =$| 1₀₁–0₀₀ and 2₀₂–1₀₁ transitions. We derived molecular abundances with respect to H₂ of (6.8 ± 1.3) × 10⁻¹¹ for H₂CN and of (3.1 ± 0.7) × 10⁻¹¹ for H₂NC, and an H₂CN/H₂NC abundance ratio of 2.2 ± 0.5. These detections confirm that the H₂CN/H₂NC ratio is ≳2 for sources with T _kin > 70 K, larger than the ∼1 ratios previously found in colder cores (T _kin ∼ 10 K). This isomeric ratio dependence on temperature cannot be fully explained with the currently proposed gas-phase formation and destruction pathways. Grain surface reactions, including the H₂NC → H₂CN isomerization, deserve consideration to explain the higher isomeric ratios and H₂CN abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms. [ABSTRACT FROM AUTHOR]