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

Abstract

The ${\rm H_2NC}$ radical is the high-energy metastable isomer of ${\rm H_2CN}$ 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 ${\rm H_2CN}$/${\rm H_2NC}$ isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature ($T_{\rm kin}$), but the shortage of them in warm sources still prevents us to confirm this hypothesis and shed light about their chemistry. In this work, we present the first detection of ${\rm H_2CN}$ and ${\rm H_2NC}$ towards a warm galactic source, the G+0.693-0.027 molecular cloud (with $T_{\rm kin} > 70 \, {\rm K}$), using IRAM 30m observations. We have detected multiple hyperfine components of the $N_{K_\text{a}K_\text{c}} = 1_{01} - 0_{00}$ and $2_{02} - 1_{01}$ transitions. We derived molecular abundances with respect to ${\rm H_2}$ of (6.8$\pm$1.3)$\times 10^{-11}$ for ${\rm H_2CN}$ and of (3.1$\pm$0.7)$\times 10^{-11}$ for ${\rm H_2NC}$, and a ${\rm H_2CN}$/${\rm H_2NC}$ abundance ratio of 2.2$\pm$0.5. These detections confirm that the ${\rm H_2CN}$/${\rm H_2NC}$ ratio is $\gtrsim$2 for sources with $T_{\rm kin} > 70 \, {\rm K}$, larger than the $\sim$1 ratios previously found in colder cores ($T_{\rm kin}\sim10 \, {\rm K}$). This isomeric ratio dependence with temperature cannot be fully explained with the currently proposed gas-phase formation and destruction pathways. Grain surface reactions, including the ${\rm H_2NC} \rightarrow {\rm H_2CN}$ isomerization, deserve consideration to explain the higher isomeric ratios and ${\rm H_2CN}$ abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms.