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

Abstract

The ${\mathrm{H}}_2\mathrm{NC}$ radical is the high-energy metastable isomer of ${\mathrm{H}}_2\mathrm{CN}$ 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 ${\mathrm{H}}_2\mathrm{CN}$/${\mathrm{H}}_2\mathrm{NC}$ isomeric ratio, likewise the HCN/HNC ratio, might increase with the kinetic temperature ($T_{\mathrm{kin}}$), 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 ${\mathrm{H}}_2\mathrm{CN}$ and ${\mathrm{H}}_2\mathrm{NC}$ towards a warm galactic source, the G+0.693-0.027 molecular cloud (with $T_{\mathrm{kin}}>70\mathrm{K}$), 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 ${\mathrm{H}}_2$ of (6.8$\pm$1.3)$\times 10^{-11}$ for ${\mathrm{H}}_2\mathrm{CN}$ and of (3.1$\pm$0.7)$\times 10^{-11}$ for ${\mathrm{H}}_2\mathrm{NC}$, and a ${\mathrm{H}}_2\mathrm{CN}$/${\mathrm{H}}_2\mathrm{NC}$ abundance ratio of 2.2$\pm$0.5. These detections confirm that the ${\mathrm{H}}_2\mathrm{CN}$/${\mathrm{H}}_2\mathrm{NC}$ ratio is $\gtrsim$2 for sources with $T_{\mathrm{kin}}>70\mathrm{K}$, larger than the $\sim$1 ratios previously found in colder cores ($T_{\mathrm{kin}}\sim 10\mathrm{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 ${\mathrm{H}}_2\mathrm{NC}\to {\mathrm{H}}_2\mathrm{CN}$ isomerization, deserve consideration to explain the higher isomeric ratios and ${\mathrm{H}}_2\mathrm{CN}$ abundances observed in warm sources, where the molecules can be desorbed into the gas phase through thermal and/or shock-induced mechanisms.

12 pages, 5 figures, 3 tables, 2 appendix - Accepted for publication in Monthly Notices of the Royal Astronomical Society