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

• Published in: arXiv.org 2023-06
• Main Authors: David San Andrés, Colzi, Laura, Rivilla, Víctor M, Juan García de la Concepción, Melosso, Mattia, Martín-Pintado, Jesús, Jiménez-Serra, Izaskun, Zeng, Shaoshan, Martín, Sergio, Requena-Torres, Miguel A
• Format: Article
• Language: English
• Subjects: Abundance; High energy astronomy; Interstellar matter; Isomerization; Molecular clouds; Quasars; Radicals; Surface reactions; Temperature dependence; Vapor phases
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2305.04611

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.