Synthesis of formamide and isocyanic acid after ion irradiation of frozen gas mixtures

¹ Astronomical Institute of Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia
e-mail: gianni@oact.inaf.it
² Dipartimento di Scienze Chimiche – Università degli Studi di Catania, Viale A. Doria 6, 95124 Catania, Italy
³ INAF–Osservatorio Astrofisico di Catania, via Santa Sofia 78, 95123 Catania, Italy
⁴ INAF–Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

Received: 7 August 2015
Accepted: 5 November 2015

Abstract

Context. Formamide (NH₂HCO) and isocyanic acid (HNCO) have been observed as gaseous species in several astronomical environments such as cometary comae and pre- and proto-stellar objects. A debate is open on the formation route of those molecules, in particular if they are formed by chemical reactions in the gas phase and/or on grains. In this latter case it is relevant to understand if the formation occurs through surface reactions or is induced by energetic processing.

Aims. We present arguments that support the formation of formamide in the solid phase by cosmic-ion-induced energetic processing of ices present as mantles of interstellar grains and on comets. Formamides, along with other molecules, are expelled in the gas phase when the physical parameters are appropriate to induce the desorption of ices.

Methods. We have performed several laboratory experiments in which ice mixtures (H₂O:CH₄:N₂, H₂O:CH₄:NH₃, and CH₃OH:N₂) were bombarded with energetic (30–200 keV) ions (H⁺ or He⁺). FTIR spectroscopy was performed before, during, and after ion bombardment. In particular, the formation of HNCO and NH₂HCO was measured quantiatively.

Results. Energetic processing of ice can quantitatively reproduce the amount of NH₂HCO observed in cometary comae and in many circumstellar regions. HNCO is also formed, but additional formation mechanisms are requested to quantitatively account for the astronomical observations.

Conclusions. We suggest that energetic processing of ices in the pre- and proto-stellar regions and in comets is the main mechanism to produce formamide, which, once it is released in the gas phase because of desorption of ices, is observed in the gas phase in these astrophysical environments.