Synthesis of new hexamethylenetetramine-based products (HMT-R) in UV-irradiated pre-cometary ice analogues

Context. Laboratory experiments mimicking irradiation of ice mantles under astrophysically relevant conditions, in particular those prevailing in dense interstellar cloud interiors and the cold zones of protoplanetary discs, drive the formation of complex organic species. A fraction of these molecules are refractory at room temperature and constitute what is known as the organic refractory residue. Aims. Because hexamethylenetetramine (HMT, (CH2)6N4) is one of the most abundant residue components, we study here the molecules based on HMT that are also present in these residues. Methods. H2O:CH3OH:NH3 ice mixtures were irradiated with a UV lamp that reproduces the emission spectrum of secondary UV photons in dense clouds. Ice formation and irradiation were conducted under high vacuum conditions at 80 K in these experiments. A solution of the residues was analysed by means of gas chromatography-mass spectroscopy (GC–MS). Results. Some HMT-based family members are reported here for the first time. These species could therefore be produced in icy dust grains exposed to radiation and warmed up to temperatures above 200 K. The side groups R in HMT-R derivatives provide information about the radicals that were present in the irradiated ice; these radicals were also the likely precursors of cometary species inferred from COSAC-Rosetta data. On the other hand, although HMT has been searched for unsuccessfully in comet 67P, it has been identified in meteorites. Moreover, some of the HMT derivatives reported in this study were previously detected in the Murchison meteorite. This work therefore opens the possibility of detecting new HMT-based molecules in pristine objects of the Solar System.