  Context. Near- and mid-infrared observations have revealed the presence of
organic refractory materials in the solar system, in cometary nuclei and on the
surface of centaurs, Kuiper-belt and trans-neptunian objects. In these
astrophysical environments, organic materials can be formed because of the
interaction of frozen volatile compounds with cosmic rays, stellar/solar
particles, and favoured by thermal processing. The analysis of laboratory
analogues of such materials gives information on their properties,
complementary to observations. Aims. We present new experiments to contribute
in the understanding of the chemical composition of organic refractory
materials in space. Methods. We bombard frozen water, methanol and ammonia
mixtures with 40 keV H$^+$ and we warm the by-products up to 300~K. The
experiments allow the production of organic residues that we analyse by means
of infrared spectroscopy and by Very High Resolution Mass Spectrometry to study
their chemical composition and their high molecular diversity, including the
presence of hexamethylenetetramine and its derivatives. Results. We find that
the accumulated irradiation dose plays a role in determining the residue's
composition. Conslusions. Based on the laboratory doses, we estimate the
astrophysical timescales to be short enough to induce an efficient formation of
organic refractory materials at the surface of icy bodies in the outer solar
system.
