Synthesis of the first nitrogen-heterocycles in interstellar ice analogs containing methylamine (CH$_3$NH$_2$) exposed to UV radiation: Formation of trimethylentriamine (TMT, c-(-CH$_2$-NH)$_3$) and hexamethylentetramine (HMT, (CH$_2$)$_6$N$_4$)

Abstract: Hexamethylentetramine has drawn a lot of attention due to its potential to produce prebiotic species. This work aims to gain a better understanding in the chemical processes concerning methylamine under astrophysically relevant conditions. In particular, this work deeps into the formation of N-heterocycles in interstellar ice analogs exposed to UV radiation, which may lead to the formation of prebiotic species. Experimental simulations of interstellar ice analogs were carried out in ISAC. ISAC is an ultra-high vacuum chamber equipped with a cryostat, where gas and vapour species are frozen forming ice samples. Infrared and ultraviolet spectroscopy were used to monitor the solid phase, and quadrupole mass spectrometry served to measure the composition of the gas phase. The variety of species detected after UV irradiation of ices containing methylamine revealed the presence of 12 species which have been already detected in the ISM, being 4 of them typically classified as complex organic molecules: formamide (HCONH2), methyl cyanide (CH$_3$CN), CH$_3$NH and CH$_3$CHNH. Warming up of the irradiated CH$_3$NH$_2$-bearing ice samples lead to the formation of trimethylentriamine (TMT), a N-heterocycle precursor of HMT, and the subsequent synthesis of HMT at temperatures above 230 K.