Hydroxylamine in astrophysical ices: Infrared spectra and cosmic-ray-induced radiolytic chemistry

Context. Gas-phase hydroxylamine (NH2OH) has recently been detected within dense clouds in the interstellar medium. However, it is also likely present within interstellar ices, as well as on the icy surfaces of outer Solar System bodies, where it may react to form more complex prebiotic molecules such as amino acids. Aims. In this work, we aim to provide infrared spectra of NH2OH in astrophysical ice analogues that will help in the search for this molecule in various astrophysical environments. Furthermore, we aim to provide quantitative information on the stability of NH2OH upon exposure to ionising radiation analogous to cosmic rays, as well as on the ensuing chemistry and potential formation of complex prebiotic molecules. Methods. Ices composed of NH2 OH, H2O, and CO were prepared by vapour deposition, and infrared spectra were acquired between 4000–500 cm−1 (2.5–20 µm) prior to and during irradiation using 15 keV protons. Results. Our spectroscopic characterisations determine that NH2OH ices deposited at 10–20 K adopt an amorphous structure, which begins to crystallise upon warming to temperatures greater than 150 K. In interstellar ice analogues, the most prominent infrared absorption band of NH2OH is that at about 1188 cm−1, which may be a good candidate to use in searches for this species in icy space environments. Calculated effective destruction cross-sections and G-values for the NH2 OH-rich ices studied show that NH2OH is rapidly destroyed upon exposure to ionising radiation (more rapidly than a number of previously studied organic molecules) and that this destruction is slightly enhanced when it is mixed with other icy species. The irradiation of a NH2OH:H2O:CO ternary ice mixture leads to a rich chemistry that includes the formation of simple inorganic molecules such as NH3, CO2, OCN−, and H2O2, as well as ammonium salts and, possibly, complex organic molecules relevant to life such as formamide, formic acid, urea, and glycine.