Organic residues in astrophysical ice analogues: Thermal processing of hydrogenated glyoxal ices under interstellar conditions

ABSTRACT Organic residues are considered as part of the chemical composition of the interstellar dust grains. They are formed under the extreme conditions of the interstellar medium and play an important role in exobiology. They may contain pre-biotic organic species such as amino acids, constituents of proteins and building blocks of DNA and RNA, key elements of life. By investigating the formation of organic residues in an astrophysical context, many groups have been focusing in the UV irradiation and subsequent warm-up of astrophysical ice analogues. This aims to suggest that organic residues are mainly formed in regions of molecular clouds exposed to UV light or cosmic rays. This study shows an organic residue formation involving glyoxal ice and H atoms. While the hydrogenation of glyoxal at 10 K leads mainly to small molecules such as CO and H2CO and CH3OH, we show that the heating of the hydrogenated ice in the 10–300 K temperature range leads to solid residues whose structure is similar to that of glycolaldehyde but they remain stable in solid phase at 300 K and atmospheric pressure. The analysis of the IR data shows that the organic residues formed through the thermal processing of CHOCHO + H reaction would be a mixture of hydroxypyruvaldehyde and methyl glyoxylate, two solid organics whose formation starts with an H-abstraction from glyoxal to form CHOCO• radical which recombines to •CH2OH and •OCH3 radicals. These latter may be formed and trapped in glyoxal ice as secondary products from H2CO + H secondary reaction.