A tracer of organic matter of prebiotic interest in space, made from UV and thermal processing of ice mantles

Aims. Our main goal was to identify an infrared spectral feature that can serve as a tracer of O and N-rich solid organic matter in space. Such material is expected to result from UV-irradiation and sublimation of icy grain mantles in certain environments, including hot cores and regions around YSOs. Our analysis of the 3.4 mu m feature, present in the spectra of organic refractory samples made from UV-irradiation of interstellar/circumstellar ice analogs, indicates that the 3.4 mu m band is a potential tracer of O and N-rich solid organic matter of prebiotic interest in space. Methods. We carried out simulation experiments of UV-photoprocessing and sublimation of ice mantles in dense clouds and circumstellar regions leading to the formation of organic refractory residues under different conditions. The analysis of the deposited ice and the organic residue products was made in situ by infrared spectroscopy. Spectral comparison of these residues to molecular standards for compositional characterization was performed. Results. For ice mixtures of different composition, UV-photon dose and frequency, the 3.4 mu m feature of the organic residue product at room temperature shows a broad double-peaked profile with subfeatures at similar to 2926 cm(-1) (3.42 mu m) and 2876 cm(-1) (3.48 mu m), mainly due to CH2 groups adjacent to OH groups. This feature profile was not reproduced in similar UV-irradiation experiments using non-realistic analogs of interstellar ice mantles. In the astrophysical context, this infrared feature is thus expected to be characteristic of the products resulting from ice UV-irradiation and sublimation; it is fully distinct from the 3.4 mu m feature observed in the diffuse interstellar medium, which is attributed to hydrogenated amorphous carbon. Conclusions. A 3.4 mu m band with a similar broad double-peaked profile tracing organic refractory matter of prebiotic interest could be searched for in regions where icy grain mantles were exposed to UV-irradiation prior to sublimation, like hot cores and regions around YSOs. This band may also be present in some cometary nuclei that preserve such grains, provided that the annealing temperatures experienced were not higher than similar to 400 K.