Synthesis of Complex Organic Molecules in Soft X-Ray Irradiated Ices

Ciaravella, A., Jiménez Escobar, A., Cecchi Pestellini, C., Huang, C. H., Sie, N. E., Caro, G. M. M., Chen, Y. J. 2019. Synthesis of Complex Organic Molecules in Soft X-Ray Irradiated Ices. Astrophysical Journal 879, 1, DOI: 10.3847/1538-4357/ab211c

We study the chemical evolution of H2O:CO:NH3 ice mixtures irradiated with soft X-rays, in the range of 250-1250 eV. We identify many nitrogen-bearing molecules such as, e.g., OCN-, NH4+, HNCO, CH3CN, HCONH2, and NH2COCONH2. Several infrared features are compatible with glycine or its isomers. During the irradiation, we detected through mass spectroscopy many species desorbing the ice. Such findings support the infrared identifications and reveal less abundant species without clear infrared features. Among them, m/z = 57 has been ascribed to methyl isocyanate (CH3NCO), a molecule of prebiotic relevance, recently detected in protostellar environments. During the warm-up after the irradiation, several infrared features including the 2168 cm(-1) band of OCN-, the 1690 cm(-1) band of formamide, and the 1590 cm(-1) band associated with three different species, HCOO-, CH3NH2, and NH3+CH2COO survive up to room temperature. Interestingly, many high masses have also been detected. Possible candidates are methylformate, (m/z = 60, HCOOCH3), ethanediamide (m/z = 88, NH2COCONH2), and N-acetyl-L-aspartic acid (m/z =175). The latter species is compatible with the presence of the m/z = 43, 70, and 80 fragments. Photodesorption of organics is relevant for the detection of such species in the gas phase of cold environments, where organic synthesis in ice mantles should dominate. We estimate the gas-phase enrichment of some selected species in the light of a protoplanetary disk model around young solar-type stars.

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