Synthesis of complex organic molecules in soft x-ray irradiated ices
  We study the chemical evolution of H2O:CO:NH3 ice mixtures irradiated with
soft X-rays, in the range 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 either the infrared identifications and reveal
less abundant species with not 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 2168 cm-1 band of
OCN-, 1690 cm-1 band of formamide, and the 1590 cm-1 band associated to three
different species, HCOO-, CH3NH2 and NH3+CH2COO survive up to room temperature.
Interestingly, many high masses have been also detected. Possible candidates
are methyl-formate, (m/z = 60, HCOOCH3), ethanediamide (m/z = 88, NH2COCONH2),
and N-acetyl-L-aspartic acid (m=z = 175). This latter species is compatible
with the presence of the m/z = 43, 70 and 80 fragments. Photo-desorption 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 disc model around young solar-type stars.
