2015ApJ...811..151P

We investigate the effects produced mainly by broadband soft X-rays up to 2 keV (plus fast (∼keV) photoelectrons and low-energy (∼eV) induced secondary electrons) in the ice mixtures containing H₂O:CO₂:NH₃:SO₂(10:1:1:1) at two different temperatures (50 and 90 K). The experiments are an attempt to simulate the photochemical processes induced by energetic photons in SO₂-containing ices present in cold environments in the ices surrounding young stellar objects (YSO) and in molecular clouds in the vicinity of star-forming regions, which are largely illuminated by soft X-rays. The measurements were performed using a high-vacuum portable chamber from the Laboratório de Astroquímica e Astrobiologia (LASA/UNIVAP) coupled to the spherical grating monochromator beamline at the Brazilian Synchrotron Light Source (LNLS) in Campinas, Brazil. In situ analyses were performed by a Fourier transform infrared spectrometer. Sample processing revealed the formation of several organic molecules, including nitriles, acids, and other compounds such as H₂O₂, H₃O⁺, SO₃, CO, and OCN^–. The dissociation cross section of parental species was on the order of (2-7)x10^–18 cm². The ice temperature does not seem to affect the stability of SO₂ in the presence of X-rays. Formation cross sections of new species produced were also determined. Molecular half-lives at ices toward YSOs due to the presence of incoming soft X-rays were estimated. The low values obtained employing two different models of the radiation field of YSOs (TW Hydra and typical T-Tauri star) reinforce that soft X-rays are indeed a very efficient source of molecular dissociation in such environments.