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تسجيل مستخدم جديدX-ray photolysis of CH$_3$COCH$_3$ ice: Implications for the radiation effects of compact objects towards astrophysical ices
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In this study, we employed broadband X-rays ($6-2000$ eV) to irradiate the frozen acetone CH$_3$COCH$_3$, at the temperature of 12 K, with different photon fluences up to $2.7times 10^{18}$ photons cm$^{-2}$. Here, we consider acetone as a representative complex organic molecule (COM) present on interstellar ice grains. The experiments were conduced at the Brazilian synchrotron facility (LNLS/CNPEN) employing infrared spectroscopy (FTIR) to monitor chemical changes induced by radiation in the ice sample. We determined the effective destruction cross-section of the acetone molecule and the effective formation cross-section for daughter species. Chemical equilibrium, obtained for fluence $2times 10^{18}$ photons cm$^{-2}$, and molecular abundances at this stage were determined, which also includes the estimates for the abundance of unknown molecules, produced but not detected, in the ice. Timescales for ices, at hypothetical snow line distances, to reach chemical equilibrium around several compact and main-sequence X-ray sources are given. We estimate timescales of 18 days, 3.6 and 1.8 months, $1.4times 10^9-6times 10^{11}$ years, 600 and $1.2times 10^7$ years, and $10^7$ years, for the Sun at 5 AU, for O/B stars at 5 AU, for white dwarfs at 1 LY, for the Crab pulsar at 2.25 LY, for Vela pulsar at 2.25 LY, and for Sagittarius A* at 3 LY, respectively. This study improves our current understanding about radiation effects on the chemistry of frozen material, in particular, focusing for the first time, the effects of X-rays produced by compact objects in their eventual surrounding ices.
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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 K 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 Laboratorio de Astroquimica e Astrobiologia (LASA/UNIVAP) coupled to the spherical grating monochromator (SGM) beamline at the Brazilian Synchrotron Light Source (LNLS) in Campinas, Brazil. In-situ analyses were performed by a Fourier transform infrared (FTIR) 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 in the order of 2-7E-18 cm2. The ice temperature seems not to affect the stability for SO2 in the presence of X-rays. Formation cross sections of produced new species were also determined. Molecular half-lives at ices towards YSOs due to the presence of incoming soft X-rays were estimated. The low obtained values, employing two different models of 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.
The physical evolution of Young Stellar Objects (YSOs) is accompanied by an enrichment of the molecular complexity, mainly triggered by the heating and energetic processing of the astrophysical ices. In this paper, a study of how the ice column densi
ty varies across the protostellar evolution has been performed. Tabulated data of H$_2$O, CO$_2$, CH$_3$OH, HCOOH observed by ground- and space-based telescopes toward 27 early-stage YSOs were taken from the literature. The observational data shows that ice column density and spectral index ($alpha$), used to classify the evolutionary stage, are well correlated. A 2D continuum radiative transfer simulation containing bare and grains covered by ices at different levels of cosmic-ray processing were used to calculate the Spectral Energy Distributions (SEDs) in different angle inclinations between face-on and edge-on configuration. The H$_2$O:CO$_2$ ice mixture was used to address the H$_2$O and CO$_2$ column density variation whereas the CH$_3$OH and HCOOH are a byproduct of the virgin ice after the energetic processing. The simulated spectra were used to calculate the ice column densities of YSOs in an evolutionary sequence. As a result, the models show that the ice column density variation of HCOOH with $alpha$ can be justified by the envelope dissipation and ice energetic processing. On the other hand, the ice column densities are mostly overestimated in the cases of H$_2$O, CO$_2$ and CH$_3$OH, even though the physical and cosmic-ray processing effects are taken into account.
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