Photodesorption of Interstellar Ices: a Wavelength-dependent Approach to Unveil Molecular Mechanisms
Abstract
In the cold and dense regions of the interstellar medium, non-thermal desorption of interstellar ices drives the ice-to-gas ratio of most molecules. More specifically, non-thermal desorption induced by UV photons has been proposed as the dominating mechanism responsible for the observation of cold molecular gas in various star-forming environments. In protoplanetary disks mid-planes, for example, UV photons from the pre-main sequence star can penetrate deep into the disk, reach the ice region and induce ice sublimation. In prestellar cores and in the outer parts of protostellar envelopes, it is the Lyman-alpha-dominated UV field generated locally by the interaction of cosmic rays with H2 that can potentially interact with ice mantles and result in observable cold molecular gas.To constrain the photodesorption mechanism of interstellar ices and predict its efficiency for various UV fields, we have developed a novel wavelength-dependent approach using the vacuum UV beamline DESIRS at the French synchrotron facility SOLEIL. Monochromatic tunable UV light in the 7 - 14 eV window is used to irradiate interstellar ice analogues and the rates at which molecules photodesorb are simultaneously measured using mass-spectrometry. The frequency resolved photodesorption spectra of pure CO and N2 ices show a clear UV-wavelength dependency, directly scaled to the absorption spectra of the condensed molecules, which hints for a Desorption Induced by Electronic Transition (DIET) process.The application of this technique to isotopically labeled layered ices and binary ice mixtures has further revealed that CO and N2 ice photodesorption is an indirect process where it is the electronic excitation of sub-surface species that leads to the desorption of surface molecules. The photodesorption efficiency of a species is thus linked to its molecular environment and photodesorption rates are different for pure and mixed ices. This has strong implications for astrochemical modeling and could potentially explain the CO and N2 (N2H+) abundance profiles in prestellar cores.
- Publication:
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American Astronomical Society Meeting Abstracts #224
- Pub Date:
- June 2014
- Bibcode:
- 2014AAS...22420504F