Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.157f&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Astronomy and Astrophysics
Astrophysics
Scientific paper
In the cold parts of star-forming regions, molecules such as H_2O and CO are expected to be completely frozen out onto dust grains. Yet these molecules are found in cold gas in dense clouds and protoplanetary disks; their presence may be explained by UV-induced photodesorption. Recent astrophysically relevant study on ice photodesorption (Westley et al. 1995 ; Öberg et al. 2007,2009) showed that photodesorption is an efficient process when inducing desorption with an H_2 discharge lamp, which UV distribution is peaked at Lyman alpha. The UV fields around solar-type pre-main sequence stars are however dominated by discrete atomic and molecular emission lines (Bergin et al. 2003). Thus it is crucial to investigate ice photodesorption as a function of irradiation wavelength in order to provide accurate photodesorption rates and constrain this molecular mechanism. For the first time, the wavelength-dependent photodesorption of pure CO and H_2O (D_2O) ice is explored in various spectral windows between 80 and 160 nm. The experiments are performed with the ultra-high vacuum setup SPICES (LPMAA-UPMC, France) using tunable synchrotron radiation (SOLEIL, France). Ice photodesorption is simultaneously probed by infrared absorption in reflection mode (RAIRS) of the ice and by quadrupole mass spectrometry of the gas phase composition. The experimental results reveal a strong wavelength dependency. CO ice photodesorption could be monitored continuously between 80 and 160 nm, and the yield is directly linked to the vibronic transition strengths of CO ice in this wavelength region. This implies a direct photodesorption mechanism initiated by electronic transition in CO ice. For both H_2O and CO pure ices, low photodesorption yields were obtained around 121.6 nm (Lyman alpha) by comparison with the high yields peaking at longer wavelengths and corresponding to the first electronic absorption band. This information is important to implement into astrochemical networks in order to accurately predict the gas and ice phase composition in photon-rich regions.
Bertin Mario
Fayolle E.
Fillion Jean-Hugues
Linnartz Harold
Michaut Xavier
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