Photon energy effect on photodesorption study of CO ice | J. Y. Chen ( Department of Physics, National Central University, Jhongli City, Taoyuan County 32054, Taiwan )

In cold and dense interstellar regions, where temperatures can be as low as 10 K, most molecules are expected to condense onto grains, and consequently be depleted in the gas phase. CO is mostly frozen out in these environments, though it is sometimes also observed in the gas phase in a few cold molecular clouds such as IC 5146, L977 and L183. Mechanisms that were proposed to explain why CO is not fully condensed in dense clouds include UV-induced photodesorption, cosmic-ray whole grain heating, cosmic-ray spot heating, and chemical desorption of weakly bound molecules. Shen et al. (2004, A&A, 415, 203) reported a model of CO desorption from cosmic-ray-induced UV photons and showed that such a yield is almost one order of magnitude larger than that directly induced by cosmic-ray particles, while other desorption mechanisms could not explain astronomical observations. A more recent study in which pure CO ice was irradiated with an MDHL at 15-18 K showed a photodesorption yield of 2.7×10^-3 molecules photon-1 (Öberg et al. 2007, ApJ, 662, L23). In contrast, Muñ oz Caro et al. (2010, A & A, 522, A108) reported photodesorption yields for CO ice irradiated with an MDHL at 8 K and 15 K of 5.4×10^-2 and 3.5×10^-2 molecules photon^-1, respectively, which is one order of magnitude larger than the values reported by Öberg et al. (2007). In this talk, we will not only present experimental study consists in the measurement of the vacuum-UV (VUV) emission spectra of a microwave-discharge hydrogen-flow lamp (MDHL), which is commonly used in astrochemistry laboratories working on ice VUV photoprocessing, but also in the measurement of VUV absorption cross section of CO and CO2 ices in order to explain the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources scanning the same energy range, due to the discrepancy of absorption cross section between parent molecules and products in Ly-α and H2 molecular emission ranges.