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Porosity measurement of genuine interstellar ices using mid-infrared optical constants and extended effective medium approximations


Remote observations of star-forming regions performed by space telescopes have provided the full inventory of interstellar ice components and the abundances of these constituents are deduced from accurate optical constants and reference spectra in the mid-infrared. Given the diversity of astronomical ice environments, as well as the low pressure, different temperatures and different gas-phase adsorption rates, one can expect a wide ice porosity range in space. Quantifying the porosity of interstellar ices from observational data remain challenging, but also key to understand the molecular complexity seen towards low mass and high mass protostars; Pores are indeed responsible of the efficiency of the adsorption, the diffusion, the reaction, and the entrapment capacities of astrochemical relevant molecules. Therefore, it is now important to address the fundamental question of porosity in space by providing for the first time quantitative information on the amount of pores present in interstellar ices. This proposal addresses this point using extended effective medium approximations (EMAs) combined with laboratory measurements of interstellar ice analogues. In this work we will be able to provide an exhaustive database of effective mid-infrared optical constants of inhomogeneous porous ice materials composed of H2O, CO, CO2, NH3, CH4 and CH3OH, i.e., the basic ingredients of unprocessed interstellar ices. Such data are strongly needed to model spectra, determine accurate band strengths, and estimate abundances of both pores and other ice constituents in interstellar clouds. This study will help to interpret previous and future infrared observations from the Infrared Space Observatory, the Spitzer Space Telescope, and the James Webb Space Telescope. (AU)

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