The effect of H2O on ice photochemistry

Abstract

UV irradiation of simple ices is proposed to efficiently produce complex organic species during star- and planet-formation. Through a series of laboratory experiments, we investigate the effects of the H2O concentration, the dominant ice constituent in space, on the photochemistry of more volatile species, especially CH4, in ice mixtures. In the experiments, thin (~40 ML) ice mixtures, kept at 20-60 K, are irradiated under ultra-high vacuum conditions with a broad-band UV hydrogen discharge lamp. Photodestruction cross sections of volatile species (CH4 and NH3) and production efficiencies of new species (C2H6, C2H4, CO, H2CO, CH3OH, CH3CHO and CH3CH2OH) in water-containing ice mixtures are determined using reflection-absorption infrared spectroscopy during irradiation and during a subsequent slow warm-up. The four major effects of increasing the H2O concentration are 1) an increase of the destruction efficiency of the volatile mixture constituent by up to an order of magnitude due to a reduction of back reactions following photodissociation, 2) a shift to products rich in oxygen e.g. CH3OH and H2CO, 3) trapping of up to a factor of five more of the formed radicals in the ice and 4) a disproportional increase in the diffusion barrier for the OH radical compared to the CH3 and HCO radicals. The radical diffusion temperature dependencies are consistent with calculated H2O-radical bond strengths. All the listed effects are potentially important for the production of complex organics in H2O-rich icy grain mantles around protostars and should thus be taken into account when modeling ice chemistry.

23 pages, 10 figures, accepted for publication in ApJ

References in corpus (5)

• Complex Chemistry in Star-Forming Regions: An Expanded Gas-Grain Warm-up Chemical Model
• Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments
• Testing grain-surface chemistry in massive hot-core regions
• Laboratory evidence for efficient water formation in interstellar ices
• Spatial mapping of ices in the Oph-F core: A direct measurement of CO depletion and the formation of CO2

Cited by in corpus (27)

• Our astrochemical heritage
• Photochemistry and astrochemistry: photochemical pathways to interstellar complex organic molecules
• Ice structures, patterns, and processes: A view across the ice-fields
• Atom addition reactions in interstellar ice analogues
• Impact of low-energy cosmic rays on star formation
• The negligible photodesorption of methanol ice and the active photon-induced desorption of its irradiation products
• An experimental study of the surface formation of methane in interstellar molecular clouds
• Prevalence of Complex Organic Molecules in Starless and Prestellar Cores within the Taurus Molecular Cloud
• Formation of interstellar methanol ice prior to the heavy CO freeze-out stage
• Comparison of UV and high-energy ion irradiation of methanol:ammonia ice
• Formation of NH2CHO and CH3CHO upon UV photoprocessing of interstellar ice analogs
• The complex chemistry of outflow cavity walls exposed: the case of low-mass protostars
• Molecular line emission from a protoplanetary disk irradiated externally by a nearby massive star
• Spectrally-resolved UV photodesorption of CH4 in pure and layered ices
• The Spatial Distribution of Organics toward the High-Mass YSO NGC 7538 IRS9
• New measurements on water ice photodesorption and product formation under ultraviolet irradiation
• The Ice Chamber for Astrophysics-Astrochemistry (ICA): A New Experimental Facility for Ion Impact Studies of Astrophysical Ice Analogues
• Desorption Kinetics and Binding Energies of Small Hydrocarbons
• Acetaldehyde binding energies: a coupled experimental and theoretical study
• The efficiency of photodissociation for molecules in interstellar ices
• Photon-induced desorption of larger species in UV-irradiated methane (CH4) ice
• SO2 and OCS toward high-mass protostars: A comparative study between ice and gas
• Signatures of Hot Molecular Hydrogen Absorption from Protoplanetary Disks: I. Non-thermal Populations
• Evaporative cooling of icy interstellar grains II. Key parameters
• The Photolysis of Aromatic Hydrocarbons Adsorbed on the Surfaces of Cosmic Dust Grains
• Photochemistry of interstellar ice forming Complex Organic Molecules
• Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains