Modeling the formation of interstellar CO_2, CO and water ice

Astronomy and Astrophysics – Astronomy

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Abstract: Observations of interstellar dust-grain ices indicate the presence of a selection of simple molecules, including H_2O, CO and CO_2. Of these, CO_2 and water are believed to form primarily on the dust grains themselves, while CO is accreted directly from the gas phase. Water and CO_2 ices have been detected along lines of sight with relatively low visual extinction (3--4 magnitudes in Taurus); however, the threshold for CO_2 detection is significantly greater (≃ 7 magnitudes). Since CO is required for the grain-surface formation of CO_2, the efficiency of CO to CO_2 conversion must vary over this range of extinctions. However, the precise mechanism of grain-surface CO_2 formation is uncertain, and previous models have been unable to reproduce observed CO_2 abundances, nor the correct threshold visual extinction. I will present the results of a new, detailed gas-grain chemical model of a quiescent molecular cloud. This model allows the individual layers of ice to be resolved, allowing the chemical composition and ``polar properties'' (i.e. H_2O content) to be traced within each ice layer. I will show that the use of a more physically accurate model of surface chemistry and ice structure allows the observed CO_2 ice abundance to be well reproduced, using uncontroversial reaction mechanisms. The observed threshold visual extinctions of H_2O, CO and CO_2 are also reproduced with this method, with the precise values largely dependent on gas density. I will show how the observed sequence of interstellar ice deposition (H_2O ... CO_2 ... CO) may be understood through a combination of chemical and physical influences. I will also consider the related behavior of ice species such as methanol (CH_3OH), which provides molecular material for the later formation of much more complex molecules during star formation.

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