Vianney Taquet, Cecilia Ceccarelli, Claudine Kahane
Mantles of iced water, mixed with CO, H2CO, and CH3OH are formed during the
so called prestellar core phase. In addition, radicals are also thought to be
formed on the grain surfaces, and to react to form complex organic molecules
later on, during the warm-up phase of the protostellar evolution. The aim of
this work is to study the formation of the grain mantles during the prestellar
core phase and the abundance of H2CO, CH3OH, and radicals trapped in them. We
have developed a macrosopic statistic multilayer model that follows the
formation of grain mantles with time and that includes two effects that may
increase the number of radicals trapped in the mantles: i) at each time of the
mantle formation, only the surface layer is chemically active rather than the
entire bulk, and ii) the porous structure of grains allows to trap reactive
particles. The model considers a network of H, O and CO forming neutral species
such as water, CO, formaldehyde, and methanol, plus several radicals. We run a
large grid of models to study the impact of the mantle multilayer nature and
grain porous structure. In addition, we explored the influence of the
uncertainty of other key parameters on the mantle composition. Our model
predicts relatively large abundances of radicals. In addition, the multilayer
approach makes it possible to follow the chemical differentiation within the
grain mantle, showing that the mantles are far from being uniform. For example,
methanol is mostly present in the outer layers of the mantles whereas CO and
other reactive species are trapped in the inner layers. The overall mantle
composition depends on the density and age of the prestellar core, and on some
microscopic parameters. Comparison with observations allows us to constrain the
value of few parameters and provide some indications on the physical conditions
during the formation of the ices.
View original:
http://arxiv.org/abs/1111.4165
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