Nautilus multi-grain model: Importance of cosmic-ray-induced desorption
  in determining the chemical abundances in the ISM
  Species abundances in the interstellar medium (ISM) strongly depend on the
chemistry occurring at the surfaces of the dust grains. To describe the
complexity of the chemistry, various numerical models have been constructed. In
most of these models, the grains are described by a single size of 0.1$\mu$m.
We study the impact on the abundances of many species observed in the cold
cores by considering several grain sizes in the Nautilus multi-grain model. We
used grain sizes with radii in the range of $0.005\mu$m to $0.25\mu$m. We
sampled this range in many bins. We used the previously published, MRN and WD
grain size distributions to calculate the number density of grains in each bin.
Other parameters such as the grain surface temperature or the
cosmic-ray-induced desorption rates also vary with grain sizes. We present the
abundances of various molecules in the gas phase and also on the dust surface
at different time intervals during the simulation. We present a comparative
study of results obtained using the single grain and the multi-grain models. We
also compare our results with the observed abundances in TMC-1 and L134N
clouds. We show that the grain size, the grain size dependent surface
temperature and the peak surface temperature induced by cosmic ray collisions,
play key roles in determining the ice and the gas phase abundances of various
molecules. We also show that the differences between the MRN and the WD models
are crucial for better fitting the observed abundances in different regions in
the ISM. We show that the small grains play a very important role in the
enrichment of the gas phase with the species which are mainly formed on the
grain surface, as non-thermal desorption induced by collisions of cosmic ray
particles is very efficient on the small grains.
