Composition and evolution of interstellar grain mantle under the effects of photodissociation

Abstract

We study the chemical evolution of interstellar grain mantle by varying the physical parameters of the interstellar medium (ISM). To mimic the exact interstellar condition, gas grain interactions via accretion from the gas phase and desorption (thermal evaporation and photo-evaporation) from the grain surface are considered. We find that the chemical composition of the interstellar grain mantle is highly dependent on the physical parameters associated with a molecular cloud. Interstellar photons are seen to play an important role towards the growth and the structure of the interstellar grain mantle. We consider the effects of interstellar photons (photo-dissociation and photo-evaporation) in our simulation under various interstellar conditions. We notice that the effects of interstellar photons dominate around the region of lower visual extinction. These photons contribute significantly in the formation of the grain mantle. Energy of the incoming photon is attenuated by the absorption and scattering by the interstellar dust. Top most layers are mainly assumed to be affected by the incoming radiation. We have studied the effects of photo-dissociation by varying the number of layers which could be affected by it. Model calculations are carried out for the static (extinction parameter is changing with the density of the cloud) as well as the time dependent case (i.e., extinction parameter and number density of the cloud both are changing with time) and the results are discussed in details. Different routes to the formation of water molecules are studied and it is noticed that around the dense region, production of water molecules via O$_3$ and H$_2$O$_2$ contributes significantly. At the end, various observational evidences for the condensed phase species are summarized with their physical conditions and are compared with our simulation results.