One Supervisor and Three Different Topics to Choose from: My Choice and Subsequent Progress

Abstract

Interstellar dust grains play a crucial role in the chemical evolution of star-forming regions. These grains exist in various sizes and also grow over time due to the formation of grain mantle. In recent times efforts were made to understand how different sized-grains take part in the formation of molecules. We considered two widely used size distributions from earlier models. To incorporate various grain sizes in the model, the distribution is divided into a number of logarithmically equal spaced ranges. Then each range is integrated to find its number density, and assigned that number density to the average size of that range. We started by studying the effect of size distribution in the production of molecular hydrogen in the diffuse interstellar medium. We found that H₂ formation efficiency is a function of grain radius and total effective surface area available for a given distribution. When we used fixed temperature for each size based on earlier works, we found that contribution from smaller grain is less since they have higher grain temperature, whereas when temperature fluctuation due to the stochastic heating of the grain is considered, smaller grains contribution increases significantly. We also found that H₂ formation rate strongly depends on the surface property and its topology, the mechanism for the reaction, as well as on the type of binding of H (e.g., without chemisorption sites H₂ formation at high temperature is very low). Molecular abundances of species, e.g., CO, H₂O, etc., in the dense clouds, are effected by the use of size distribution and grain growth over time. However, when the comparison between model abundances and observed abundances are made, it did not lead to the better agreement, and more study is needed.