Understanding the impact of diffusion of CO in the astrochemical models

Abstract

The mobility of lighter species on the surface of interstellar dust grains plays a crucial role in forming simple through complex molecules. Carbon monoxide is one of the most abundant molecules, its surface diffusion on the grain surface is essential to forming many molecules. Recent laboratory experiments found a diverse range of diffusion barriers for CO on the grain surface, their use can significantly impact the abundance of several molecules. The impact of different diffusion barriers of CO, in the astrochemical models, is studied to understand its effect on the abundance of solid CO and the species for which it is a reactant partner. A gas-grain network is used for three different physical conditions; cold-core and warm-up models with slow and fast heating rates. Two different ratios (0.3 and 0.5) between diffusion and desorption barrier are utilized for all the species. For each physical condition and ratio, six different models are run by varying diffusion barriers of CO. Solid CO abundance for the models with the lowest diffusion barrier yields less than 0.1% of water ice for cold clouds and a maximum of 0.4% for slow and fast warm-up models. Also, solid CO${}_2$ in dense clouds is significantly overproduced (140 % of water). The abundance of H${}_2$CO and CH${}_3$OH showed an opposite trend, and HCOOH, CH${}_3$CHO, NH${}_2$CO, and CH${}_3$COCH${}_3$ are produced in lower quantities for models with low diffusion barriers for CO. Considerable variation in abundance is observed between models with the high and low diffusion barrier. Models with higher diffusion barriers provide a relatively better agreement with the observed abundances when compared with the models having lower diffusion barriers.

Accepted for publication in PASA

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• Research on the Interstellar Medium and Star Formation in the Galaxy: An Indian Perspective
• CO Diffusion on Interstellar Amorphous Solid Water: A Computational Study
• Experimental investigation of O2 diffusion and entrapment in interstellar amorphous solid water (ASW)