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Chemical evolution during the formation of molecular clouds

To study the chemical evolution during the formation of molecular clouds, we model three types of clouds with different density structures: collapsing spherical, collapsing ellipsoidal, and static spherical profiles. The collapsing models are better than the static models in matching the observational characteristics in typical molecular clouds. This is mainly because the gravity can speed up the formation of some important molecules (e.g., H$_2$, CO, OH) by increasing the number density during collapse. The different morphologies of prolate, oblate, and spherical clouds lead to differences in chemical evolution, which are mainly due to their different evolution of number density. We also study the effect of initial chemical compositions on chemical evolution, and find that H atoms can accelerate OH formation by two major reactions: O + H $\rightarrow$ OH in gas phase and on dust grain surfaces, leading to the models in which hydrogen is mainly atomic initially better match observations than the models in which hydrogen is mainly molecular initially. Namely, to match observations, initially hydrogen must be mostly atomic. The CO molecules are able to form even without the pre-existence of H$_2$. We also study the influence of gas temperature, dust temperature, intensity of interstellar radiation field and cosmic-ray ionization rate on chemical evolution in static clouds. The static CO clouds with high dust temperature, strong radiation field, and intensive cosmic rays are transient due to rapid CO destruction.