CHEMICAL EVOLUTION OF INTERSTELLAR CLOUDS AND VARIATIONS OF MOLECULAR ABUNDANCES

The abundances of simple molecules are examined in terms of the time-dependent cloud evolution. The formation and destruction mechanisms of $H_2CO$ are reviewed. The average value of the fractional abundance of $H_2CO$ is derived to be in the range of $10^{-10}\;to\;5{\times}10^{-9}$. This is comparable to the observed values. The expected variations of the molecules formed from or destroyed by CO, CI, and $C^+$ whose abundances depend on the evolutionary state of the cloud are discussed.