Cyclic models of molecular clouds: ice retention and the C:O ratio

The gas in molecular clouds containing low-mass stars is cycled between high-density clump and low-density interclump phases. The cycling is generally assumed to remove mantles which are frozen out on dust in the dense phase. In this paper we explore the consequences of a more realistic assumption: that the mantles are not completely removed in each cycle. We consider two cases: a non-selective case, in which all molecules in the mantle are treated uniformly; and a selective case, in which H2O is preferentially retained in the mantle, while other molecules are released. We find that in these cyclic models the C:O ratio, representing the ratio of the total available carbon and oxygen in the gas phase, depends on the cycle number, the cosmic ray ionization rate, and the nature of the desorption process. For non-selective desorption and high values of the cosmic ray ionization rate, the C:O ratio tends to decline with age as the number of cycles increases. For selective desorption, the C:O ratio tends to increase with age as the number of cycles increases. The range of values of the C:O ratio predicted in these models is approximately 0.3–1.5, i.e., very oxygen-rich to very carbon-rich. The consequences of the changes in the C:O ratio for the chemistry are explored in detail. Results are given for dense and diffuse phases of molecular clouds. The observational consequences are briefly discussed.