Chemical Variations Across the TMC-1 Boundary: Molecular Tracers from the Translucent Phase to the Dense Phase

We investigated the chemical evolutions of gas-phase and grain-surface species across the Taurus molecular cloud-1 (TMC-1) filament from the translucent phase to the dense phase. By comparing observations with modeling results from an up-to-date chemical network, we examined the conversion processes for the carbon-, oxygen-, nitrogen-, and sulfur-bearing species, i.e., from their initial atomic form to their main molecular reservoir form both in the gas phase and on the grain surface. The conversion processes were found to depend on the species and A V . The effect of initial carbon-to-oxygen elemental abundances ratio (C/O) by varying O on the chemistry was explored, and an initial carbon elemental abundance of 2.5 × 10−4 and a C/O ratio of 0.5 could best reproduce the abundances of most observed molecules at TMC-1 CP, where more than 90 molecules have been identified. Based on the TMC-1 condition, we predicted a varied grain ice composition during the evolutions of molecular clouds, with H2O ice as the dominant ice composition at A V > 4 mag, CO2 ice as the dominant ice composition at A V <4 mag, while CO ice severely decreased at A V around 4–5 mag.