Molecular Evolution in Collapsing Prestellar Cores III: Contraction of A Bonnor-Ebert Sphere

The gravitational collapse of a spherical cloud core is investigated by numerical calculations. The initial conditions of the core lie close to the critical BonnorEbert sphere with a central density of ∼ 10 cm in one model (α = 1.1), while gravity overwhelms pressure in the other (α = 4.0), where α is the internal gravity-to-pressure ratio. The α = 1.1 model shows reasonable agreement with the observed velocity field in prestellar cores. Molecular distributions in cores are calculated by solving a chemical reaction network that includes both gas-phase and grain-surface reactions. When the central density of the core reaches 10 cm, carbon-bearing species are significantly depleted in the central region of the α = 1.1 model, while the depletion is only marginal in the other model. The two different approaches encompass the observed variations of molecular distributions in different prestellar cores, suggesting that molecular distributions can be probes of contraction or accumulation time scales of cores. The central enhancement of the NH3/N2H + ratio, which is observed in some prestellar cores, can be Present address: Department of Physics, UMIST, PO Box 88, Manchester M60 1QD, UK