Formation scenarios of dense starless cores from their ice composition

Dense, dark, starless cores of molecular clouds form in large-scale processes. The evolution of physical conditions in forming dense cores is often regulated by external phenomena and cannot be easily reproduced with models considering only the core and its immediate environment. The aim is to determine likely formation paths for dense cores that, when applied in an astrochemical model, produce column-density ratios of interstellar ice species in agreement with observations. Thus, we chemically determined how a typical core's central density evolves during its formation period. Compliance with observations of calculated ice composition was investigated for five central density growth paths: exponential, linear, sigmoid (S-type), asymptotic growth, and gravitational infall. Timescales of 1, 2, and 3 Myr were considered. An increase of the central density was accompanied by a contraction of the 1D spherical molecular cloud with a constant mass. Chemistry was simulated with a multi-grain multi-layer model with dynamical desorption energy for surface species. Exponential, infall, linear, and sigmoid density-growth models were apparently able to replicate observed ice composition, each with its own timescale. The results indicate that core formation likely occurs on a 2,Myr timescale that includes a 1,Myr stage of active contraction. Gravitational infall produces an agreement with observations when it is delayed by a factor of about 0.3. The possibility of ice photoprocessing in steady-state cores after their formation, as well as other local conditions, induces uncertainty in the actual formation path of individual objects. The study presents and tests a simple, optimisable way of creating dense cores for astrochemical modelling purposes.