摘要：Context. Pre-stellar cores are the basic unit for the formation of stars and stellar systems. The anatomy of the physical and chemical structures of pre-stellar cores is critical for understanding the star formation process. Aims. L1544 is a prototypical pre-stellar core that shows significant chemical differentiation surrounding the dust peak. We aim to constrain the physical conditions at the different molecular emission peaks. This study allows us to compare the abundance profiles predicted from chemical models with the classical density structure of the Bonnor–Ebert (BE) sphere. Methods. We conducted multi-transition pointed observations of CH3OH, c-C3H2, and HNCO with the IRAM 30m telescope toward the dust peak and the respective molecular peaks of L1544. Using this data set, with nonlocal-thermodynamic-equilibrium radiative transfer calculations and a one-dimensional model, we revisit the physical structure of L1544 and benchmark the observations with the abundance profiles from current chemical models. Results. We find that the HNCO, c-C3H2, and CH3OH lines in L1544 trace progressively higher-density gas, from ∼104 to several times 105 cm−3 . Particularly, we find that to produce the observed intensities and ratios of the CH3OH lines, a local gas density enhancement above that of the BE sphere is required. This suggests that the physical structure of an early-stage core may not necessarily follow a smooth decrease in gas density profile locally, but can be intercepted by clumpy substructures that surround the gravitational center. Conclusions. Multiple transitions of molecular lines from different molecular species can provide a tomographic view of the density structure of pre-stellar cores. The local gas density enhancement deviating from the BE sphere may reflect the impact of accretion flows that appear asymmetric and are enhanced at the meeting point of large-scale cloud structures.
