2017A&A...597A..74J

Pre-stellar cores within molecular clouds provide the very initial conditions in which stars are formed. FeSt 1-457 is a prototypical starless core and the most chemically evolved among those isolated, embedded in the most pristine part of the Pipe nebula, the bowl. We use the IRAM 30 m telescope and the PdBI to study the chemical and physical properties of the starless core FeSt 1-457 (Core 109) in the Pipe nebula. We fit the hyperfine structure of the N₂H⁺ (1-0) IRAM 30 m data. This allowed us to measure with high precision the velocity field, line widths and opacity and derive the excitation temperature and column density in the core. We used a modified Bonnor-Ebert sphere model adding a temperature gradient towards the center to fit the 1.2mm continuum emission and visual extinction maps. Using this model, we have estimated the abundances of the N₂H⁺ and the rest of molecular lines detected in the 30 GHz wide line survey performed at 3mm with IRAM 30 m using ARTIST software. The core presents a rich chemistry with emission from early (C₃H₂, HCN, CS) and late-time molecules (e.g., N₂H⁺), with a clear chemical spatial differentiation for nitrogen (centrally peaked), oxygen (peaking to the southwest) and sulfurated molecules (peaking to the east). For most of the molecules detected (HCN, HCO⁺, CH₃OH, CS, SO, ¹³CO and C¹⁸O), abundances are best fit with three values, presenting a clear decrease of abundance of at least one or two orders of magnitude towards the center of the core. The Bonnor-Ebert analysis indicates the core is gravitationally unstable and the magnetic field is not strong enough to avoid the collapse. Depletion of molecules onto the dust grains occurs at the interior of the core, where dust grain growth and dust depolarization also occurs. This suggests that these properties may be related. On the other hand, some molecules exhibit asymmetries in their integrated emission maps, which appear to be correlated with a previously reported submillimetre polarization asymmetry. These asymmetries could be due to a stronger interstellar radiation field in the western side of the core.