The observed chemical structure of L1544

Context. Prior to star formation, pre-stellar cores accumulate matter towards the centre. As a consequence, their central density increases while the temperature decreases. Understanding the evolution of the chemistry and physics in this early phase is crucial to study the processes governing the formation of a star. Aims. We aim to study the chemical differentiation of a prototypical pre-stellar core, L1544, by detailed molecular maps. In contrast with single-pointing observations, we performed a deep study on the dependencies of chemistry on physical and external conditions. Methods. Here we present the emission maps of 39 different molecular transitions belonging to 22 different molecules in the central 6.25 arcmin2 of L1544. We classified our sample into five families, depending on the location of their emission peaks within the core. To systematically study the correlations among different molecules, we have performed the principal component analysis (PCA) on the integrated emission maps. The PCA allows us to reduce the amount of variables in our dataset. Finally, we have compared the maps of the first three principal components with the H2 column density map, and the Tdust map of the core. Results. The results of our qualitative analysis is the classification of the molecules in our dataset in the following groups: (i) the c-C3H2 family (carbon chain molecules like C3H and CCS); (ii) the dust peak family (nitrogen-bearing species like N2H+); (iii) the methanol peak family (oxygen-bearing molecules like methanol, SO and SO2); (iv) the HNCO peak family (HNCO, propyne and its deuterated isotopologues). Only HC18O+ and 13CS do not belong in any of the above mentioned groups. The principal component maps allow us to confirm the (anti-)correlations among different families that were described in a first qualitative analysis, but also point out the correlation that could not be inferred before. For example, the molecules belonging to the dust peak and the HNCO peak families correlate in the third principal component map, hinting on a chemical and/or physical correlation. Conclusions. The principal component analysis has shown to be a powerful tool to retrieve information about the correlation of different molecular species in L1544, and their dependence on physical parameters previously studied in the core