The spatial distribution of complex organic molecules in the L1544 pre-stellar core

The detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T < 10 K) has challenged our understanding of the formation processes of COMs in the interstellar medium. Recent modeling on COM chemistry at low temperatures has provided new insight into these processes predicting that COM formation depends strongly on parameters such as visual extinction and the level of CO freeze out. We report deep observations of COMs toward two positions in the L1544 pre-stellar core: the dense, highly extinguished continuum peak with A ≥ 30 mag within the inner 2700 au; and a low-density shell with average A ∼ 7.5-8 mag located at 4000 au from the core's center and bright in CHOH. Our observations show that CHO, CHOCH, and CHCHO are more abundant (by factors of ∼2-10) toward the low-density shell than toward the continuum peak. Other COMs such as CHOCHO, c-CHO, HCCCHO, CHCHCN, and HCCNC show slight enhancements (by factors ≤3), but the associated uncertainties are large. This suggests that COMs are actively formed and already present in the low-density shells of pre-stellar cores. The modeling of the chemistry of O-bearing COMs in L1544 indicates that these species are enhanced in this shell because (i) CO starts freezing out onto dust grains driving an active surface chemistry; (ii) the visual extinction is sufficiently high to prevent the UV photo-dissociation of COMs by the external interstellar radiation field; and (iii) the density is still moderate to prevent severe depletion of COMs onto grains.Observations carried out under projects 129-14 and 001-16. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). Max Planck SocietyThis research has received funding from the People Programme (Marie Curie Actions) of the EU's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement PIIF-GA-2011-301538, and from the STFC through an Ernest Rutherford Fellowship (proposal ST/L004801/1). A.V. and P.C. acknowledge support from the European Research Council (ERC; Project PALs 320620). L.T. acknowledges partial support from the Italian Ministero dell'Istruzione, Università e Ricerca through the grant Progetti Premiali 2012—iALMA (CUP C52I13000140001) and from Gothenburg Centre of Advanced Studies in Science and Technology through the program Origins of habitable planets. N.M. acknowledges funding support from the European Research Council (ERC grant 610256: NANOCOSMOS)