The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core

International audienceThe detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T V ≥ 30 mag within the inner 2700 au; and a low-density shell with average A V ∼ 7.5-8 mag located at 4000 au from the core’s center and bright in CH3OH. Our observations show that CH3O, CH3OCH3, and CH3CHO are more abundant (by factors of ∼2-10) toward the low-density shell than toward the continuum peak. Other COMs such as CH3OCHO, c-C3H2O, HCCCHO, CH2CHCN, 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