title: The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core.
abstract: 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 modelling 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 AV≥30 mag within the inner 2700 au; and a low-density shell with average AV~7.5-8 mag located at 4000 au from the core's center and bright in CH3OH. Our observations show that CH3O, CH3OCH3and CH3CHO are more abundant (by factors ~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 modelling 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.
authors.affiliation: Astronomy Unit, School of Physics & Astronomy, Queen Mary University of London, Mile End Road, E1 4NS, London (UK); Department of Physics & Astronomy, University College London, 132 Hampstead Road, NW1 2PS London (UK); European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching (Germany).
authors.forename: Izaskun
authors.initials: I
authors.lastname: Jiménez-Serra
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstr., 85741 Garching (Germany); Ural Federal University, Ekaterinburg, Russia.
authors.forename: Anton I
authors.initials: AI
authors.lastname: Vasyunin
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstr., 85741 Garching (Germany).
authors.forename: Paola
authors.initials: P
authors.lastname: Caselli
authors.affiliation: INAF, Osservatorio de Radioastronomia, Via P. Gobetti 101, 40129 Bologna, (Italy); Instituto de Ciencia de Materiales de Madrid, CSIC, C/ Sor Juana Inés de la Cruz 3, 28049 Cantoblanco (Spain).
authors.forename: Nuria
authors.initials: N
authors.lastname: Marcelino
authors.affiliation: Instituto de Radioastronomía Milimétrica, Avenida Divina Pastora 7, E-18012 Granada (Spain).
authors.forename: Nicolas
authors.initials: N
authors.lastname: Billot
authors.affiliation: Department of Physics & Astronomy, University College London, 132 Hampstead Road, NW1 2PS London (UK).
authors.forename: Serena
authors.initials: S
authors.lastname: Viti
authors.affiliation: European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching (Germany); INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Testi
authors.affiliation: Université de Toulouse, UPS-OMP, IRAP, F-31400 Toulouse, France; CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Vastel
authors.affiliation: Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France; CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Bertrand
authors.initials: B
authors.lastname: Lefloch
authors.affiliation: Observatorio Astronómico Nacional (OAN, IGN), Calle Alfonso XII 3, E-28014 Madrid, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Bachiller
keywords: astrochemistry — ISM; formation — ISM; individual objects (L1544); molecules — stars