Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon

Abstract

(Abridged) Complex organic molecules (COMs) are considered essential precursors to prebiotic species. While COMs were once expected to be efficiently destroyed under UV-irradiated conditions, detections in photodissociation regions (PDRs) have challenged this view. However, the mechanisms by which UV radiation contributes to their formation are still uncertain. Here, we present moderately resolved maps of simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula, obtained by combining ALMA and IRAM 30m single-dish observations at $\sim 15^{\prime \prime }$ resolution. We analyze the spatial distribution of species such as C${}^{17}$O, CH${}_2$CO, CH${}_3$CHO, HNCO, CH${}_3$CN, and HC${}_3$N. By incorporating previous C${}^{17}$O and C${}^{18}$O single-dish data as well as PdBI maps of H${}_2$CO and CH${}_3$OH, we derive profiles of gas density, temperature, thermal pressure, and column densities of the organic species as a function of distance from the UV source. Our results show that most organic species$-$particularly H${}_2$CO, CH${}_2$CO, CH${}_3$CHO, HNCO, and CH${}_3$CN$-$exhibit enhanced column densities at the UV-illuminated edge compared to cloud interiors, possibly indicating efficient dust-grain surface chemistry driven by the diffusion of atomic C and radicals produced via photodissociation of CO and CH${}_3$OH, as supported by recent laboratory experiments. The exceptions, HC${}_3$N and CH${}_3$OH, can be attributed to inefficient formation on dust grains and ineffective non-thermal desorption into the gas phase, respectively. Additionally, contributions from gas-phase hydrocarbon photochemistry, possibly seeded by grain-surface products, cannot be ruled out. Further chemical modeling is needed to confirm the efficiency of these pathways for the studied species, which could have important implications for other cold, UV-irradiated environments such as protoplanetary disks.

21 pages, 4 tables, 10 figures; V3: minor changes, matches the accepted proof for A&A publication

References in corpus (71)

• CASA, the Common Astronomy Software Applications for Radio Astronomy
• Complex Chemistry in Star-Forming Regions: An Expanded Gas-Grain Warm-up Chemical Model
• Non-thermal desorption from interstellar dust grains via exothermic surface reactions
• Photochemistry and astrochemistry: photochemical pathways to interstellar complex organic molecules
• Testing grain-surface chemistry in massive hot-core regions
• Astrochemistry During the Formation of Stars
• The cometary composition of a protoplanetary disk as revealed by complex cyanides
• Complex organic molecules in protoplanetary disks
• Astrochemistry and compositions of planetary systems
• The Spatial Distribution of Complex Organic Molecules in the L1544 Pre-stellar Core
• Photodissociation and X-Ray Dominated Regions
• Formation of complex organic molecules in hot molecular cores through nondiffusive grain-surface and ice-mantle chemistry
• Low sulfur depletion in the Horsehead PDR
• Formation of Complex Organic Molecules in Cold Interstellar Environments through non-diffusive grain-surface and ice-mantle chemistry
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• Questioning the spatial origin of complex organic molecules in young protostars with the CALYPSO survey
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• The Perseus ALMA Chemistry Survey (PEACHES). I. The Complex Organic Molecules in Perseus Embedded Protostars
• Modeling the Lukewarm Corino Phase - is L1527 unique?
• Simultaneous Hydrogenation and UV-photolysis Experiments of NO in CO-rich Interstellar Ice Analogues; linking HNCO, OCN-, NH2CHO and NH2OH
• The interstellar chemistry of C3H and C3H2 isomers
• Low Temperature Surface Formation of NH3 and HNCO: hydrogenation of nitrogen atoms in CO-rich interstellar ice analogues
• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk
• An experimental study of the surface formation of methane in interstellar molecular clouds
• Spatially resolved l-c3h+ emission in the horsehead photodissociation region: Further evidence for a top-down hydrocarbon chemistry
• HCO mapping of the Horsehead : Tracing the illuminated dense molecular cloud surfaces
• Molecules with ALMA at Planet-forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H, and H2CO
• Molecules with ALMA at Planet-forming Scales (MAPS). IX. Distribution and Properties of the Large Organic Molecules HC${}_3$N, CH${}_3$CN, and $c$-C${}_3$H${}_2$
• An ALMA Survey of H${}_2$CO in Protoplanetary Disks
• Complex Organic Molecules Towards Embedded Low-Mass Protostars
• Complex organic molecules in strongly UV-irradiated gas
• Gas-phase formation of acetaldehyde: review and new theoretical computations
• An unbiased ALMA spectral survey of the LkCa 15 and MWC 480 protoplanetary disks
• Upper limits on CH${}_3$OH in the HD 163296 protoplanetary disk: evidence for a low gas-phase CH${}_3$OH/H${}_2$CO ratio
• Formation of complex molecules in translucent clouds: Acetaldehyde, vinyl alcohol, ketene, and ethanol via nonenergetic processing of C2H2 ice
• Carbon Atom Reactivity with Amorphous Solid Water: H${}_2$O Catalyzed Formation of H${}_2$CO
• Carbon-Chain Chemistry in the Interstellar Medium
• Hydrogenation of accreting C-atoms and CO molecules -- simulating ketene and acetaldehyde formation under dark and translucent cloud conditions
• C18O, 13CO, and 12CO abundances and excitation temperatures in the Orion B molecular cloud: An analysis of the precision achievable when modeling spectral line within the Local Thermodynamic Equilibrium approximation
• New N-bearing species towards OH231.8+4.2: HNCO, HNCS, HC${}_3$N and NO
• The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks
• A cold accretion flow onto one component of a multiple protostellar system
• Tracing snowlines and C/O ratio in a planet-hosting disk: ALMA molecular line observations towards the HD169142 disk
• The Distribution and Origin of C${}_2$H in NGC 253 from ALCHEMI
• Complex organic molecules during low-mass star formation: Pilot survey results
• Surface Diffusion of Carbon Atoms as a Driver of Interstellar Organic Chemistry
• Discovery of thiofulminic acid with the QUIJOTE line survey: A study of the isomers of HNCS and HNCO in TMC-1
• A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry
• Investigation of HNCO isomers formation in ice mantles by UV and thermal processing: an experimental approach
• A New "Non-energetic" Route to Complex Organic Molecules in Astrophysical Environments: The C + H${}_2$O $\to$ H${}_2$CO Solid-state Reaction
• Protostellar Interferometric Line Survey of the Cygnus X region (PILS-Cygnus) -- First results: observations of CygX-N30
• Methane formation in cold regions from carbon atoms and molecular hydrogen
• Dust evolution across the Horsehead Nebula
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• PDRs4All XI. Detection of infrared CH${}^{+}$ and CH${}_3^{+}$ rovibrational emission in the Orion Bar and disk d203-506: evidence of chemical pumping
• The extremely sharp transition between molecular and ionized gas in the Horsehead nebula
• Photodesorption of acetonitrile CH3CN in UV-irradiated regions of the Interstellar Medium: an experimental evidence
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• The rapid formation of macromolecules in irradiated ice of protoplanetary disk dust traps
• ALMA reveals thermal and non-thermal desorption of methanol ice in the HD 100546 protoplanetary disk
• Radial and vertical constraints on the icy origin of H${}_2$CO in the HD 163296 Protoplanetary Disk
• Collisional excitation of methyl (iso)cyanide by He atoms: rate coefficients and isomerism effects
• JWST observations of the Horsehead photon-dominated region I. First results from multi-band near- and mid-infrared imaging
• Interstellar Chemistry of CN Radicals on Ices: The formation of CH3CN and CH3NC and potential connection to acetamide
• Vertical and radial distribution of atomic carbon in HD 163296
• Exploring the chemistry induced by energetic processing of the H2-bearing, CO-rich apolar ice layer
• CoCCoA: Complex Chemistry in hot Cores with ALMA. The chemical evolution of acetone from ice to gas
• Toward a robust physical and chemical characterization of heterogeneous lines of sight: The case of the Horsehead nebula
• Spectral survey of the diffuse gas toward BL Lac in the Q band
• Weak, extended water vapor emission in the Horsehead nebula