PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs

Abstract

We present subarcsecond-resolution ALMA mosaics of the Orion Bar PDR in [CI] 609um, C2H (4-3), and C18O (3-2) emission lines complemented by JWST images of H2 and aromatic infrared band (AIB) emission. The rim of the Bar shows very corrugated structures made of small-scale H2 dissociation fronts (DFs). The [CI] 609 um emission peaks very close (~0.002 pc) to the main H2-emitting DFs, suggesting the presence of gas density gradients. These DFs are also bright and remarkably similar in C2H emission, which traces "hydrocarbon radical peaks" characterized by very high C2H abundances, reaching up to several x10^-7. The high abundance of C2H and of related hydrocarbon radicals, such as CH3, CH2, and CH, can be attributed to gas-phase reactions driven by elevated temperatures, the presence of C+ and C, and the reactivity of FUV-pumped H2. The hydrocarbon radical peaks roughly coincide with maxima of the 3.4/3.3 um AIB intensity ratio, a proxy for the aliphatic-to-aromatic content of PAHs. This implies that the conditions triggering the formation of simple hydrocarbons also favor the formation (and survival) of PAHs with aliphatic side groups, potentially via the contribution of bottom-up processes in which abundant hydrocarbon radicals react in situ with PAHs. Ahead of the DFs, in the atomic PDR zone (where [H]>>[H2]), the AIB emission is the brightest, but small PAHs and carbonaceous grains undergo photo-processing due to the stronger FUV field. Our detection of trace amounts of C2H in this zone may result from the photoerosion of these species. This study provides a spatially resolved view of the chemical stratification of key carbon carriers in a PDR. Overall, both bottom-up and top-down processes appear to link simple hydrocarbon molecules with PAHs in molecular clouds; however, the exact chemical pathways and their relative contributions remain to be quantified.

Accepted for publication in A&A. 23 pages including Appendices. Abridged abstract. This is v2. The English language has been edited

References in corpus (52)

• Detection of Two Interstellar Polycyclic Aromatic Hydrocarbons via Spectral Matched Filtering
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• Pure hydrocarbon cycles in TMC-1: Discovery of ethynyl cyclopropenylidene, cyclopentadiene and indene
• One Relation for All Wavelengths: The Far-Ultraviolet to Mid-Infrared Milky Way Spectroscopic R(V) Dependent Dust Extinction Relationship
• Photodissociation and X-Ray Dominated Regions
• On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR
• PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• The penetration of FUV radiation into molecular clouds
• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk
• PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars
• Discovery of two cyano derivatives of acenaphthylene (C${}_{12}$H${}_8$) in TMC-1 with the QUIJOTE line survey
• Spatially resolved l-c3h+ emission in the horsehead photodissociation region: Further evidence for a top-down hydrocarbon chemistry
• Mixed aliphatic and aromatic composition of evaporating very small grains in NGC 7023 revealed by the 3.4/3.3 $\mu$m ratio
• SpeX near-infrared spectroscopic extinction curves in the Milky Way
• Expanding bubbles in Orion A: [CII] observations of M42, M43, and NGC 1977
• Laboratory Photo-chemistry of PAHs: Ionization versus Fragmentation
• Complex organic molecules in strongly UV-irradiated gas
• The Orion HII Region and the Orion Bar in the Mid-Infrared
• C${}_2$H observations toward the Orion Bar
• PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula
• ALMA imaging of C2H emission in the disk of NGC1068
• Rotationally Warm Molecular Hydrogen in the Orion Bar
• IRAM-30m large scale survey of ${}^{12}$CO(2-1) and ${}^{13}$CO(2-1) emission in the Orion molecular cloud
• Herschel / HIFI spectral line survey of the Orion Bar - Temperature and density differentiation near the PDR surface
• The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines
• Molecular globules in the Veil bubble of Orion. IRAM 30m 12CO, 13CO, and C18O 2-1 expanded maps of Orion A
• Destruction of refractory carbon grains drives the final stage of proto-planetary disk chemistry
• The C--H Stretching Features at 3.2--3.5 Micrometer of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups
• Formation of interstellar SH${}^{+}$ from vibrationally excited H${}_2$: Quantum study of S${}^{+}$ + H${}_2$ $\rightleftarrows$ SH${}^{+}$ + H reactions and inelastic collisions
• A Near-infrared Survey of UV-excited Molecular Hydrogen in Photodissociation Regions
• Spatial distribution of FIR rotationally excited CH+ and OH emission lines in the Orion Bar PDR
• Photodissociation of aliphatic PAH derivatives under relevant astrophysical conditions
• High angular resolution near-IR view of the Orion Bar revealed by Keck/NIRC2
• Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
• HCN emission from translucent gas and UV-illuminated cloud edges revealed by wide-field IRAM 30m maps of Orion B GMC: Revisiting its role as tracer of the dense gas reservoir for star formation
• Impact of PAH photodissociation on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs
• Dynamical effects of the radiative stellar feedback on the H I-to-H2 transition
• PDRs4All VI: Probing the Photochemical Evolution of PAHs in the Orion Bar Using Machine Learning Techniques
• The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR
• PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon
• Quantum study of reaction O(3P) + H2(v,j) $\to$ OH + H: OH formation in strongly UV-irradiated gas
• PDRs4All VII. The 3.3 $\mu$m aromatic infrared band as a tracer of physical properties of the ISM in galaxies
• Observational study of hydrocarbons in the bright photodissociation region of Messier 8
• PDRs4All. V. Modelling the dust evolution across the illuminated edge of the Orion Bar
• Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions
• Impact of HAC evolution on the formation of small hydrocarbons in the Orion Bar and the Horsehead PDRs
• Photocleavage of aliphatic C--C bonds in the interstellar medium
• Imaging Resonance Effects in C + H${}_2$ Collisions using a Zeeman Decelerator

Cited by in corpus (11)

• PDRs4All XV: CH radical and H${}_3^{+}$ molecular ion in the irradiated protoplanetary disk d203-506
• PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar
• Aromatic Species in the Molecular Universe
• Abundant hydrocarbons in a buried galactic nucleus with signs of carbonaceous grain and polycyclic aromatic hydrocarbon processing
• PDRs4All XVII: Formation and excitation of HD in photodissociation regions. Application to the Orion Bar
• H3+ in irradiated protoplanetary disks: Linking far-ultraviolet radiation and water vapor
• Detection of CO${}_2$ ice in the planetary nebula NGC 6302
• Acenaphthene Derivatives as Signatures of C${}_{11}$H${}_9^{+}$ Reactivity with Methylated Naphthalenes
• PDRs4All XIX. The 6 to 9 $\mu$m region as a probe of PAH charge and size in the Orion Bar
• Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon
• PDRs4All XVIII. JWST-NIRCam Photometric properties of protoplanetary disks in the Orion Nebula Cluster