Merged H/H2 and C+/C/CO transitions in the Orion Bar

Abstract

High-resolution ALMA images towards the Orion Bar show no discernible offset between the peak of H2 emission in the photodissociation region (PDR) and the CO(3-2) and HCO+(4-3) emission in the molecular region. This implies that positions of H2 and CO dissociation fronts are indistinguishable in the limit of ALMA resolution. We use the chemo-dynamical model MARION to show that the ALMA view of the Orion Bar, namely, no appreciable offset between the CO(3-2) and HCO+(4-3) peaks, merged H2 and CO dissociation fronts, and high intensity of HCO+(4-3) emission, can only be explained by the ongoing propagation of the dissociation fronts through the molecular cloud, coupled to the dust motion driven by the stellar radiation pressure, and are not reproduced in the model where the dissociation fronts are assumed to be stationary. Modelling line intensities, we demonstrate that after the fronts have merged, the angular separation of the CO(3-2) and HCO+(4-3) peaks is indeed unresolvable with the ALMA observations. Our model predictions are consistent with the results of the ALMA observations about the relation of the bright HCO+(4-3) emission to the compressed gas at the border of the PDR in the sense that the theoretical HCO+(4-3) peak does correspond to the gas density enhancement, which naturally appears in the dynamical simulation, and is located near the H2 dissociation front at the illuminated side of the CO dissociation front.

arXiv commentaccepted to MNRAS

References in corpus (35)

• Dust Grain Size Distributions and Extinction in the Milky Way, LMC, and SMC
• A computer program for fast non-LTE analysis of interstellar line spectra
• An atomic and molecular database for analysis of submillimetre line observations
• The distance to the Orion Nebula
• Photodissociation and photoionisation of atoms and molecules of astrophysical interest
• The Critical Density and the Effective Excitation Density of Commonly Observed Molecular Dense Gas Tracers
• The 2013 Release of Cloudy
• The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-Violet at R~2500
• The Luminosity & Mass Function of the Trapezium Cluster: From B stars to the Deuterium Burning Limit
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• A PDR-Code Comparison Study
• Triggered star formation on the borders of the Galactic HII region RCW 120
• The cosmic ray ionization rate in the Galactic disk, as determined from observations of molecular ions
• Compression and ablation of the photo-irradiated cloud the Orion Bar
• The gas temperature in flaring disks around pre-main sequence stars
• 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
• Structure of photodissociation fronts in star-forming regions revealed by observations of high-J CO emission lines with Herschel
• Orion's Bar: Physical Conditions across the Definitive H+ / H0 / H2 Interface
• Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. I. A Mode of Triggered Star Formation
• Mopra CO Observations of the Bubble HII Region RCW120
• The Orion HII Region and the Orion Bar in the Mid-Infrared
• Infrared Emission and the Destruction of Dust in HII regions
• Physical Conditions in Orion's Veil
• Dust dynamics and evolution in expanding HII regions. I. Radiative drift of neutral and charged grains
• First evidence for dusty disks around Herbig Be stars
• Star formation around the HII region Sh2-235
• Modelling clumpy PDRs in 3D - Understanding the Orion Bar stratification
• Dust dynamics and evolution in HII regions - II. Effects of dynamical coupling between dust and gas
• Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium
• Rotationally Warm Molecular Hydrogen in the Orion Bar
• Quantum Calculation of Inelastic CO Collisions with H: I. rotational quenching of low-lying rotational levels
• Herschel / HIFI spectral line survey of the Orion Bar - Temperature and density differentiation near the PDR surface
• Pumping up the [N I] nebular lines
• Photoevaporating PDR models with the Hydra PDR Code
• Molecular tracers of radiative feedback in Orion (OMC-1). Widespread CH+ (J=1-0), CO (10-9), HCN (6-5), and HCO+ (6-5) emission

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