High angular resolution near-IR view of the Orion Bar revealed by Keck/NIRC2

Abstract

Nearby Photo-Dissociation Regions (PDRs), where the gas and dust are heated by the far UV-irradiation emitted from stars, are ideal templates to study the main stellar feedback processes. With this study we aim to probe the detailed structures at the interfaces between ionized, atomic, and molecular gas in the Orion Bar. This nearby prototypical strongly irradiated PDR will be among the first targets of the James Webb Space Telescope (JWST) within the framework of the PDRs4All Early Release Science program. We employed the sub-arcsec resolution accessible with Keck-II NIRC2 and its adaptive optics system to obtain the most detailed and complete images, ever performed, of the vibrationally excited line H${}_2$ 1-0 S(1) at 2.12~$\mu$m, tracing the dissociation front, and the [FeII] and Br$\gamma$ lines, at 1.64 and 2.16~$\mu$m respectively, tracing the ionization front. We obtained narrow-band filter images in these key gas line diagnostic over $\sim 40^{\prime \prime }$ at spatial scales of $\sim$0.1${}^{\prime \prime }$ ($\sim$0.0002~pc or $\sim$40~AU at 414~pc). The Keck/NIRC2 observations spatially resolve a plethora of irradiated sub-structures such as ridges, filaments, globules and proplyds. A remarkable spatial coincidence between the H${}_2$ 1-0 S(1) vibrational and HCO${}^{+}$ J=4-3 rotational emission previously obtained with ALMA is observed. This likely indicates the intimate link between these two molecular species and highlights that in high pressure PDR the H/H${}_2$ and C${}^{+}$/C/CO transitions zones come closer as compared to a typical layered structure of a constant density PDR. This is in agreement with several previous studies that claimed that the Orion Bar edge is composed of very small, dense, highly irradiated PDRs at high thermal pressure immersed in a more diffuse environment.

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Cited by in corpus (10)

• PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar
• PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula
• 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
• 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
• PDRs4All VI: Probing the Photochemical Evolution of PAHs in the Orion Bar Using Machine Learning Techniques
• PDRs4All. X. ALMA and JWST detection of neutral carbon in the externally irradiated disk d203-506: Undepleted gas-phase carbon
• PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
• Near infrared view on the photodissociation regions S255, S257, NGC7538 and S140
• PDRs4All XVII: Formation and excitation of HD in photodissociation regions. Application to the Orion Bar