Habart, and 9 co-authors, reported in mid-2022 that the 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.
Aims: With this study, the authors aimed 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.
Methods: The authors 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 H2 1-0 S(1) at 2.12 µm, tracing the dissociation front, and the [FeII] and Brγ lines, at 1.64 and 2.16 µm respectively, tracing the ionization front. The former is a powerful tracer of the FUV radiation field strength and gas density distribution at the PDR edge, while the last two trace the temperature and density distribution from the ionized gas to the PDR. We obtained narrow-band filter images in these key gas line diagnostic over ∼ 4000 at spatial scales of ∼0.100 (∼0.0002 pc or ∼40 AU at 414 pc).
Results: The Keck/NIRC2 observations spatially resolve a plethora of irradiated sub-structures such as ridges, filaments, globules and proplyds. This portends what JWST should accomplish and how it will complement the highest-resolution ALMA maps of the molecular cloud. A remarkable spatial coincidence between the H2 1-0 S(1) vibrational and HCO+ J=4-3 rotational emission previously obtained with ALMA is observed. This likely indicates theintimate link between these two molecular species and highlights that in high pressure PDR the H/H2 and C+/C/CO transitions zones come closer as compared to a typical layered structure of a constant density PDR. The H/H2 dissociation front appears as a highly structured dense region with over-dense sub-structures with a typical thickness of a few ∼10−3 pc. 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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