Astronomers have captured the most detailed photos of the radiation zone in Orion's'sword' to date

Credit: Habart et al./W. M. Keck Observatory

Astronomers using the W. M. Keck Observatory on Hawaii Island got the most comprehensive and complete photos yet shot of the zone where the famous constellation of Orion has blasted with ultraviolet (UV) radiation from big young stars from Maunakea.

This irradiated neutral zone, known as a Photo-Dissociation Region (PDR), is situated in the Orion Bar of the Orion Nebula, an active star-forming hotspot located in the center of the "sword" hanging from Orion's "belt." When observed with the naked eye, the nebula is sometimes misidentified as one of the stars in the constellation; when examined via a telescope, the picturesque nebula is shown to be a luminous gaseous stellar nursery 1,350 light years from Earth.

Carlos Alvarez, a staff astronomer at Keck Observatory and co-author of the paper, stated, "It was thrilling to be the first to see the sharpest images of the Orion Bar ever taken in the near-infrared, together with my colleagues from the 'PDRs4All' James Webb Space Telescope team."

Because the Orion Nebula is the nearest massive star formation zone to us and may be analogous to the environment in which our solar system formed, investigating its PDR (the area heated by sunlight) is an ideal spot to gather information about how stars and planets develop.

According to Emilie Habart, associate professor at Institut d'Astrophysique Spatiale at Paris-Saclay University and lead author of a paper on this topic, studying photo-dissociation areas is like peering into our history. These areas are significant because they allow us to understand how young stars impact the gas and dust clouds in which they are created, particularly at places where stars such as the sun develop.

The paper has been approved for publication in the journal Astronomy & Astrophysics and is now accessible as a preprint on These trailblazing observations aided in the development of the James Webb Space Telescope (JWST) Early Release Science (ERS) program PDRs4All: Radiative Feedback of Massive Stars (ID1288). The PDRs4All program is described in a Publication of the Astronomical Society of the Pacific paper by Berné, Habart, Peeters et al. (2022).

Left: Hubble Space Telescope mosaic of the Orion Bar. Credit: NASA/STScI/Rice Univ./C.O’Dell et al. The NIRC2 wide camera Field of View is shown in the yellow square. Right: Infrared heat map of the Orion Bar obtained with Keck Observatory’s NIRC2 instrument reveals substructures such as proplyds. Credit: Habart et al./W. M. Keck Observatory

The PDRs4All team used Keck Observatory's second generation Near-Infrared Camera (NIRC2) in conjunction with the Keck II telescope's adaptive optics technology to investigate Orion's PDR. They were able to spatially identify and differentiate the Orion Bar's distinct substructures such as ridges, filaments, globules, and proplyds (externally lighted photoevaporating disks around newborn stars) that developed when starlight blasted and shaped the nebula's combination of gas and dust.

Never before, according to Habart, have we been able to examine at such a microscopic scale how interstellar matter formations rely on their surroundings, notably how planetary systems might emerge in settings heavily irradiated by huge stars. This may help us better comprehend the interstellar medium's legacy on planetary systems, notably our beginnings.

Massive newborn stars generate vast amounts of UV radiation, which affects the physics and chemistry of their immediate environment; how this rush of energy that the stars inject into their native cloud influences and molds star formation is unknown. The new Keck Observatory photos of the Orion Bar will help astronomers better comprehend this process by revealing in detail where gas in the PDR transitions from hot ionized gas to warm atomic gas to cold molecular gas. Mapping this conversion is critical since dense, cold molecular gas is required for star formation.

These new Keck Observatory observations have influenced preparations for JWST observations of the Orion Bar, which is one of JWST's objectives and will be viewed in the next weeks.

One of the most fascinating elements of this study, according to Alvarez, is watching Keck play a key role in the JWST era. JWST will be able to delve further into the Orion Bar and other PDRs, and Keck will be critical in verifying JWST's preliminary scientific findings. Together, the two telescopes can give unique insight into the gas and chemical makeup of PDRs, allowing us to better grasp the nature of these interesting star-blasted areas.

Journal Information: High angular resolution near-IR view of the Orion Bar revealed by Keck/NIRC2, arXiv:2206.08245v1 [astro-ph.GA]

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