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arXiv · 2608.03140

PDRs4All XXII. Near-Infrared continuum in the Orion Bar

Takashi Onaka·Emmanuel Dartois·Els Peeters·Olivier Berne·Emilie Habart·Christiaan Boersma·Jan Cami·Asuncion Fuente·Javier R. Goicoechea·Ozan Lacinbala·Yoko Okada·Alexander G. G. M. Tielens·Dries Van De Putte·Francois Boulanger·Thomas Pino·Yong Zhang
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Abstract

Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3um in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program "PDRs4All." Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1--4.5um. The continuum at wavelengths longer than 2.7um shows good correlations with the 3.3um AIB, while the correlation of the continuum at 1.2um is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27um, which are attributed to the presence in the spectra of water ice and CO2 ice. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

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Takashi Onaka, Emmanuel Dartois, Els Peeters, Olivier Berne, Emilie Habart, Christiaan Boersma, Jan Cami, Asuncion Fuente, Javier R. Goicoechea, Ozan Lacinbala, Yoko Okada, Alexander G. G. M. Tielens, Dries Van De Putte, Francois Boulanger, Thomas Pino, Yong Zhang. 2026-08-17. PDRs4All XXII. Near-Infrared continuum in the Orion Bar. https://arxiv.org/abs/2608.03140

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