Spitzer reveals what's behind Orion's Bar
Robert H. Rubin, Janet P. Simpson, C. R. O'Dell, Ian A. McNabb, Sean, W. J. Colgan, Scott Y. Zhuge, Gary J. Ferland, Sergio A. Hidalgo

TL;DR
This study uses Spitzer Space Telescope data to analyze the ionization and elemental abundances, especially neon and sulfur, in the Orion Nebula's extended regions, providing new insights into the nebula's ionization structure and elemental ratios.
Contribution
First detailed IR spectroscopic mapping of the Orion Nebula's outer regions, revealing high-ionization states and precise elemental abundance ratios beyond the inner nebula.
Findings
Ne/H abundance ratio is (1.01±0.08)×10⁻⁴.
Gas-phase S/H abundance ratio is (7.68±0.30)×10⁻⁶.
Ne++ ions are present up to 12' from theta1OriC, indicating extended high-ionization regions.
Abstract
We present Spitzer Space Telescope observations of 11 regions SE of the Bright Bar in the Orion Nebula, along a radial from the exciting star theta1OriC, extending from 2.6 to 12.1'. Our Cycle 5 programme obtained deep spectra with matching IRS short-high (SH) and long-high (LH) aperture grid patterns. Most previous IR missions observed only the inner few arcmin. Orion is the benchmark for studies of the ISM particularly for elemental abundances. Spitzer observations provide a unique perspective on the Ne and S abundances by virtue of observing the dominant ionization states of Ne (Ne+, Ne++) and S (S++, S3+) in Orion and H II regions in general. The Ne/H abundance ratio is especially well determined, with a value of (1.01+/-0.08)E-4. We obtained corresponding new ground-based spectra at CTIO. These optical data are used to estimate the electron temperature, electron density, optical…
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