Astrochemistry

Searching For Missing Interstellar Oxygen In The Far-infrared

By Keith Cowing
Status Report
astro-ph.GA
September 3, 2025
Filed under , , , , ,
Searching For Missing Interstellar Oxygen In The Far-infrared
Efficiency factors of crystalline ice grains with carbon inclusions for grain radii of 1 (red), 2 (blue), 5 (green), and 10 µm (brown). The inclusion fraction is set as 0.1. (a) The solid lines indicate the extinction efficiency factors in the 3 µm region. The scattering and absorption efficiency factors for a 2 µm grain are also shown with the dotted and dashed lines. (b) Absorption efficiency factors in the FIR. The colors are the same as in (a). — astro-ph.EP

Study of interstellar elemental depletion poses an important problem in the interstellar matter that at least a quarter of the total oxygen (∼160 ppm relative to hydrogen) is not accounted for in any known form of oxygen in the translucent or dense interstellar medium (ISM).

Detailed analysis of the absorption feature of water ice at 3 μm suggests that one fifth of the missing oxygen may reside in 3 μm-sized water ice grains. However, the 3 μm feature becomes complex and weak for grains larger than 3 μm, and thus the NIR spectroscopy is not the best means to study the presence of large ice grains reliably.

Here we show that sensitive observations of the far-infrared (FIR) features of water ice at 44 and 62 μm enable us to constrain the amount of crystalline water ice grains up to 5 μm or even larger sizes unambiguously. Oxygen is one of the key elements in the ISM chemistry, and [O I] 63 μm is a dominant cooling line in the neutral ISM. Understanding the actual form of the missing oxygen in the ISM is crucial for the study of the ISM and star-formation process.

To detect the FIR features of the crystalline water ice over the expected strong continuum, a sensitive FIR spectrograph represented by PRIMA/FIRESS is indispensable. Since the feature is broad, the low spectral resolution of R∼130 is sufficient, but accurate relative calibration better than 1% is required.

Takashi Onaka, Itsuki Sakon, Takashi Shimonishi, Mitsuhiko Honda

Comments: This paper is part of the JATIS special issue focused on the PRobe Infrared Mission for Astrophysics (PRIMA) probe mission concept. The issue is edited by Matt Griffin and Naseem Rangwala (JATIS VOL. 11 NO. 3 | July 2025)
Subjects: Astrophysics of Galaxies (astro-ph.GA); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2509.01846 [astro-ph.GA] (or arXiv:2509.01846v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2509.01846
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Journal reference: Journal of Astronomical Telescopes, Instruments, and Systems, 11, 031602 (2025)
Related DOI:
https://doi.org/10.1117/1.JATIS.11.3.031602
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Submission history
From: Takashi Onaka
[v1] Tue, 2 Sep 2025 00:08:09 UTC (405 KB)
https://arxiv.org/abs/2509.01846
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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