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How JWST Revealed H₃⁺’s Long-Sought Rotational Spectrum
JWST/MIRI detected H₃⁺’s previously unseen pure rotational spectrum in HH 168; the lines trace hot shocked molecular hydrogen, not ionized gas. The study reports 26 H₃⁺ rotational lines detected above 5σ in one or both HH 168 fields.
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Key takeaways
- JWST/MIRI detected H₃⁺’s previously unseen pure rotational spectrum in HH 168; the lines trace hot shocked molecular hydrogen, not ionized gas.
- The study reports 26 H₃⁺ rotational lines detected above 5σ in one or both HH 168 fields.
- Shock models do not yet give a single clear answer: a J type model comes closer to the H₃⁺ brightness, while a C type model better fits the H₂ lines.
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openai.comCreate a landscape editorial hero image for this Studio Global article: How did David Neufeld, Joel Green, Paule Sonnentrucker, and Dan Watson use the James Webb Space Telescope’s MIRI spectrometer under program. Article summary: The result is a detection of H₃⁺’s previously unseen pure rotational spectrum—not the first detection of H₃⁺ itself—and its emission follows hot shocked molecular hydrogen rather than ionized gas. [9] The shock-model com. Topic tags: general, academic, education, general web, government. Style: premium digital editorial illustration, source-backed research mood, clean composition, high detail, modern web publication hero. Use reference image context only for broad subject, composition, and topical grounding; do not copy the exact image. Avoid: logos, brand marks, copyrighted characters, real person likenesses, fake screenshots, UI text, readable text, watermarks
JWST’s Mid-Infrared Instrument (MIRI) has revealed a long-sought signal from H₃⁺, the trihydrogen cation: its faint, “forbidden” pure rotational spectrum. In observations of the shocked region HH 168, the lines follow hot molecular hydrogen rather than ionized gas. The detection opens a new window on molecular shocks, but uncertain H₃⁺ collision physics means the signal is not yet a straightforward measure of ionization. 1
What JWST detected in HH 168
The research team used MIRI to observe HH 168, a shock in Cepheus A West. The observations were carried out under JWST program 7084, designed to resolve the interface between a jet and a cloud shock. 7 The study reports 26 H₃⁺ rotational lines detected above five times the estimated statistical uncertainty in one or both of two fields.
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This is the first reported detection of H₃⁺’s pure rotational spectrum—not the first detection of H₃⁺ itself. The important clue is where the emission appears: the lines share the spatial distribution and velocity of hot, shocked molecular hydrogen (H₂), rather than those of ionized gas. That connects the signal to the molecular part of the shock. 1
Why H₃⁺’s rotational spectrum is called “forbidden”
H₃⁺ is important in interstellar chemistry because it can transfer a proton to other molecules, initiating reaction pathways that produce new molecular ions. It helps drive chemical complexity in space; it does not directly create complex organic molecules by itself. 
The difficulty in detecting its pure rotational spectrum comes from the ion’s symmetry. In its equilibrium shape, the three hydrogen nuclei form an equilateral triangle. This arrangement gives H₃⁺ no permanent electric dipole moment, so the usual electric-dipole rotational transitions are strongly suppressed.
“Forbidden” does not mean impossible. Rotation can distort the molecule through centrifugal forces, allowing extremely weak transitions that are absent in the idealized, perfectly symmetric case. Those faint lines are what make the JWST result significant: the observations reveal emission that had been difficult to detect.
What the shock models say—and what they do not
The study compares two broad kinds of shock models. In a C-type shock, physical conditions change more smoothly; in a dissociative J-type shock, molecules are destroyed at the shock and can reform later in the post-shock gas. 17
The models do not offer a simple winner. The C-type model fits the observed H₂ lines better, but predicts H₃⁺ emission about five to six times fainter than observed. The J-type model predicts H₃⁺ emission closer to the observed strength—within roughly a factor of 1.5—but does not fit the H₂ lines as well. 1
That tension matters. The study notes that excitation of H₃⁺ by energetic collisions is poorly known, making it difficult to interpret the line strength decisively or use it as a calibrated measure of ionization. 1
A second setting: archival observations of HH 211
The researchers also report finding H₃⁺’s forbidden rotational spectrum in archival MIRI data from the HH 211 bow shock. 35 This additional detection suggests that the signal is not unique to HH 168 and supports looking for it in other shocked molecular outflows.
The clearest next targets would be regions with strong warm-H₂ emission, where shocked molecular gas is prominent. But detecting H₃⁺ is only a first step: better data on how collisions excite the ion are needed before its lines can reliably quantify ionization in shocked gas. 1
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What is the short answer to "How JWST Revealed H₃⁺’s Long-Sought Rotational Spectrum"?
JWST/MIRI detected H₃⁺’s previously unseen pure rotational spectrum in HH 168; the lines trace hot shocked molecular hydrogen, not ionized gas.
What are the key points to validate first?
JWST/MIRI detected H₃⁺’s previously unseen pure rotational spectrum in HH 168; the lines trace hot shocked molecular hydrogen, not ionized gas. The study reports 26 H₃⁺ rotational lines detected above 5σ in one or both HH 168 fields.
What should I do next in practice?
Shock models do not yet give a single clear answer: a J type model comes closer to the H₃⁺ brightness, while a C type model better fits the H₂ lines.
Sources
arxiv.orgFirst detection of the forbidden rotational spectrum of H+3{}_{3}^{+}: JWST/MIRI observations of the shocked gas in HH 168
stsci.edu7084 - Measuring the Thickness of a J-Shock in Cep A West
newsminimalist.comJWST Detects First-Ever 'Forbidden' Signal from Key Molecule ...