# JWST detects forbidden rotational lines of H3+ for the first time, in the HH 168 shock

- Published: 2026-11-13 05:44 UTC
- Section: Sky & Space › Astronomy
- Tags: H3+ rotational spectrum, forbidden transitions, JWST MIRI, HH 168, Cepheus A West, HH 211, Herbig-Haro object, interstellar chemistry, shocked molecular gas
- URL: https://realtech.media/news/rt-34e5acc444953cdb6153fa2e_jwst_detects_forbidden/

**JWST/MIRI recorded 26 lines of the pure rotational spectrum of H3+ toward HH 168, unseen before in a lab or in space. They give a new probe of ionization in shocked gas.**

![JWST detects forbidden rotational lines of H3+ for the first time, in the HH 168 shock](https://realtech.media/media/2026/11/08da8ffa458a0308.webp)
*ScienceAlert*

A team using the MIRI instrument of the James Webb Space Telescope has detected the pure rotational spectrum of the H3+ ion toward the Herbig-Haro object HH 168 in Cepheus A West, according to a paper on arXiv (Cornell University) [1]. Twenty-six lines exceed 5 σ in one or both of two observed fields. The strongest is the (J,K) = (5,0) → (4,3) transition at 16.325 μm [1]. The work was led by David Neufeld of Johns Hopkins University.

## Why this spectrum was missing

H3+ is a cornerstone of interstellar ion-molecule chemistry. Its vibrational transitions have long been observed in absorption along many interstellar sight lines and in emission from the upper atmospheres of the giant planets [1]. The pure rotational spectrum, however, consists only of "forbidden" transitions made possible by centrifugal distortion. It had not been observed in the laboratory or in any astronomical source [1].

The ion is an equilateral triangle of three hydrogen nuclei and has no permanent dipole moment. In 1986, Fu-Shih Pan and Takeshi Oka of the University of Chicago calculated that rotation slightly distorts this shape and creates a tiny dipole, allowing very weak emission. HH 168 is a region where jets from a young star shock the surrounding molecular gas and heat it.

## What the lines show

The H3+ lines share the velocity and spatial distribution of the hot shocked H2, not of the ionized gas [1]. A rotation diagram gives a rotational temperature of about 730 K. The level populations, though, are not thermal and are best explained by subthermal excitation in gas hotter than 2000 K [1].

The authors compared the data with models of shocks propagating into mildly irradiated molecular gas. Constrained by the H2 rotational lines, a dissociative J-type shock predicts H3+ emission within a factor of about 1.5 of that observed [1]. A nondissociative C-type shock fits the H2 lines better but underpredicts the H3+ emission by a factor of 5–6. The authors note that the excitation of H3+ by energetic collisions is poorly known [1].

Archival MIRI data show the same forbidden rotational spectrum of H3+ in the HH 211 bow shock [1]. According to the authors, the H3+ lines provide a new probe of ionization in shocked molecular gas.

## Sources

1. [arXiv (Cornell University)](https://doi.org/10.48550/arXiv.2611.10122)
2. [ScienceAlert](https://www.sciencealert.com/jwst-detects-a-forbidden-signal-never-before-seen-in-the-universe-not-even-in-a-lab)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-34e5acc444953cdb6153fa2e_jwst_detects_forbidden/