The James Webb Space Telescope (JWST) has detected for the first time the "forbidden" rotational spectrum of the trihydrogen ion, H3+, in the Herbig-Haro object 168 (HH 168), in the Cepheus A star-forming region. The result comes from a team led by David A. Neufeld of Johns Hopkins University, using Webb's MIRI instrument, and the paper has been accepted for publication in The Astrophysical Journal Letters.
A molecule with no allowed spectrum
H3+ is considered the cornerstone of ion-molecule chemistry in interstellar space: it forms when cosmic rays ionize molecular hydrogen and kick-starts the reactions that produce most of the molecules in interstellar clouds. Its vibrational transitions have been observed for decades, but its pure rotational spectrum had never been seen — not in the laboratory, not in space. Because the molecule forms an equilateral triangle, with no permanent dipole moment, these transitions are "forbidden" by the usual rules of molecular radiation and only occur through a tiny centrifugal distortion. The possibility was calculated by Fu-Shih Pan and Takeshi Oka in 1986, 40 years ago.
26 lines in HH 168
Observing HH 168 — a jet from a young star slamming into the surrounding gas and heating it in a shock — the team identified 26 spectral lines above 5 sigma, with the strongest, the (J,K) = (5,0) to (4,3) transition at 16.325 micrometers, recorded at 72 sigma. The lines follow the velocity and spatial distribution of the hot shocked molecular hydrogen, not the ionized gas. The same signature was also found in archival MIRI data of the object HH 211, showing the phenomenon is not a one-off. The observations were made on August 1, 2026, in JWST GO program 7084, and the paper is available on arXiv (arXiv:2611.10122).
A new gauge of ionization
The H3+ lines give astronomers a new way to measure ionization in shocked molecular gas, a parameter that is hard to obtain by other means. The estimated rotational temperature is about 730 K, but the level populations point to subthermal excitation in gas hotter than 2,000 K.
Webb keeps revealing the Universe in unprecedented detail. Read also: Webb's red dots may hide primordial black holes.
Image credit: HH 211, captured by the James Webb Space Telescope — ESA/Webb, NASA, CSA, T. Ray (Dublin Institute for Advanced Studies), via Wikimedia Commons (CC BY 4.0). The same H3+ signature was also found in archival MIRI data of this object.
Sources: ScienceAlert; arXiv:2611.10122
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