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Filling a 40-Year Observational Gap: Forbidden Pure Rotational Spectrum of the H₃⁺ Ion Detected for the First Time

Source: IT之家 Published: Science & Research 中文
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Using the James Webb Space Telescope (JWST), astronomers have detected for the first time the forbidden rotational spectrum of the trihydrogen cation (H₃⁺) in the HH 168 object in Cepheus A West — a spectrum predicted theoretically about forty years ago by Pan and Oka (1986) but never previously observed in the laboratory or in astronomical observations. With Webb's mid-infrared instruments, the team captured 26 relevant spectral lines with significance above 5σ, the strongest at 16.325 microns, indicating gas at temperatures of at least about 2,000 Kelvin, and they also found spectra of the same type in archival Webb data of HH 211.

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Filling a 40-Year Observational Gap: The Forbidden Pure Rotational Spectrum of the Trihydrogen Cation H₃⁺ Captured for the First Time

IT之家, November 14 — Astronomers using the James Webb Space Telescope (JWST) have detected, for the first time, the forbidden rotational spectrum of the trihydrogen cation (H₃⁺) predicted by theory about forty years ago, in the HH 168 object in Cepheus A West.

Pan and Oka calculated the forbidden rotational spectrum of H₃⁺ in 1986 and proposed that it could be used to detect interstellar H₃⁺. In the roughly forty years since, this type of spectrum had never been detected in the laboratory or in astronomical observations. This study reports the first detection of this type of spectrum in HH 168 using the Webb Space Telescope.

IT之家 note: "Forbidden" is a spectroscopy term referring to a situation where a certain energy-level transition is restricted by specific rules and cannot absorb or emit light through the most conventional, most likely pathway. Atoms and molecules can only exist in certain specific energy states; changing from one state to another is called a "transition." Emitting light means that when it transitions from a higher energy level to a lower one, it releases energy in the form of photons.

In quantum mechanics there are "selection rules" that, based on conditions such as angular momentum and symmetry, restrict which transitions can occur. The most common type is called "electric dipole transition"; transitions that do not meet the corresponding conditions are called "forbidden transitions."

The equilibrium structure of the trihydrogen cation (H₃⁺) is a symmetric equilateral triangle with no permanent electric dipole moment—that is, it lacks the kind of charge-distribution asymmetry that normally favors the production of rotational radiation. Therefore, under the ideal rigid-structure approximation, it has no ordinary electric dipole pure rotational spectrum.

But real molecules are not completely rigid; centrifugal distortion caused by rotation can induce a tiny electric dipole moment, allowing the originally suppressed rotational transitions to produce a faint spectrum. Such faint signals had previously been neither measured in the laboratory nor observed in astronomical objects. The related research was made public on the preprint platform arXiv on November 6.

Using the Webb Space Telescope's mid-infrared instrument, astronomers captured a total of 26 related spectral lines with significance exceeding 5σ across two observation regions, with the strongest one at 16.325 micrometers. The spatial distribution and velocities of these signals are consistent with high-temperature hydrogen molecules impacted by shocks, indicating that they come from hot molecular gas. Analysis shows that the gas temperature exciting these signals is at least about 2,000 Kelvin.

The team also found spectra of the same type in Webb archival data of HH 211. This means the new method is expected to help researchers trace the ionization and chemical changes of interstellar gas after it is shocked.

  • First detection of the forbidden rotational spectrum of H + 3 : JWST/MIRI observations of the shocked gas in HH 168

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