JWST detects forbidden rotational signal from trihydrogen cation

JWST detects forbidden signal from trihydrogen cation

Astronomers led by David Neufeld of Johns Hopkins University have used the JWST to find a faint rotational light from the trihydrogen cation, H3+, predicted in 1986 and never seen before, even in laboratories. The signal came from gas thousands of light-years away, in a newborn star's shock waves.

H3+ is among the most important molecules in space. It underpins almost all gas-phase interstellar chemistry, including the formation of the complex organic molecules that supply the ingredients for life. It is often called "the molecule that made the Universe." It has been detected lurking in interstellar clouds and is partially responsible for Jupiter's auroral glow.

Why the light was thought impossible

H3+ has three hydrogen nuclei sharing just two electrons. They form an equilateral triangle, so the molecule has no permanent dipole moment and no allowed pure rotational spectrum. As the molecule spins, its electrical charge remains evenly distributed, with no shifting imbalance to generate the electromagnetic radiation that molecules ordinarily emit as they rotate.

In 1986, astrophysicists Fu-Shih Pan and Takeshi Oka of the University of Chicago calculated that trihydrogen should be able to emit a very faint amount of light as it rotates. As the molecule spins, centrifugal forces distort its perfectly symmetrical shape ever so slightly. This creates a tiny electric dipole moment, allowing it to emit radiation after all. The resulting transitions are so extraordinarily weak that they are considered "forbidden" – not because they violate the laws of physics, but because they are prohibited by the usual rules governing molecular radiation.

For four decades, that prediction remained unconfirmed. The signal was simply too faint to detect, even in laboratory experiments.

What Neufeld's team found

The breakthrough came from pairing the JWST with a Herbig-Haro object, one of the galaxy's most spectacular phenomena. These form when baby stars throw violent outbursts. A forming star feeds on material swirling around it, but not all of that material ends up in the star. Powerful magnetic fields help fling some of it back out into space in tightly focused jets, traveling at hundreds of kilometers per second. When those jets slam into the surrounding molecular cloud, they generate immense shock waves, heating the gas to thousands of degrees and causing it to blaze with light.

Neufeld and his colleagues were examining JWST observations of an object named Herbig-Haro 168, or HH 168, when something was a little peculiar. Buried among the infrared emissions from the hot, shocked gas were a series of extraordinarily faint spectral lines. Their wavelengths matched those of the forbidden rotational transitions of the trihydrogen cation, calculated by Pan and Oka 40 years earlier.

The team identified 26 distinct lines, all detected with high statistical confidence. The strongest of those lines registered a staggering 72 sigma. The emission closely tracked the emission of ordinary molecular hydrogen (H₂), suggesting that both molecules were responding to the same violent conditions.

When the researchers tried to reproduce the observations using shock wave models, neither approach worked perfectly. One model did a good job of reproducing the emission from ordinary molecular hydrogen, but underestimated the trihydrogen signal by a factor of five or six. Another came much closer to matching the trihydrogen emission, but relied on conditions outside the range for which the model had been validated.

When the team examined archival JWST observations of another Herbig-Haro object, HH 211, they found the same telltale emission there, too.

The research, which has been accepted for publication in The Astrophysical Journal Letters, is available on preprint server arXiv.

Sources

• tech.yahoo.com: JWST Detects a 'Forbidden' Signal Never Before Seen in The Universe – Not Even in a Lab
• sciencealert.com: JWST Detects a 'Forbidden' Signal Never Before Seen in The Universe – Not Even in a Lab : ScienceAlert
• journalarta.com: JWST Detects Forbidden Signal From Molecule Predicted in 1986 — JournalArta Global

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