The H₃⁺ ion holds a special place in astrochemistry. It is regarded as the cornerstone of ion-molecule chemistry in the interstellar medium. 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. Its pure rotational spectrum, however, had remained unseen.
Now it has been detected for the first time. The team used MIRI, the Mid-Infrared Instrument on the James Webb Space Telescope. The result is so far available as an arXiv preprint.
Why the rotational lines of H₃⁺ are so hard to see
Most molecules have a rotational spectrum because their polar bonds give them a dipole moment. For H₃⁺ such transitions are "forbidden". They become possible only through centrifugal distortion: a rapidly rotating molecule slightly changes its shape. That makes the lines extremely faint. They had not been observed in the laboratory or in any astronomical source.
Where and what they found
The target was HH 168 in Cepheus A West. It is a Herbig-Haro object, a region where a jet from a young star slams into surrounding gas and heats it with a shock. In two observed fields, astronomers detected 26 lines above 5σ. The strongest is the (J,K) = (5,0) → (4,3) transition at 16.325 μm.
The H₃⁺ lines share the velocity and spatial distribution of the hot shocked molecular hydrogen, H₂. They do not match the ionized gas. So the emission comes from the hot molecular gas.
A rotation diagram gives a temperature of about 730 K. But the level populations are not thermal. The authors find this is best explained by subthermal excitation in gas hotter than 2000 K. In other words, the levels do not have time to settle into a thermal distribution.
Two shock models
The observations were compared with models of shocks propagating into mildly irradiated molecular gas. The models were constrained by the H₂ lines.
- A dissociative J-type shock, in which molecules are partly destroyed, predicts H₃⁺ emission within a factor of ~1.5 of the observed value.
- A nondissociative C-type shock, in which molecules survive, fits the H₂ lines better. But it underpredicts the H₃⁺ emission by a factor of 5-6.
How solid is the result
The detection itself is secure: 26 lines above 5σ. The interpretation is less certain. The excitation of H₃⁺ by energetic collisions is poorly known, so the comparison with models rests on incomplete knowledge of these processes. The factor of 5-6 shortfall for the C-type shock therefore does not prove that such a shock is ruled out in HH 168. The work is also a preprint that has not yet been peer reviewed.
What's new
This is the first detection of the H₃⁺ rotational spectrum in an astronomical source. The same forbidden spectrum appears in archival MIRI data of the HH 211 bow shock, so HH 168 is not a one-off.
The main consequence is practical. H₃⁺ lines become a new probe of ionization in shock-heated molecular gas. Ionization governs how the gas couples to magnetic fields and what type of shock forms. It can now be measured through an ion that is central to the chemistry of this gas. For this to work reliably, the way energetic collisions excite H₃⁺ needs to be better understood.
