Webb catches a molecular ion shedding the energy of its formation

Making a molecule can leave it spinning. In interstellar gas, that rotational energy usually becomes difficult to separate from the effects of later collisions. But some molecules radiate so quickly that their light retains a record of their formation.

In an arXiv preprint, David Neufeld and colleagues report such a record from HH 168, a shock in the Cepheus A West region, roughly 700 parsecs away. Their observations with the James Webb Space Telescope identify highly excited rotational emission from CH+, the methylidyne cation: a carbon atom bonded to hydrogen, carrying a positive charge.

The result is not the first detection of CH+ rotational emission. Its novelty is the detection of much higher rotational states, allowing the researchers to reconstruct how ions enter the lowest vibrational state at different rotation levels.

Identifying the molecular emitter

CH+ forms efficiently when positively charged carbon reacts with molecular hydrogen, producing CH+ and a hydrogen atom. The reaction requires an energy input of about 0.40 electronvolts. Cold gas cannot readily supply it; shocks, relative motion between ions and neutral molecules, or internally excited hydrogen can.

The team used Webb’s Mid-Infrared Instrument to observe two fields along HH 168. A family of initially unidentified spectral lines followed a regularly changing spacing characteristic of a light, rotating molecule. Fitting that sequence gave a rotational constant closely matching laboratory measurements of CH+.

Neighboring lines appeared where the identification predicted. Eleven lines exceeded five times their estimated measurement uncertainty in both fields. The emission also closely followed hot molecular hydrogen in position and matched its motion along the line of sight.

The identified sequence spans approximately 13.7–21.3 micrometres and reaches rotational level J = 32. Here, J labels rotational angular momentum. The highest detected level carries about 1.65 electronvolts of rotational energy.

To compare the lines fairly, the researchers brought their maps to a common angular resolution, measured matching regions and corrected for dust extinction using hydrogen emission.

Counting photons from a downward cascade

The crucial feature is speed. The observed CH+ levels radiate away their energy in about 6–26 milliseconds. At plausible densities in HH 168, collisions cannot populate or redistribute these levels fast enough to explain the emission. The ions must instead enter the rotational ladder already highly excited: a process called chemical pumping.

Within the lowest vibrational state, an ion descends one rotational step at a time, emitting a photon at each step. A line therefore counts all the ions entering that ladder at its upper level or above. Comparing neighboring lines reveals how the entry rate changes between levels.

The measured photon intensity falls approximately linearly as J increases. One simple explanation is a roughly equal entry rate into successive levels, extending to around J = 33.

A second description fits slightly better: a Boltzmann distribution, with an effective formation temperature near 6,200 kelvin. Neither description is excluded. The first is an empirical pattern, not a reaction calculation; the second assumes a particular distribution of reactant energies and a fixed fraction transferred into rotation.

That formation temperature is not a measurement of the gas temperature. It combines the reactants’ energy distribution with the fraction appearing as product rotation.

Energetic chemistry, with important gaps

Producing CH+ in J = 32 requires at least 2.05 electronvolts: 1.65 for rotation plus the reaction’s 0.40-electronvolt energy cost. This can come from collision energy, internal excitation of hydrogen, or both. It does not establish that all the surrounding gas is extremely hot.

If the hydrogen starts in low internal states, relative ion–hydrogen motion of at least about 15 kilometres per second could supply that energy. The observations do not resolve such drift directly or determine which shock structure supplies the energy.

There are also gaps in the molecular accounting. Lower rotational levels are poorly constrained or outside the relevant spectral coverage, and some lines overlap unrelated emission. Ions can enter the observed ladder after decaying from vibrationally or electronically excited states. Consequently, the cascade measures entry into that ladder, not the complete distribution immediately after every formation event. The inferred formation rate is a lower limit to the total.

The spectra also expose inaccuracies in predicted high-J CH+ wavelengths. Discrepancies reach the equivalent of roughly 150 kilometres per second, but the comparison with hydrogen lines indicates errors in predicted molecular energies rather than an additional gas velocity.

The immediate achievement is a measurement of how energetic reaction products shed rotation. Explaining that pattern fully will require reaction calculations at higher collision energies and shock models that connect formation, excitation and destruction.