PDRs4All
Bibliographic record
Abstract
Context . The methylidyne cation (CH + ) and the methyl cation (CH 3 + ) are building blocks of organic molecules in the ultraviolet (UV) irradiated gas, yet their coupled formation and excitation mechanisms mostly remain unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules. Aims . Our goal is to use the first detection of CH + and CH 3 + infrared rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506 to probe their formation and excitation mechanisms and constrain the physico-chemical conditions of the environment. Methods . We used spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments on board JWST to study the infrared CH + and CH 3 + spatial distribution at very small scales (down to 0.1′′) and compared it to excited H 2 emission. We studied their excitation in detail, and in the case of CH + , we compared the observed line intensities with chemical formation pumping models based on recent quantum dynamical calculations. Throughout this study, we compare the emission of these molecules in two environments: the Bar a photodissociation region – and a protoplanetary disk (d203-506), both of which are irradiated by the Trapezium cluster. Results . We detected CH + and CH 3 + vibrationally excited emission both in the Bar and d203-506. These emissions originate from the same region as highly excited H 2 (high rotational and rovibrational levels) and correlate less with the lower rotational levels of H 2 ( J ′ < 5) or the emission of aromatic and aliphatic infrared bands. Our comparison between the Bar and d203-506 revealed that both CH + and CH 3 + excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH + and CH 3 + rovibrational lines is around T ∼ 1500 K in the Bar and T ∼ 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1% of the total known (CH + ) and expected (CH 3 + ) column densities. These different results show that CH + and CH 3 + level populations strongly deviate from local thermodynamical equilibrium. The CH + rovibrational supra-thermal emission ( v = 1 and v = 2) can be explained by chemical formation pumping with excited H 2 via C + + H 2 * = CH + + H. The difference in the population distribution of the H 2 * energy levels between the Orion Bar and d203-506 then result in different excitation temperatures. These results support a gas phase formation pathway of CH + and CH 3 + via successive hydrogen abstraction reactions. However, we do not find any evidence of CH 3 + emission in the JWST spectrum, which may be explained by the fact its spectroscopic signatures could be spread in the JWST spectra. Finally, the observed CH + intensities coupled with a chemical formation pumping model provide a diagnostic tool to trace the local density. Conclusions . Line emission from vibrationally excited CH + and CH 3 + provides new insight into the first steps of hydrocarbon gas-phase chemistry in action. This study highlights the need for extended molecular data of detectable molecules in the interstellar medium in order to analyze the JWST observations.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.003 |
| Meta-epidemiology (narrow) | 0.002 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.006 | 0.002 |
| Open science | 0.003 | 0.004 |
| Research integrity | 0.003 | 0.002 |
| Insufficient payload (model declined to judge) | 0.657 | 0.653 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".