The mid-infrared aliphatic bands associated with complex hydrocarbons
Bibliographic record
Abstract
Context. The mid-infrared emission features commonly attributed to polycyclic aromatic hydrocarbons (PAHs) vary in profile and peak position. These profile variations form the basis of their classification: Classes A, B, C reflect profiles with increasing central wavelength while Class D has similar central wavelength as Class B but a similar broad shape as Class C. A well-known empirical relationship exists between the central wavelength of these emission features in circumstellar environments and the effective temperature of their central stars. One posited explanation is that the presence of aliphatic hydrocarbons contributes to the variations in the shapes and positions of the features. Aims. We aim to test this hypothesis by characterising the aliphatic emission bands at 6.9 and 7.25 µm and identifying relationships between these aliphatic bands and the aromatic features. Methods. We have examined 5–12 µm spectra of 63 astronomical sources exhibiting hydrocarbon emission which have been observed by ISO/SWS, Spitzer/IRS, and SOFIA/FORCAST. We measured the intensities and central wavelengths of the relevant features and classified the objects based on their 7–9 µm emission complex. We examined correlations between the intensities and central wavelengths of the features, both aliphatic and aromatic, and investigated the behaviour of the aliphatic features based on the object type and hydrocarbon emission class. Results. The presence of the 6.9 and 7.25 µm aliphatic bands depends on (aromatic) profile class, with aliphatic features detected in all Class D sources, 26% of the Class B sources, and no Class C sources. The peak position of the aliphatic features varies, with more variability seen in Class B sources than Class D sources, mimicking the degree of variability of the aromatic features in these classes. Variations are observed within Class D 6–9 µm profiles, but are significantly smaller than those in Class B. While a linear combination of Classes B and C emission can reproduce the Class D emission features at 6.2 and 7.7–8.6 µm, it cannot reproduce the aliphatic bands or the 11–14 µm hydrocarbon features. A correlation is found between the intensities of the two aliphatic bands at 6.9 and 7.25 µm, and between these aliphatic features and the 11.2 µm feature, indicating that conditions required for a population of neutral hydrocarbon particles are favourable for the presence of aliphatic material. A comparison with experimental data suggests a different assignment for the aliphatic 6.9 µm band in Class D and (some) Class B environments. Finally, we discuss evolutionary scenarios between the different classes.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
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".