Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
Bibliographic record
Abstract
We have used archival far-ultraviolet spectra from observations made by HST STIS and FUSE to determine the column densities and rotational excitation temperatures for carbon monoxide and molecular hydrogen, respectively, along 23 sight lines to Galactic O and B stars. The reddening values range from E ( B - V ) = 0.07 to 0.62, sampling the diffuse to translucent interstellar medium (ISM). We find that the H 2 column densities range from 5 × 10 18 to 8 × 10 20 cm -2 and the CO from upper limits around 2 × 10 12 cm -2 to detections as high as 1.4 × 10 16 cm -2 . CO increases with increasing H 2 , roughly following a power law of factor ~2. The CO/H 2 column density ratio is thus not constant, ranging from 10 -7 to 10 -5 , with a mean value of 3 × 10 -6 . The sample segregates into "diffuse" and "translucent" regimes, the former with molecular fraction ≲0.25 and A V / d < 1 mag kpc -1 . The mean CO/H 2 for these two regimes are 3.6 × 10 -7 and 9.3 × 10 -6 , respectively, significantly lower than the canonical dark cloud value of 10 -4 . Six sight lines show the isotopic variant 13 CO, and the isotopic ratio we observe (~50-70) is consistent with, if perhaps a little below, the average 12 C/ 13 C for the ISM at large. The average H 2 rotational excitation temperature is 74 ± 24 K, agreeing well with previous studies, and the average CO temperature is 4.1 K, with some sight lines showing temperatures as high as 6.4 K. The higher excitation CO is observed with higher column densities, consistent with the effects of photon trapping in clouds with densities in the 20-100 cm -3 range. We discuss the implications for the structure of the diffuse/translucent regimes of the ISM and the estimation of molecular mass in galaxies.
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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 | 0.000 |
| 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) | 0.001 | 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".