Bright C<sub>2</sub>H emission in protoplanetary discs in Lupus: high volatile C/O > 1 ratios
Bibliographic record
Abstract
Context. Recent ALMA surveys in different star-forming regions have shown that CO emission in protoplanetary discs is much fainter than expected. Accordingly, CO-based gas masses and gas to dust ratios are orders of magnitude lower than previously thought. This may be explained either as fast gas dispersal, or as chemical evolution and locking up of volatiles in larger bodies leading to the low observed CO fluxes. The latter processes lead to enhanced C/O ratios in the gas, which may be reflected in enhanced abundances of carbon-bearing molecules like C 2 H. Aims. The goal of this work is to use C 2 H observations to understand whether low CO fluxes are caused by volatile depletion or by fast gas dissipation. Methods. We present ALMA Cycle 4 C 2 H ( N = 3 – 2, J = 7∕2 – 5∕2, F = 4 – 3 and F = 3 – 2) observations of a subsample of nine sources in the Lupus star-forming region. The integrated C 2 H emission is determined and compared to previous CO isotopologue observations and physical-chemical model predictions. Results. Seven out of nine discs are detected in C 2 H, whose line emission is almost as bright as 13 CO. All detections are significantly brighter than the typical sensitivity of the observations, hinting at a bimodal distribution of the C 2 H line intensities. This conclusion is strengthened when our observations are compared with additional C 2 H observations of other discs. When compared with physical-chemical models, the observed C 2 H fluxes can be reproduced only if some level of volatile carbon and oxygen depletion is allowed and [C]/[O] > 1 in the gas. Models with reduced gas-to-dust ratios near unity however fail to reproduce the observed C 2 H line luminosity. A steeper than linear correlation between C 2 H and CN emission line is found for the Lupus discs. This is linked to the fact that C 2 H emission lines are affected more strongly by [C]/[O] variations than CN lines. Ring-like structures are detected both in C 2 H and in continuum emission but, as for CN, they do not seem to be connected. The source Sz 71 shows ring-shaped emission in both C 2 H and CN with the location of the peak intensity coinciding, within our 30 au resolution. Conclusions. Our new ALMA C 2 H observations favour volatile carbon and oxygen depletion rather than fast gas dispersal to explain the faint CO observations for most of the discs. This result has implications for disc-evolution and planet-formation theories, as disc gas masses may be larger than expected if CO is considered to be the main carbon carrier in the gas phase.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".