2019A&A...631A..69M

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 C₂H.
Aims. The goal of this work is to use C₂H observations to understand whether low CO fluxes are caused by volatile depletion or by fast gas dissipation.
Methods. We present ALMA Cycle 4 C₂H (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 C₂H emission is determined and compared to previous CO isotopologue observations and physical-chemical model predictions.
Results. Seven out of nine discs are detected in C₂H, whose line emission is almost as bright as ¹³CO. All detections are significantly brighter than the typical sensitivity of the observations, hinting at a bimodal distribution of the C₂H line intensities. This conclusion is strengthened when our observations are compared with additional C₂H observations of other discs. When compared with physical-chemical models, the observed C₂H 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 C₂H line luminosity. A steeper than linear correlation between C₂H and CN emission line is found for the Lupus discs. This is linked to the fact that C₂H emission lines are affected more strongly by [C]/[O] variations than CN lines. Ring-like structures are detected both in C₂H 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 C₂H and CN with the location of the peak intensity coinciding, within our 30 au resolution.
Conclusions. Our new ALMA C₂H 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.