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Description
Recent ALMA surveys in different star-forming regions have shown that CO emission in protoplanetary disks is much fainter than expected. Accordingly, CO-based gas masses and gas/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. The goal of this work is to employ C_2_H observations to understand whether low CO fluxes are caused by volatile depletion, or by fast gas dissipation. 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. Seven out of nine disks 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 disks. When compared with physical-chemical models run with DALI, 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/dust ratios near unity fail instead 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 disks. 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. 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. 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 disks. This result has implications for disk-evolution and planet-formation theories, as disk 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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