  ALMA surveys 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 molecules like C2H. The goal of this work is to employ C2H
observations to understand whether low CO fluxes are caused by volatile
depletion, or by fast gas dissipation. We present ALMA Cycle 4 C2H observations
of a subsample of nine sources in Lupus. The integrated C2H emission is
determined and compared to previous CO observations and model predictions.
Seven out of nine disks 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. When compared with DALI 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/dust ratios fail instead
to reproduce the observations. A steeper than linear correlation between C2H
and CN emission line is found. 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. Sz 71 shows ring shaped emission in both C2H
and CN with the location of the peak intensity coinciding. 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 disks.
