Lupus disks with faint CO isotopologues: low gas/dust or large carbon
  depletion?
  An era has started in which gas and dust can be observed independently in
protoplanetary disks, thanks to the recent surveys with ALMA. The first
near-complete high-resolution disk survey in both dust and gas in a single
star-forming region has been carried out in Lupus, finding surprisingly low
gas/dust ratios. The goal of this work is to fully exploit CO isotopologues
observations in Lupus, comparing them with physical-chemical model results, in
order to obtain gas masses for a large number of disks. We have employed
physical-chemical models to analyze continuum and CO isotopologues observations
of Lupus disks, including isotope-selective processes and freeze-out. Employing
also the ALMA 13CO-only detections, disk gas masses have been calculated for a
total of 34 sources, expanding the sample of 10 disks studied by Ansdell et al.
(2016), where also C18O was detected. We confirm that overall gas-masses are
very low, often smaller than 1 $M_{\rm J}$, if volatile carbon is not depleted.
Accordingly, global gas/dust ratios predominantly between 1 and 10. Low
CO-based gas masses and gas/dust ratios may indicate rapid loss of gas, or
alternatively chemical evolution, e.g. via sequestering of carbon from CO to
more complex molecules, or carbon locked up in larger bodies. Current ALMA
observations cannot distinguish between these two hypotheses. We have simulated
both scenarios, but chemical model results do not allow us to rule out one of
the two. Assuming that all Lupus disks have evolved mainly due to viscous
processes over the past few Myr, the observed correlation between the current
mass accretion rate and dust mass found by Manara et al. (2016) implies a
constant gas-to-dust ratio, which is close to 100 based on the observed $M_{\rm
disk}/\dot{M}_{\rm acc}$ ratio. This in turn points to a scenario in which
carbon depletion is responsible for the low CO isotopologue line luminosities.
