An ALMA Survey of CO isotopologue emission from Protoplanetary Disks in
  Chamaeleon I
  The mass of a protoplanetary disk limits the formation and future growth of
any planet. Masses of protoplanetary disks are usually calculated from
measurements of the dust continuum emission by assuming an interstellar
gas-to-dust ratio. To investigate the utility of CO as an alternate probe of
disk mass, we use ALMA to survey $^{13}$CO and C$^{18}$O J = $3-2$ line
emission from a sample of 93 protoplanetary disks around stars and brown dwarfs
with masses from 0.03 -- 2 M$_{\odot}$ in the nearby Chamaeleon I star-forming
region. We detect $^{13}$CO emission from 17 sources and C$^{18}$O from only
one source. Gas masses for disks are then estimated by comparing the CO line
luminosities to results from published disk models that include CO freeze-out
and isotope-selective photodissociation. Under the assumption of a typical ISM
CO-to-H$_2$ ratios of $10^{-4}$, the resulting gas masses are implausibly low,
with an average gas mass of $\sim$ 0.05 M$_{Jup}$ as inferred from the average
flux of stacked $^{13}$CO lines. The low gas masses and gas-to-dust ratios for
Cha I disks are both consistent with similar results from disks in the Lupus
star-forming region. The faint CO line emission may instead be explained if
disks have much higher gas masses, but freeze-out of CO or complex C-bearing
molecules is underestimated in disk models. The conversion of CO flux to CO gas
mass also suffers from uncertainties in disk structures, which could affect gas
temperatures. CO emission lines will only be a good tracer of the disk mass
when models for C and CO depletion are confirmed to be accurate.
