C$^{18}$O emission as an effective measure of gas masses of
  protoplanetary disks
  Many astrochemical models of observed CO isotopologue line emission, earlier
considered a good proxy measure of H$_2$ and hence disk gas mass, favor large
deviations in the carbon and oxygen gas phase abundances and argue that severe
gas phase CO depletion makes it a poor mass tracer. Here, we show that
C$^{18}$O line emission is an effective measure of the gas mass, and despite
its complex chemistry, a possibly better tracer than HD. Our models are able to
reproduce C$^{18}$O emission from recent ALMA surveys and the TW Hya disk to
within a factor of $\sim 2-3$ using carbon and oxygen abundances characteristic
of the interstellar medium (C/H$=1.4 \times 10^{-4}$; O/H$=3.2\times 10^{-4}$)
without having to invoke unusual chemical processing. Our gas and dust disk
structure calculations considering hydrostatic pressure equilibrium and our
treatment of the CO conversion on grains are primarily responsible for the very
different conclusions on disk masses and CO depletion. As did previous studies,
we find that a gas phase C/O of $\sim 1-2$ can explain observed hydrocarbon
emission from the TW Hya disk; but significantly, we find that CO isotopologue
emission is only marginally affected by the C/O ratio. We therefore conclude
that C$^{18}$O emission provides estimates of disk masses that are uncertain
only to within a factor of a few, and describe a simplified modeling procedure
to obtain gas disk masses from C$^{18}$O emission lines.
