  Accurate disk mass measurements are necessary to constrain disk evolution and
the timescale of planet formation, but such measurements are difficult to make
and are very dependent on assumptions. Here we look at the assumption that the
disk is optically thin at radio wavelengths and the effect of this assumption
on measurements of disk dust mass. We model the optical to radio spectral
energy distributions (SEDs) of 41 protoplanetary disks located in the young
(~1-3 Myr old) Lupus star-forming region, including 0.89 mm, 1.33 mm, and 3 mm
flux densities when available. We measure disk dust masses that are ~1.5-6
times higher than when using the commonly adopted disk dust mass equation under
the assumption of optically thin emission in the (sub-)millimeter. The cause of
this discrepancy is that most disks are optically thick at millimeter
wavelengths, even up to 3 mm, demonstrating that observations at longer
wavelengths are needed to trace the fully optically thin emission of disks.
