Detection of Substructures in Young Transition Disk WL 17
  WL 17 is a young transition disk in the Ophiuchus L1688 molecular cloud
complex. Even though WL 17 is among the brightest disks in L1688 and massive
enough to expect dust self-scattering, it was undetected in polarization down
to ALMA's instrument sensitivity limit. Such low polarization fractions could
indicate unresolved polarization within the beam or optically thin dust
emission. We test the latter case by combining the high sensitivity 233 GHz
Stokes I data from the polarization observations with previous ALMA data at 345
GHz and 100 GHz. We use simple geometric shapes to fit the observed disk
visibilities in each band. Using our simple models and assumed dust temperature
profiles, we estimate the optical depth in all three bands. The optical depth
at 233 GHz peaks at $\tau_{233} \sim 0.3$, which suggests the dust emission may
not be optically thick enough for dust self-scattering to be efficient. We also
find the higher sensitivity 233 GHz data show substructure in the disk for the
first time. The substructure appears as brighter lobes along the major axis, on
either side of the star. We attempt to fit the lobes with a simple geometric
model, but they are unresolved in the 233 GHz data. We propose that the disk
may be flared at 1 mm such that there is a higher column of dust along the
major axis than the minor axis when viewed at an inclination. These
observations highlight the strength of high sensitivity continuum data from
dust polarization observations to study disk structures.
