Dust Polarization Toward Embedded Protostars in Ophiuchus with ALMA.
  III. Survey Overview
  We present 0.25 arcsec resolution (35 au) ALMA 1.3 mm dust polarization
observations for 37 young stellar objects (YSOs) in the Ophiuchus cloud. These
data encompass all the embedded protostars in the cloud and several Flat and
Class II objects to produce the largest, homogeneous study of dust polarization
on disk scales to date. The goal of this study is to study dust polarization
down to disk scales. We find that 14/37 (38%) of the YSOs are detected in
polarization. Nine of these sources have uniform polarization angles and four
sources have azimuthal polarization. The sources with uniform polarization tend
to have steeper inclinations (> 60 degree) than those with azimuthal
polarization (< 60 degree). The majority (9/14) of the detected sources have
polarization morphologies and disk properties consistent with dust
self-scattering in optically thick disks. The remaining sources may be instead
tracing magnetic fields. Their inferred field directions from rotating the
polarization vectors by 90 degree are mainly poloidal or hourglass shaped. We
find no evidence of a strong toroidal field component toward any of our disks.
For the 23 YSOs that are undetected in polarization, roughly half of them have
3-sigma upper limits of < 2%. These sources also tend to have inclinations < 60
degree and they are generally compact. Since lower inclination sources tend to
have azimuthal polarization, these YSOs may be undetected in polarization due
to unresolved polarization structure within our beam. We propose that disks
with inclinations > 60 degree are the best candidates for future polarization
studies of dust self-scattering as these systems will generally show uniform
polarization vectors that do not require very high resolution to resolve. We
release the continuum and polarization images for all the sources with this
publication. Data from the entire survey can be obtained from Dataverse.
