The Eccentric Cavity, Triple Rings, Two-Armed Spirals, and Double Clumps
  of the MWC 758 Disk
  Spatially resolved structures in protoplanetary disks hint at unseen planets.
Previous imaging observations of the transitional disk around MWC 758 revealed
an inner cavity, a ring-like outer disk, emission clumps, and spiral arms, all
possibly generated by companions. We present ALMA dust continuum observations
of MWC 758 at 0.87 millimeter (mm) wavelength with 43$\times$39 mas angular
resolution (6.9$\times$6.2 AU) and 20 $\mu$Jy beam$^{-1}$ rms. The central
sub-mm emission cavity is revealed to be eccentric; once deprojected, its outer
edge can be well-fitted by an ellipse with an eccentricity of 0.1 and one focus
on the star. The broad ring-like outer disk is resolved into three narrow rings
with two gaps in between. The outer two rings tentatively show the same
eccentricity and orientation as the innermost ring bounding the inner cavity.
The two previously known dust emission clumps are resolved in both the radial
and azimuthal directions, with radial widths equal to $\sim$4$\times$ the local
scale height. Only one of the two spiral arms previously imaged in
near-infrared (NIR) scattered light is revealed in ALMA dust emission, at a
slightly larger stellocentric distance owing to projection effects. We also
submit evidence of disk truncation at $\sim$100 AU based on comparing NIR
imaging observations with models. The spirals, the north clump, and the
truncated disk edge are all broadly consistent with the presence of one
companion exterior to the spirals at roughly 100 AU.
