The Disk Substructures at High Angular Resolution Project (DSHARP): III.
  Spiral Structures in the Millimeter Continuum of the Elias 27, IM Lup, and
  WaOph 6 Disks
  We present an analysis of ALMA 1.25 millimeter continuum observations of
spiral structures in three protoplanetary disks from the Disk Substructures at
High Angular Resolution Project. The disks around Elias 27, IM Lup, and WaOph 6
were observed at a resolution of $\sim40-60$ mas ($\sim6-7$ au). All three
disks feature $m=2$ spiral patterns in conjunction with annular substructures.
Gas kinematics established by $^{12}$CO $J=2-1$ observations indicate that the
continuum spiral arms are trailing. The arm-interarm intensity contrasts are
modest, typically less than 3. The Elias 27 spiral pattern extends throughout
much of the disk, and the arms intersect the gap at $R\sim69$ au. The spiral
pattern in the IM Lup disk is particularly complex-it extends about halfway
radially through the disk, exhibiting pitch angle variations with radius and
interarm features that may be part of ring substructures or spiral arm
branches. Spiral arms also extend most of the way through the WaOph 6 disk, but
the source overall is much more compact than the other two disks. We discuss
possible origins for the spiral structures, including gravitational instability
and density waves induced by a stellar or planetary companion. Unlike the
millimeter continuum counterparts of many of the disks with spiral arms
detected in scattered light, these three sources do not feature high-contrast
crescent-like asymmetries or large ($R>20$ au) emission cavities. This
difference may point to multiple spiral formation mechanisms operating in
disks.
