  The brightest transitional protoplanetary disks are often azimuthally
asymmetric: their mm-wave thermal emission peaks strongly on one side. Dust
overdensities can exceed $\sim$100:1, while gas densities vary by factors less
than a few. We propose that these remarkable ALMA observations---which may bear
on how planetesimals form---reflect a gravitational global mode in the gas
disk. The mode is (1) fast---its pattern speed equals the disk's mean Keplerian
frequency; (2) of azimuthal wavenumber $m=1$, displacing the host star from the
barycenter; and (3) Toomre-stable. We solve for gas streamlines including the
indirect stellar potential in the frame rotating with the pattern speed, under
the drastic simplification that gas does not feel its own gravity. Near
co-rotation, the gas disk takes the form of a horseshoe-shaped annulus. Dust
particles with aerodynamic stopping times much shorter or much longer than the
orbital period are dragged by gas toward the horseshoe center. For intermediate
stopping times, dust converges toward a $\sim$45$^\circ$-wide arc on the
co-rotation circle. Particles that do not reach their final accumulation points
within disk lifetimes, either because of gas turbulence or long particle drift
times, conform to horseshoe-shaped gas streamlines. Our mode is not
self-consistent because we neglect gas self-gravity; still, we expect that
trends between accumulation location and particle size, similar to those we
have found, are generically predicted by fast modes and are potentially
observable. Unlike vortices, global modes are not restricted in radial width to
the pressure scale height; their large radial and azimuthal extents may better
match observations.
