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The brightest transitional protoplanetary disks are often azimuthallyasymmetric: their mm-wave thermal emission peaks strongly on one side. Dustoverdensities can exceed $\sim$100:1, while gas densities vary by factors lessthan a few. We propose that these remarkable ALMA observations---which may bearon how planetesimals form---reflect a gravitational global mode in the gasdisk. The mode is (1) fast---its pattern speed equals the disk's mean Keplerianfrequency; (2) of azimuthal wavenumber $m=1$, displacing the host star from thebarycenter; and (3) Toomre-stable. We solve for gas streamlines including theindirect stellar potential in the frame rotating with the pattern speed, underthe drastic simplification that gas does not feel its own gravity. Nearco-rotation, the gas disk takes the form of a horseshoe-shaped annulus. Dustparticles with aerodynamic stopping times much shorter or much longer than theorbital period are dragged by gas toward the horseshoe center. For intermediatestopping times, dust converges toward a $\sim$45$^\circ$-wide arc on theco-rotation circle. Particles that do not reach their final accumulation pointswithin disk lifetimes, either because of gas turbulence or long particle drifttimes, conform to horseshoe-shaped gas streamlines. Our mode is notself-consistent because we neglect gas self-gravity; still, we expect thattrends between accumulation location and particle size, similar to those wehave found, are generically predicted by fast modes and are potentiallyobservable. Unlike vortices, global modes are not restricted in radial width tothe pressure scale height; their large radial and azimuthal extents may bettermatch observations.

Fast Modes and Dusty Horseshoes in Transitional Disks