Multi-wavelength ALMA Imaging of HD 34282: Dust-trapping Signatures of a Vortex Candidate
Azimuthal arcs in millimeter continuum emission from protoplanetary disks are often attributed to dust-trapping vortices, but definitive observational confirmation of vortices remains lacking. We present sub-0.1" resolution ALMA continuum observations of the HD 34282 disk at 0.9, 1.3, 2.1, and 3.1 mm. These observations resolve a bright azimuthal arc superposed on a compact double-gap, triple-ring morphology, most clearly at shorter wavelengths, and enable us to probe the physical origin of the arc. It exhibits a lower spectral index than the surrounding rings, consistent with enhanced grain growth and/or higher dust surface density of a dust-trapping vortex. Its azimuthal width decreases with increasing wavelength, consistent with tighter confinement of larger grains, or lower optical depths at longer wavelengths. These observations probe dust with Stokes numbers St < 0.03. Vortex models predict negligible peak shifts in this regime, consistent with the 1.3 to 3.1 mm data. At 0.9 mm, however, the arc peak is offset by 15 +/- 4 degree in the direction of disk rotation relative to longer wavelengths, and the near-side ring emission is locally dimmer compared to the far-side, likely reflecting optical-depth or temperature effects. These observations are consistent with azimuthal dust trapping, potentially associated with a vortex-induced pressure maximum.