Cm-wavelength observations of MWC758: resolved dust trapping in a vortex and intra-cavity signal

The large crescents imaged by ALMA in transition disks suggest that azimuthal dust trapping concentrates the larger grains, but centimetre-wavelengths continuum observations are required to map the distribution of the largest observable grains. A preliminary detection at ~1cm of an unresolved clump along the outer ring of MWC758 (Clump1), and buried inside more extended sub-mm continuum, motivates followup VLA observations. Deep multiconfiguration integrations reveal the morphology of Clump 1 and additional cm-wave components which we characterize via comparison with a deconvolution of recent 342~GHz data (~1mm). Clump1, which concentrates ~1/3 of the whole disk flux density at ~1cm, is resolved as a narrow arc with a deprojected aspect ratio chi>5.3, and with half the azimuthal width than at 342 GHz. The spectral trends in the morphology of Clump1 are quantitatively consistent with the Lyra-Lin prescriptions for dust trapping in an anticyclonic vortex, provided with a very elongated chi~16 and cold T~20K vortex. Such physical conditions imply an appreciably optically thick continuum even in cm-waves. A secondary and shallower peak at 342GHz is ~twice fainter relative to Clump1 at 33GHz. Clump2 appears to be less efficient at trapping large grains. We marginally detect intra-cavity signal at 33GHz, which we propose could be due to spinning dust radiation (also called anomalous or excess microwave emission, AME or EME).