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 previous detection at \sim 1 cm of an unresolved clump along the outer ring of MWC 758 ( Clump 1 ) , 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 ( \sim 1 mm ) .
Clump 1 , which concentrates \sim 1 / 3 of the whole disk flux density at \sim 1 cm , is resolved as a narrow arc with a deprojected aspect ratio \chi > 5.6 , and with half the azimuthal width than at 342 GHz .
The spectral trends in the morphology of Clump 1 are quantitatively consistent with the Lyra-Lin prescriptions for dust trapping in an anticyclonic vortex , provided with porous grains ( f \sim 0.2 \pm 0.2 ) in a very elongated ( \chi \sim 14 \pm 3 ) and cold ( T \sim 23 \pm 2 K ) vortex .
The same prescriptions constrain the turbulence parameter \alpha and the gas surface density \Sigma _ { g } through \log _ { 10 } \left ( \alpha \times \Sigma _ { g } / \mathrm { g cm } ^ { -2 } \right ) \sim - 2.3 \pm 0.4 , thus requiring values for \Sigma _ { g } larger than a factor of a few compared to that reported in the literature from the CO isotopologues , if \alpha \lesssim 10 ^ { -3 } .
Such physical conditions imply an appreciably optically thick continuum even at cm-wavelengths ( \tau _ { 33 \mathrm { GHz } } \sim 0.2 ) .
A secondary and shallower peak at 342 GHz is about twice fainter relative to Clump 1 at 33 GHz .
Clump 2 appears to be less efficient at trapping large grains .