Previous high-angular-resolution 225GHz (~1.3mm) continuum observations of the transitional disk DM Tau have resolved an outer ring at 20-120au radii that is weakly azimuthally asymmetric. We aim to examine dust growth and filtration in the outer ring of DM Tau. We performed the ~0.06" (~8.7au) resolution Karl G. Jansky Very Large Array (JVLA) 40-48GHz (~7mm; Q band) continuum observations and the complementary observations at lower frequencies. In addition, we analyzed the archival JVLA observations that were taken since 2010. Intriguingly, the Q band image resolved the azimuthally highly asymmetric, knotty dust emission sources close to the inner edge of the outer ring. Fitting the 8-700GHz spectral energy distribution (SED) with two dust components indicates that the maximum grain size (a_max_) in these knotty dust emission sources is likely >~300um while it is ~225GHz observations, owing to a high optical depth at the ring. In other words, the apparent symmetric or asymmetric morphology of the transitional disks may be related to the optical depths of those disks at the observing frequency

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