2025ApJS..277...45C

We have compiled the ∼4-400 GHz broad spectra of 32 Class II protoplanetary disks (PPDs) in the Taurus-Auriga region, which represents the brightest one-third of sources detected in the submillimeter band in this region. The spectra at >20 GHz frequency can be described with a piecewise function: (1) a power law with a spectral index of ∼2 at >200 GHz, (2) a power law with a spectral index in the range of 0.3-4.2 at 20-50 GHz, and (3) a transition region in between these two power laws, which can be characterized by a sigmoid function. This suggests that the flux densities at >200 GHz and <50 GHz are dominated by distinct emission components. At >200 GHz, the emission is likely dominated by the optically thick dust thermal emission in the bulk of the disks. In some sources that were not detected at 6.8 GHz or 10 GHz, embedded high-density dust substructures may contribute to a significant fraction of the flux densities at 30-50 GHz, and the spectral indices are mostly consistent with 2.0. Although, at 30-50 GHz, free-free and/or synchrotron emission may be significant, and some sources in our sample have spectral indices <2.0. Based on these results, we hypothesize that high-density dust substructures (e.g., vortices) are often found in resolved Class II PPDs, and are a precursor to the formation of kilometer-sized planetesimals and rocky planets. They may not present high contrast at >200 GHz frequencies owing to the high optical depth. To probe these dust substructures, high angular resolution observations at <100 GHz are necessary to distinguish them from free-free and synchrotron emission sources. Otherwise, in the analyses of the spatially unresolved spectra, one needs to simultaneously constrain the flux densities of free-free, synchrotron, and dust emission with the observations at ∼5-50 GHz.