The 198–358 GHz spectral index (α_198−358) versus 337 GHz flux density (F_337GHz). The values of F_{337 GHz} have been rescaled to a 140 pc distance, according to the distances of the target sources listed in Table 1. The histograms on the right have been normalized such that the area under each of them is 1.0. We excluded the extended disks, DL Tau, CI Tau, GM Tau, AB Aur, DM Tau, AA Tau, and GO Tau from the sample of C.-Y. Chung et al. (2024).