Spectral index images of W51-IRS2 in band 3 (top), band 6 (middle), and calculated from B3/B6 (bottom). As in Figures H.12 and H.13, for the top two rows, the left panel is the tt0 term, the middle panel is tt1, and the right panel is the derived spectral index α. The compact sources along the south end of the image are clearly dust-dominated, with α ∼ 3 − 4 in both bands. The brightest compact source, W51 north, is evidently optically thick at 1 mm but thin at 3 mm, with α_1mm ≈ 2 and α_3mm ≈ 4, as has been previously observed (Ginsburg et al. 2017; Goddi et al. 2020). See also Fig. 2m of Paper I; Motte et al. (2022).