W51-E B3 Spectral Index maps distinguish HII regions from dust-dominated objects. In each panel, the left image shows the tt0 (Taylor term 0), which is our approximation of the continuum level and is used to create a mask. The middle panel shows the tt1 image. The rightmost panel shows , the spectral index, and we have truncated the display to a plausible physical range −2 < α < 4; values beyond this range most likely represent measurement errors. (top) W51 e1/e8. The circular object to the right is an ultracompact HII region. (top-middle) W51 e2 B3. This source splits into e2e, the dust-dominated (α ∼ 4) source to the left, and e2w, the hypercompact HII region with α ∼ 0 − 1. W51 e2w shows signs of a changing spectral index across the band, as it appears to be the source of the symmetric ringing errors that span the image. (bottom-middle) W51 e2 B6. The HII region e2w remains relatively flat, though somewhat more positively sloped than a pure free-free source; it contains at least some dust. W51 e2e has a slightly shallower slope than at B3, indicating that it is optically thick (α = 2). (bottom) W51 IRS1 / Main, the extended HII region that dominates the overall image. There is no clear detection in the tt1 term, suggesting α ∼ 0, which is expected for an optically thin HII region. See also Fig. 2k in Paper I; Motte et al. (2022).