Spectral indices of the mm-wave emission. Green dots refer to the outer disk, while blue squares and red crosses refer to the central emission. For the first three targets – GGTau, HD 100546, and HD 142527 – the red crosses indicate α_extrap, while the blue squares indicate α_{3 mm-cm} (see text for further details). The dashed grey line shows the value of the spectral index in the ISM, α = 3.7, while the black line shows α = 2.