Two-dimensional map of the spectral index α around the protostar IRAS15398-3359, the position of which is shown by a white star. Pixels are non-zero where both 1.2 mm and 3.1 mm emission is present at 3σ level, at least. The lower panel profile has been obtained by azimuthally averaging over the map, between 50 and 500 au (disk radius is ∼ 9 au). It shows α ∼ 3.7 along the dusty cavity walls of IRAS15398-3359. The dashed green line represents ISM-like values.