Derived millimeter spectral index for MP Mus between 1.3 and 2.2 mm. Top: spectral index map. Only pixels with S/N > 5 are considered. The solid and dashed contours correspond to α_{1.3−2.2 mm} of 2 and 2.5, respectively. The image beam is shown on the bottom left corner. Bottom: corresponding de-projected α_{1.3−2.2 mm} radial profile and uncertainties (solid red line and area). The beam FWHM is shown as a horizontal red line. The dashed line marks the α = 2 transition.