Continuum flux maps at 3 mm obtained with the MUSTANG-2 bolometer used to compute the dust emissivity spectral index β_0.85-3mm. Left panel: flux at 3 mm in the TMC 1-C prestellar core. Contours correspond to levels 5σ × [1,3,5,7,9], with σ = 10⁻⁵ Jy beam⁻¹. Right panel: flux at 3 mm in the NGC 1333-C7 sector. Contours correspond to levels 5σ × [1,3,5,7,9], with σ = 3.4 × 10⁻⁵ Jy beam⁻¹. The fluxes were obtained using the MUSTANG-2 bolometer of the Green Bank Telescope. The origin (0,0) of these maps is the same as in Fig. 8.