Left panel: dust absorption opacity as a function of wavelength for grain size distributions characterized by (a) ∝ a^−3.0 and increasing maximum grain size (a_max). Right panel: dust opacity spectral index (β) calculated between 1.3 and 2.7 mm wavelengths as a function of the maximum grain size. Black solid line is the β_env value from the parametric model and the black dashed lines are the uncertainties. Green region shows an upper limit for a_max in the envelope of Per-emb-50.