Dust properties considered in this work. Panel a: mass absorption coefficient at λ = 1.3 mm as a function of $a_{max}^{\text{eff\hspace{0.17em}}}$. Panel b: opacity slope β_{1.3–3 mm} between 1.3 mm and 3 mm. The grain size distribution follows a power law $\mathrm{d}n(a_{\text{eff\hspace{0.17em}}})\propto a_{\text{eff\hspace{0.17em}}}^{-3.5}\mathrm{d}{a}_{\text{eff\hspace{0.17em}}}$ with a fixed $a_{min}^{\text{eff\hspace{0.17em}}}$ = 0.01 μm. Panel c: mass absorption coefficient as a function of wavelength for $a_{max}^{\text{eff\hspace{0.17em}}}$ = 1 mm. The black lines show the result for compact grains, whereas the case for the porous grains with a porosity of 𝒫 = 0.8 is indicated with blue lines.