Top panel: variation with radius of the molecular hydrogen density, n (dash-dotted line), the column density, N (dotted line), and the resulting continuum opacity at 350 GHz, τ (solid line) for a simple spherical isothermal (12 K) core model. Bottom panel: variation of the gas mass (dash-dotted line) and 350 GHz flux (solid line) as a function of radius for the model displayed in the top panel. The dashed gray line shows the expected cumulative flux distribution for a pure blackbody (optically thick emission) at the same temperature. The observed data toward B1b-N and B1b-S are displayed in blue and red, respectively, including the ALMA data from 0.05′′ to 3′′ and JCMT and SMA data between 3′′ and 12′′ (Pezzuto et al. 2012; Hirano & Liu 2014; I-Hsiu Li et al. 2017).