Left, optical depth map of the dust emission at 160 μm (τ160), which traces the mass of the shell. The two large circles indicate the extent of the shell used to determine the mass of the limb-brightened shell. The small circle (‘OMC1’) circumscribes the Huijgens region associated with the Trapezium stars. We estimated the mass that is enclosed between the two large circles, excluding the Huijgens region. Right, SED of the dust emission observed for different positions in Orion; Fλ is the observed flux. These SEDs are analysed to determine the dust and gas mass. Data and curves represent observed SEDs and model fits for β = 2, respectively. The legend shows the resulting dust temperature Td and τ160. These SED fits were analysed for each spatial point and the resulting τ160 values were used to construct the map shown in the left panel.