Giant Grains in Orion

A modification of Larson's model for optically thick dust clouds is used to fit the infrared and submillimetre spectrum of the Orion nebula. For a spherically symmetric, uniform dust cloud in which the grains are composed of silicates and have radius 0·2 µm, the optical depth in the ultraviolet has to exceed 1000, and the infrared cluster would have to lie by chance in front of the centre of the Orion molecular and submillimetre cloud. This conclusion would not appear to be greatly altered if the cloud is flattened or has a density gradient, or if complex ices are used instead of silicates. On the other hand a consistent picture of the whole spectrum is obtained if silicate grains of radius ~100 µm are used, with the infrared cluster supplying the energy to the rest of the cloud. The total mass in grains would be $$270\,{M}_{\odot}$$ , implying either an unusually low gas-to-dust ratio or a molecular hydrogen number-density greater than $${10}^{6}\,\text{cm}^{-3}$$. Even if the grains have their theoretical maximum absorption efficiency, a radius of ~ 3 µm is required to fit the whole spectrum. It seems plausible to assume that there is either a bimodal distribution of grain sizes, or a continuous distribution ranging from small to large grains, in the Orion cloud.