All those transitions with J < J* are both optically thick and have level populations in LTE. Deep within the cloud, the photons at these frequencies transport energy by diffusion from one region to another, but only escape at the cloud surface. Here, the flux is FCO(J + 1, J) = nBv(Tg)AvJ+1j, where the Planckian specific intensity Bv(Tg) is evalu-
ated at line center. That is,
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