Figure 11.19. Frequency dependence of optical depth t(v) (left) and specific intensity I(v,t) (right) for an observer at some fixed position x in the cloud. I follows from t (v) and the solution (44) of the radiative transfer differential equation. Arbitrary spectral shapes for the source (cloud) spectrum Is and the background spectrum I0 are shown as dashed lines. Emission lines are expected if the foreground gas is optically thin, t ^ 1, and hotter than the background source (c) or, in the limiting case, there is no background source (a). Absorption lines are expected if the background source is hotter than the foreground source, again if the cloud is optically thin (d). If the cloud is optically thick, the continuum spectrum of the cloud is observed (b,e).
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