Four absorption lines on top of spectra of the ground-state ortho-HO 212−101 line (black), with continuum in Orion S. Assuming that the OPRs are 1, 2, 2.5, and 3, the derived optical depth of 0.08, 0.16, 0.20, and 0.24 are presented, respectively, as green, red, yellow, and blue.
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