Spectra of the [CII] and [¹³CII] F = 2– 1 lines toward EFF4 (left) and EFF5 (right). The [¹³CII] line has beenscaled by the ¹³C abundance ([¹²C/¹³C] = 67, Langer & Penzias 1990), and the relative strength of its hyperfine component (0.625, Ossenkopf et al. 2013). The scaled [¹³CII] line is brighter than the [CII] line, which suggests that the latter is optically thick, or that there is self-absorption. In the left panel we show the results of modeling the [CII] emission considering self-absorption of the background emission by a foreground component. In this figure, we averaged the [¹³ CII] line to a channel width of 0.8 km s⁻¹.