Example of apparently highly supersonic CO spectra in molecular clouds. Upper panel: characteristic supersonic (ℳ ~ 10) spectrum in B213-L1495 observed in C¹⁸O (1−0) (blue; Hacar et al. 2013), and ¹³CO (red) and ¹²CO (1−0) (black) lines (Goldsmith et al. 2008). Mid panel: predicted ¹³CO (red solid line) and ¹²CO (1−0) (black solid line) emission from the fitted C¹⁸O (1−0) line (blue) assuming pure opacity broadening (see text). Within the velocity range where the C¹⁸O is detected, its saturated emission reproduces most of the observed linewidths of both ¹³CO and ¹²CO lines. In the case of the two additional components detected in the most abundant ¹³CO and ¹²CO isotopologues, the expected C¹⁸O emission is consistent with noise (dashed lines). The central velocity of the 7 velocity components detected in the spectra are indicated by vertical blue lines at the top of the middle panel. Lower panel: ¹²CO (black line) and ¹³CO (red histogram) emission residuals after the subtraction of the corresponding saturated C¹⁸O emission. In this last case, the observed C¹⁸O (1−0) emission (blue dotted line) is superposed for illustrative purposes. We note how our simplified LTE treatment overestimates the self-absorption of the optically thick ¹³CO and, particularly, the ¹²CO lines. The remaining emission found between some of the modeled components observed in the ¹³CO wings (marked by vertical red lines in the lower panel) might suggest the presence of a diffuse substratum (i.e., inter-fiber medium; green dashed line; see Sect. 4.2). The contribution of Gaussian noise with rms = 0.2 K was included in all modeled spectra.