Comparison between different parameters and the ¹²CO detectability of our targets. (a) Scale height vs. distance for our targets. The solid line represents a viewing angle of ±90°, the dotted line represents a viewing angle of ±30°, and the dashed line represents a viewing angle of 0°. The non-detected in ¹²CO are shown as red circles, sources detected in ¹²CO (J = 2−1) line are shown as green squares, and sources detected in both ¹²CO lines are shown as blue stars. (b) IRAS [25]-[60] vs. [12]-[25] two-colour diagram restricted to those regions in which our sources are located (i.e. regions I, II, IIIa, VIa, and VII). The colour scheme is the same as (a). (c) Histogram with the distribution of the NUV magnitudes of our sample. The sources detected in ¹²CO are shown in white and sources non-detected in ¹²CO are shown in grey. (d) Same as (c) for FUV magnitudes. (e) Comparison between NUV and FUV luminosities. The solid line represents the relation R_FUV/NUV = 1, the dashed line represents the relation R_FUV/NUV = 0.1, and the dotted line represents the relation R_FUV/NUV = 0.01. Solid circles correspond to well measured luminosities and empty triangles to upper limits in one of the luminosities; the colour scheme is the same as (a).