Spatial distribution in the Galactic plane of observed sources. The CO-selected cores with and without detections of C₂ H are denoted by the yellow and blue stars, respectively. The background image represents the H_α emission (Finkbeiner 2003) in unit of R (10⁶/4π photons cm⁻² s⁻¹ sr⁻¹). The green contours represent CO (1–0) integrated emission detected by Planck HFI (Planck Collaboration XIII 2014). The red contours show the Planck 353 μm continuum emission. The contour levels are (0.05, 0.1, 0.3, 0.5, 0.7, 0.9) × maximum value. Famous star forming regions such as Taurus-Perseus-California (Lombardi et al. 2010), Cepheus, Orion complex (Dame et al. 2001) are sketched with the black line.