The evolution of CH in Planck Galactic cold clumps

i narrow self-absorption (HINSA), and H₂ column densities inferred from thermal dust emission. Our results reveal ubiquitous subsonic velocity dispersions of CH, in contrast to ¹3CO, which is predominantly supersonic. The findings suggest that subsonic CH emissions may trace dense, low-turbulent gas structures in PGCCs. To investigate how environmental parameters, particularly the cosmic-ray ionization rate (CRIR), affect the evolution of CH in PGCCs, we estimated upper limits for the CRIR using HINSA. The derived values span (8. 1±4. 7) to (2. 0±0. 8) s^-1 over an H₂ column density range of (1. 7±0. 2) to (3. 6±0. 4) cm^-2. This result favors theoretical predictions of a cosmic-ray attenuation model, in which the interstellar spectra of low-energy CR protons and electrons match Voyager measurements, although alternative models cannot yet be ruled out. The abundance of CH decreases with increasing column density, while showing a positive dependence on the CRIR, which requires atomic oxygen not heavily depleted to dominate CH destruction in PGCCs. By fitting the abundance of CH with an analytic formula, we placed constraints on atomic O abundance (2. 4±0. 4 with respect to H column density) and C^+ abundance (7. 4±0. 7 ζ₂/n_ ̊m H₂). These findings indicate that CH formation is closely linked to the C^+ abundance, regulated by cosmic-ray ionization, while other processes, such as turbulent diffusive transport, might also contribute a non-negligible effect to CH formation.