2025A&A...703A.159L

Methylidyne (CH) has long been considered a reliable tracer of molecular gas in the low-to-intermediate extinction range. Although extended CH 3.3 GHz emission is commonly observed in diffuse and translucent clouds, observations in cold, dense clumps are rare. In this work, we conducted high-sensitivity CH observations toward 27 Planck Galactic cold clumps (PGCCs) with the Arecibo 305 m telescope. Toward each source, the CH data were analyzed in conjunction with ¹³CO (1-0) emission, H 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) × 10^–18 to (2.0 ± 0.8) × 10^–16 s^–1 over an H₂ column density range of (1.7 ± 0.2) × 10²¹ to (3.6 ± 0.4) × 10²² 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 × 10^–4 with respect to H column density) and C⁺ abundance (7.4 ± 0.7 × 10¹³ζ₂/n_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.