Abstract
ABSTRACT Single-pointing observations of 73 Planck cores from the Early Cold Core Catalogue with the Nanshan 26-m telescope are presented, targeting the H$_2$CO ($1_{1,0}$–$1_{1,1}$) 4.8 GHz ($\lambda \sim 6$ cm) absorption line. H$_2$CO absorption has been detected in 51 sources (69.9 per cent), with 24 sources (32.9 per cent) also showing components with hyperfine structure (HFS). In these 51 detected cores, non-thermal velocity dispersion dominates over thermal line broadening ($\sigma _{\rm TH}/\sigma _{\rm NT} \lt 1$), with 96 per cent exhibiting supersonic turbulence ($\mathcal {M} \gt 1$). A weak correlation between $\sigma _{\rm NT}$ and $T_{\rm kin}$ suggests that turbulence contributes to gas heating. A strong $\sigma _{\rm NT}$–ortho-H$_2$CO column density correlation highlights the importance of both turbulence and gravity. For the 24 sources with resolved HFS, the derived excitation temperatures range from 2.08 to 2.59 K (mean 2.37 K). Follow-up mapping of four high signal-to-noise ratio cores with regions of resolved HFS components reveals widespread gas with $T_{\rm ex} \approx 2.36$–2.64 K. Cores with resolved HFS exhibit narrower line widths, lower Mach numbers, higher column densities, and larger optical depths, indicating dynamically quiescent gas in clouds moving towards early gravitational collapse. In contrast, non-HFS regions surrounding the HFS regions in these sources display broader lines and stronger non-thermal motions, suggesting a dynamically complex environment where gravity begins to influence the earliest stages of star formation.