2026AJ....171..145Y

We present high-resolution (∼1000 au) 3 mm observations with the NOrthern Extended Millimeter Array toward the DR21 South Filament (DR21SF), aiming to reveal its internal fragmentation and search for deeply embedded star-forming activities. Both the continuum and molecular line emissions align well with the filament axis traced by the low-resolution (∼18″) column density map. The 3 mm continuum, CS (2-1), and HCO⁺ (1-0) emissions reveal continuous and diffuse structures with measured full width half maximum widths of 0.054, 0.029, and 0.030 pc, respectively. In contrast, the H¹³CO⁺ (1-0) emission appears more clumpy and localized. The non-thermal motion in the filament is predominantly subsonic to transonic. We detect 13 dense cores in NH₂D (1₁₁-1₀₁), three of which coincide with continuum peaks; virial analysis suggests most are gravitationally bound. Using a friend-of-friend algorithm, we identify 32, 34, and 22 velocity-coherent fibers from the CS, HCO⁺, and H¹³CO⁺ data, respectively. Compared to fibers traced by CS and HCO⁺, H¹³CO⁺ fibers are more frequently associated with NH₂D cold cores and exhibit higher average mass-per-unit-length values. Differences among CS, HCO⁺, and H¹³CO⁺ emissions likely arise from variations in effective critical densities. These results are consistent with a hierarchical structure, in which the 3.6 pc DR21SF contains velocity-coherent fibers and gravitationally bound dense cores.