[Paper Review] Unveiling Fiber Networks and Core Formation in the DR21 South Filament

K. S. Yang, Keping Qiu+1 more|arXiv (Cornell University)|Jan 5, 2026

Astrophysics and Star Formation Studies0 citations

TL;DR

The study uses ~1000 AU resolution NOEMA 3 mm observations to reveal velocity-coherent fibers and gravitationally bound dense cores embedded in the quiescent DR21 South Filament, linking large-scale filaments to core formation.

ABSTRACT

We present high-resolution ($\sim$1000 AU) 3 mm observations with the NOrthern Extended Millimeter Array toward the DR21 South Filament, aiming to reveal its internal fragmentation and search for deeply embedded star-forming activities. Both the continuum and molecular line emissions align well with the filament axis traced by the low-resolution ($\sim$18$^{\prime\prime}$) column density map. The 3 mm continuum, CS (2$-$1), and HCO$^+$ (1$-$0) emissions reveal continuous and diffuse structures with measured FWHM widths of 0.054, 0.029, and 0.030 pc, respectively. In contrast, the H$^{13}$CO$^+$ (1$-$0) emission appears more clumpy and localized. The non-thermal motion in the filament is predominantly subsonic to transonic. We detect 13 dense cores in NH$_2$D (1$_{11}-1_{01}$), three of which coincide with continuum peaks; virial analysis suggests most are gravitationally bound. Using a friend-of-friend algorithm, we identify 32, 34, and 22 velocity-coherent fibers from the CS, HCO$^+$, and H$^{13}$CO$^+$ data, respectively. Compared to fibers traced by CS and HCO$^+$, H$^{13}$CO$^+$ fibers are more frequently associated with NH$_2$D cold cores and exhibit higher average mass-per-unit-length values. Differences among CS, HCO$^+$, and H$^{13}$CO$^+$ 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.

Motivation & Objective

• Investigate the internal fragmentation and quiet, star-formation–unbiased structure of the DR21 South Filament (DR21SF).
• Identify velocity-coherent fiber substructures within the filament across multiple tracers.
• Characterize the physical and dynamical properties of detected fibers and embedded dense cores.
• Assess how different tracers (CS, HCO+, H13CO+) map to varying density regimes and excitation conditions.

Proposed method

• Conduct high-resolution (∼1000 AU) 3 mm continuum and line observations of DR21SF with NOEMA over a 16-point mosaic in D configuration.
• Identify velocity components by Gaussian fitting of CS (2−1), HCO+ (1−0), and H13CO+ (1−0) data cubes.
• Apply a two-step friend-of-friend (FoF) algorithm to segment velocity-coherent fibers from the fitted data.
• Extract filament skeletons with FilFinder to trace the central axes of fibers.
• Estimate fiber masses and line masses from molecular column densities under LTE and optically thin assumptions (with caveats about optical depth and filling factor).
• Use DisPerSE on smoothed maps to delineate filamentary skeletons and measure filament widths via perpendicular profiles.

Experimental results

Research questions

• RQ1 What are the physical properties (length, mass per unit length, width) of the velocity-coherent fibers within DR21SF?
• RQ2 How do different molecular tracers trace filaments and substructures, and what do their Mach numbers imply about turbulence?
• RQ3 Are the identified fibers gravitationally bound or transient, and how do they relate to the dense NH2D cores and continuum peaks?
• RQ4 What is the relationship between fibers and the embedded dense cores in the context of filament–core evolution?

Key findings

• DR21SF hosts 32, 34, and 22 velocity-coherent fibers traced by CS, HCO+, and H13CO+, respectively.
• H13CO+ fibers are more clumpy, more frequently associated with NH2D cold cores, and have higher average mass-per-unit-length values than CS and HCO+ fibers.
• Most measured Mach numbers are subsonic to transonic (0 < M ≤ 2) across tracers, indicating a quiescent dynamical state for the filament.