The Dynamical State of The Serpens South Filamentary Infrared Dark Cloud

TL;DR

This study investigates the physical and magnetic properties of the Serpens South filament, revealing that magnetic support and ambipolar diffusion likely play key roles in cluster formation, contrasting with turbulence-driven models.

Contribution

It provides detailed observational evidence linking magnetic fields and core dynamics to cluster formation in an infrared dark cloud.

Findings

Clumps are magnetically supported with low virial ratios.

The region shows low-velocity infall, not free-fall collapse.

The northern clump is a pre-protocluster candidate.

Abstract

We present the results of N$_2$H$^+$ ($J=1-0$) observations toward Serpens South, the nearest cluster-forming, infrared dark cloud. The physical quantities are derived by fitting the hyperfine structure of N$_2$H$^+$. The Herschel and 1.1-mm continuum maps show that a pc-scale filament fragments into three clumps with radii of $0.1-0.2$ pc and masses of $40-230M_\odot$. We find that the clumps contain smaller-scale ($\sim 0.04$ pc) structures, i.e., dense cores. We identify 70 cores by applying CLUMPFIND to the N$_2$H$^+$ data cube. In the central cluster-forming clump, the excitation temperature and line-width tend to be large, presumably due to protostellar outflow feedback and stellar radiation. However, for all the clumps, the virial ratios are evaluated to be $0.1-0.3$, indicating that the internal motions play only a minor role in the clump support. The clumps exhibit no free-fall, but low-velocity infall, and thus the clumps should be supported by additional forces. The most promising force is the globally-ordered magnetic field observed toward this region. We propose that the Serpens South filament was close to magnetically-critical and ambipolar diffusion triggered the cluster formation. We find that the northern clump, which shows no active star formation, has a mass and radius comparable to the central cluster-forming clump, and therefore, it is a likely candidate of a {\it pre-protocluster clump}. The initial condition for cluster formation is likely to be a magnetically-supported clump of cold, quiescent gas. This appears to contradict the accretion-driven turbulence scenario, for which the turbulence in the clumps is maintained by the accretion flow.