The fragmentation and stability of hierarchical structure in Serpens South

TL;DR

This study investigates the hierarchical fragmentation and dynamical state of filamentary molecular gas in Serpens South, revealing complex kinematics, subsonic motions, and the insufficiency of magnetic support against collapse.

Contribution

It provides detailed observational analysis of filament fragmentation, hierarchical structure, and gas dynamics in Serpens South, highlighting deviations from spherical models and the role of magnetic fields.

Findings

Hierarchical structures are better explained by cylindrical fragmentation models.

Gas motions are predominantly sub- or trans-sonic across scales.

Magnetic support appears insufficient to prevent collapse, indicating active infall.

Abstract

Filamentary structures are ubiquitous in molecular clouds, and have been recently argued to play an important role in regulating the size and mass of embedded clumps through fragmentation and mass accretion. Here, we reveal the dynamical state and fragmentation of filamentary molecular gas associated with the Serpens South protocluster through analysis of wide (~4 x 4 pc) observations of NH3 (1,1) and (2,2) inversion transitions with the Green Bank Telescope. Detailed modeling of the NH3 lines reveals that the kinematics of the cluster and surrounding filaments are complex. We identify hierarchical structure using a dendrogram analysis of the NH3 emission. The distance between neighbour structures that are embedded within the same parent structure is generally greater than expected from a spherical Jeans analysis, and is in better agreement with cylindrical fragmentation models. The NH3 line width-size relation is flat, and average gas motions are sub- or trans-sonic over all physical scales observed. Subsonic regions extend far beyond the typical 0.1 pc scale previously identified in star-forming cores. As a result, we find a strong trend of decreasing virial parameter with increasing structure mass in Serpens South. Extremely low virial parameters on the largest scales probed by our data suggest that the previously observed, ordered magnetic field is insufficient to support the region against collapse, in agreement with large radial infall motions previously measured toward some of the filaments. A more complex magnetic field configuration in the dense gas, however, may be able to support the filaments.