Are fibres in molecular cloud filaments real objects?

TL;DR

This study uses numerical simulations to investigate the nature of fibres in star-forming filaments, revealing that many observed fibres may be line-of-sight superpositions rather than distinct physical objects, with magnetic fields influencing their structure.

Contribution

It demonstrates that fibres in molecular cloud filaments are often projection effects and highlights the role of magnetic fields in filament morphology and kinematics.

Findings

Fibres are often line-of-sight superpositions, not separate objects.

Dense filament regions contain most of the mass within fibres.

Magnetic fields align with filament substructures and influence accretion.

Abstract

We analyse the morphology and kinematics of dense filamentary structures produced in a numerical simulation of a star--forming cloud of $1.4 \times 10^4 \, \Msun$ evolving under their own self--gravity in a magnetized medium. This study is motivated by recent observations of velocity--coherent substructures ("fibres") in star-forming filaments. We find such "fibres" ubiquitously in our simulated filament. We found that a fibre in one projection is not necessarily a fibre in another projection, and thus, caution should be taken into account when considering them as real objects. We found that only the densest parts of the filament ($\sim$30\% of the densest volume, which contains $\sim$70\% of the mass) belong to fibres in 2 projections. Moreover, it is quite common that they are formed by separated density enhancements superposed along the line of sight. Observations of fibres can yield insight into the level of turbulent substructure driven by gravity, but care should be taken in interpreting the results given the problem of line of sight superposition. We also studied the morphology and kinematics of the 3D skeleton (spine), finding that subfilaments accrete structured material mainly along the magnetic field lines, which are preferentially perpendicular to the skeleton. The magnetic field is at the same time dragged by the velocity field due to the gravitational collapse.