# The formation of filamentary bundles in turbulent molecular clouds

Abstract

The classical picture of a star-forming filament is a near-equilibrium structure, with collapse dependent on its gravitational criticality. Recent observations have complicated this picture, revealing filaments as a mess of apparently interacting subfilaments, with transsonic internal velocity dispersions and mildly supersonic intra-subfilament dispersions. How structures like this form is unresolved. Here we study the velocity structure of filamentary regions in a simulation of a turbulent molecular cloud. We present two main findings: first, the observed complex velocity features in filaments arise naturally in self gravitating hydrodynamic simulations of turbulent clouds without the need for magnetic or other effects. Second, a region that is filamentary only in projection and is in fact made of spatially distinct features can displays these same velocity characteristics. The fact that these disjoint structures can masquerade as coherent filaments in both projection and velocity diagnostics highlights the need to continue developing sophisticated filamentary analysis techniques for star formation observations.

# Introduction

Lada et al. (2010) (see also e.g. Enoch et al., 2007) demonstrated that the fraction of gas in the dense molecular web is of order 10 percent of the total molecular mass. Interestingly, the mass fraction of protostars to dense ($$n > 10^4$$ cm$$^{-3}$$) molecular gas is also a constant of the same order. Burkert et al. (2013) showed that this requires new filamentary segments to continuously form from the diffuse intra-filament medium on a gravitational collapse timescale.
More recently, observations by (citation not found: 2013A&A...554A..55H) revealed that filaments are often compact bundles of thin spaghetti-like subfilaments. They presented observations of a prominent filamentary feature in Taurus, dominated by the L1495 cloud (Lynds, 1962) and several dark patches (Barnard, 1927). They observed the region in the moderate density tracer C$$^{18}$$O, obtaining spectra along a $$\sim 10$$ pc length of the region. Analysing the richest $$\sim 3$$ pc section of the filament in the resultant position–position–velocity space, they found the intriguing result that the gas along the ridge is organized in velocity-coherent filamentary structures, with typical lengths $$\sim0.5$$ pc. Each filament is internally subsonic or transsonic, though the collection of filaments is characterised by a mildly supersonic interfilamentary dispersion of $$\sim 0.5$$ km s$$^{-1}$$. They describe the collection of velocity detections as “elongated groups that lie at different ‘heights’ (velocities) and present smooth and often oscillatory patterns”. Multiple velocity components along a single line of sight have also been observed in Serpens South (Tanaka et al., 2013); this feature may therefore be common for many young star forming sites.