Filament fragmentation in high-mass star formation

Abstract

Context. Filamentary structures in the interstellar medium are crucial ingredients of the star formation process. They fragment to form individual star-forming cores, and at the same time they may also funnel gas toward the central gas cores, providing an additional gas reservoir. Aims. We want to resolve the length scales for filament formation and fragmentation (resolution ≤0.1 pc), in particular the Jeans length and cylinder fragmentation scale. Methods. We have observed the prototypical high-mass star-forming filament IRDC 18223 with the Plateau de Bure Interferometer (PdBI) in the 3.2 mm continuum and N 2 H + (1-0) line emission in a ten-field mosaic at a spatial resolution of ∼4 (∼14 000 au). Results. The dust continuum emission resolves the filament into a chain of at least 12 relatively regularly spaced cores. The mean separation between cores is ∼0.40(±0.18) pc. While this is approximately consistent with the fragmentation of an infinite, isothermal, and gravitationally bound gas cylinder, a high mass-to-length ratio of M/l ≈ 1000 M pc −1 requires additional turbulent and/or magnetic support against radial collapse of the filament. The N 2 H + (1−0) data reveal a velocity gradient perpendicular to the main filament. Although rotation of the filament cannot be excluded, the data are also consistent with the main filament being comprised of several velocity-coherent subfilaments. Furthermore, this velocity gradient perpendicular to the filament resembles results toward Serpens south that are interpreted as signatures of filament formation within magnetized and turbulent sheet-like structures. Lower-density gas tracers ([CI] and C 18 O) reveal a similar red-and blueshifted velocity structure on scales around 60 east and west of the filament. This may tentatively be interpreted as a signature of the large-scale cloud and the smaller scale filament being kinematically coupled. We do not identify a velocity gradient along the axis of the filament. This may be due to no significant gas flows along the filamentary axis, but it may also be partly caused by a low inclination angle of the filament with respect to the plane of the sky minimizing such a signature. Conclusions. The IRDC 18223 3.2 mm continuum data are consistent with thermal fragmentation of a gravitationally bound and compressible gas cylinder. However, the high mass-to-length ratio requires additional support -most likely turbulence and/or magnetic fields -against collapse. The N 2 H + spectral line data indicate a kinematic origin of the filament, but we cannot conclusively differentiate whether it has formed out of (pre-existing) velocity-coherent subfilaments, whether magnetized converging gas flows, a larger-scale collapsing cloud, or even whether rotation played a significant role during filament formation.

Morphology and Kinematics of Filaments in the Serpens and Perseus Molecular Clouds

“…This case is similar to that observed by Beuther et al (2015) in the dense filament IRDC 18223, and by Moeckel & Burkert (2015) in simulations.…”

Filamentary flow and magnetic geometry in evolving cluster-forming molecular cloud clumps

“…Magnetic fields can offer pressure support to filaments, and were observed to be somewhat "puffier" than their unmagnetised counterparts, with broader radial profiles and lower central densities (for details see Kirk et al 2015). Turbulence was also observed to play a critical role in supporting the filament against collapse, consistent with the observations by Beuther et al (2015) of the massive, turbulent filament. This paper is the follow-on and extension of the investigations into simulated filaments begun in Kirk et al (2015).…”

“…Bally et al 1987;Schneider et al 2010), while a few new observations, such as from the CARMA Large Area Star Formation Survey (CLASSy) (Storm et al 2014), are high enough resolution to see velocity gradients across filaments (Fernández-López et al 2014). Beuther et al (2015) recently imaged a massive filamentary infrared dark cloud (IRDC 18223), observed in 3.2mm continuum and in molecular line data. This massive filament has a line mass of about 1000 M /pc.…”

Resolving the fragmentation of high line-mass filaments with ALMA: the integral shaped filament in Orion A

“…The applicability of the nearequilibrium cylindrical models to highly super-critical filaments (such as the ISF) has not been established. It has been argued on qualitative grounds, and commonly assumed in literature, that non-thermal motions can provide microturbulent pressure support to super-critical filaments, effectively increasing their critical line-mass (e.g., Fiege & Pudritz 2000a;Jackson et al 2010;Fischera & Martin 2012;Heitsch 2013;Hernandez et al 2012;Beuther et al 2015a). However, we need to recognise that the nature of non-thermal motions within highly super-critical filaments (including the ISF) has not been established.…”

“…Specifically, filaments that have line-masses greatly in excess to the critical value of the self-gravitating, thermally supported, non-magnetised, infinitely long equilibrium model, i.e., ≫16 M ⊙ pc −1 (Ostriker 1964), contain large enough mass reservoirs to give birth to high-mass stars and star clusters (e.g., Pillai et al 2006;Beuther et al 2010Beuther et al , 2015aHenning et al 2010;Schneider et al 2012;Kainulainen et al 2013;Stutz & Gould 2016;Contreras et al 2016). This makes understanding their fragmentation and gravitational collapse important for Galactic-scale star formation.…”