[Paper Review] Initial Conditions for Star Formation: A Physical Description of the Filamentary ISM

A. Hacar, S. E. Clark+5 more|arXiv (Cornell University)|Mar 17, 2022

Astrophysics and Star Formation StudiesPhysics and Astronomy34 citations

TL;DR

The paper reviews filamentary structure in the ISM across scales, assembles a large filament census, and analyzes how accretion and evolution set initial conditions for star formation.

ABSTRACT

The interstellar medium contains filamentary structure over a wide range of scales. Understanding the role of this structure, both as a conduit of gas across the scales and a diagnostic tool of local physics, is a major focus of star formation studies. We review recent progress in studying filamentary structure in the ISM, interpreting its properties in terms of physical processes, and exploring formation and evolution scenarios. We include structures from galactic-scale filaments to tenth-of-a-parsec scale filaments, comprising both molecular and atomic structures, from both observational and theoretical perspectives. In addition to the literature overview, we assemble a large amount of catalogue data from different surveys and provide the most comprehensive census of filamentary structures to date. Our census consists of 22 803 filamentary structures, facilitating a holistic perspective and new insights. We use our census to conduct a meta-analysis, leading to a description of filament properties over four orders of magnitudes in length and eight in mass. Our analysis emphasises the hierarchical and dynamical nature of filamentary structures. Filaments do not live in isolation, nor they generally resemble static structures close to equilibrium. We propose that accretion during filament formation and evolution sets some of the key scaling properties of filaments. This highlights the role of accretion during filament formation and evolution and also in setting the initial conditions for star formation. Overall, the study of filamentary structures during the past decade has been observationally driven. While great progress has been made on measuring the basic properties of filaments, our understanding of their formation and evolution is clearly lacking. In this context, we identify a number of directions and questions we consider most pressing for the field.

Motivation & Objective

• Summarize the role of filamentary structures as conduits and diagnostic tools for local ISM physics.
• Synthesize observational and theoretical progress on filament formation and evolution across scales.
• Provide a comprehensive census of filament properties to enable meta-analyses and scaling relations.
• Highlight how accretion during filament evolution influences the initial conditions for star formation.

Proposed method

• Review a wide range of observational and theoretical studies of filamentary structures from sub-parsec to kiloparsec scales.
• Assemble and analyze a large catalogue of filamentary structures (22,803 filaments) from diverse surveys.
• Perform a meta-analysis to describe filament properties over four orders of magnitude in length and eight in mass.
• Discuss parameterizations of column density profiles (Gaussian and Plummer-like) and derive related quantities.
• Incorporate hydrostatic and isothermal cylinder theory to discuss critical line masses and stability considerations.
• Examine the role of magnetic fields and non-thermal motions in filament stability and fragmentation.

Experimental results

Research questions

• RQ1 What are the characteristic properties and scaling relations of filaments across four orders of length and eight orders of mass?
• RQ2 How do accretion and dynamical evolution shape the initial conditions for star formation within filaments?
• RQ3 To what extent do magnetic fields and non-thermal motions influence filament stability and fragmentation across different environments?
• RQ4 How do observational surveys across scales converge on a unified description of filament morphology and dynamics?

Key findings

• Filaments exhibit a hierarchical and dynamic nature, not static or isolated structures.
• A large census (22,803 filaments) enables meta-analysis over wide scales, revealing consistent scaling trends.
• Accretion during formation and evolution appears to set some of the key scaling properties of filaments, influencing initial star-formation conditions.
• Filament properties such as line mass, length, and radial profiles span broad ranges, with typical inner widths around 0.1 pc in nearby clouds, though this is debated.
• Filaments tend to be sub- to trans-sonic in some cases, and magnetic fields can provide support that depends on field orientation, impacting stability against gravity.
• There is an emphasis on observational progress, with ongoing gaps in understanding formation mechanisms and evolution across environments.