Formation and evolution of interstellar filaments; Hints from velocity dispersion measurements

TL;DR

This study examines the velocity dispersions of interstellar filaments, revealing two regimes linked to their gravitational state and proposing an evolutionary scenario involving gravitational contraction and accretion.

Contribution

It provides observational evidence distinguishing subcritical and supercritical filaments based on velocity dispersion and column density, and suggests an evolutionary model for filament development.

Findings

Subcritical filaments have transonic velocity dispersions independent of column density.

Supercritical filaments show velocity dispersions scaling with the square root of column density.

Supercritical filaments likely undergo gravitational contraction and accrete mass while maintaining constant inner widths.

Abstract

We investigate the gas velocity dispersions of a sample of filaments recently detected as part of the Herschel Gould Belt Survey in the IC5146, Aquila, and Polaris interstellar clouds. To measure these velocity dispersions, we use 13CO, C18O, and N2H+ line observations obtained with the IRAM 30m telescope. Correlating our velocity dispersion measurements with the filament column densities derived from Herschel data, we show that interstellar filaments can be divided into two regimes: thermally subcritical filaments, which have transonic velocity dispersions (c_s ~< \sigma_tot < 2 c_s) independent of column density, and are gravitationally unbound; and thermally supercritical filaments, which have higher velocity dispersions scaling roughly as the square root of column density (\sigma_tot ~ \Sigma^0.5), and are self-gravitating. The higher velocity dispersions of supercritical filaments may not directly arise from supersonic interstellar turbulence but may be driven by gravitational contraction/accretion. Based on our observational results, we propose an evolutionary scenario whereby supercritical filaments undergo gravitational contraction and increase in mass per unit length through accretion of background material while remaining in rough virial balance. We further suggest that this accretion process allows supercritical filaments to keep their approximately constant inner widths (~ 0.1 pc) while contracting.