Formation and evolution of interstellar filaments

Abstract

We investigate the gas velocity dispersions of a sample of filaments recently detected as part of the Herschel Gould Belt Survey in the IC 5146, Aquila, and Polaris interstellar clouds. To measure these velocity dispersions, we use 13 CO, C 18 O, and N 2 H + line observations obtained with the IRAM 30 m 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 < ∼ σ 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 (σ tot ∝ Σ 0 0.5 ) and which 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.

Magnetohydrodynamical simulation of the formation of clumps and filaments in quiescent diffuse medium by thermal instability

“…Firstly, these internal velocity dispersions are on the order of those measured from observations (Arzoumanian et al 2011) -TI-driven filament formation can reproduce the "turbulent" velocities measured in filaments (e.g. by Arzoumanian et al 2013). Secondly, the fact that different filaments have different central velocities, whilst still being part of a larger bundle of filaments or even projection of interconnected sheets highlights that it is now impossible to conclude from observations of filaments with different central velocities that they are separate filaments -in this work they could equally be part of a TI-formed corrugated sheet, non-uniform in density, temperature and velocity.…”

“…Line emission observations of C 18 O and N2H + and other molecules towards star forming filaments (Zuckerman & Palmer 1974;Arzoumanian et al 2013;Hacar et al 2013;Furuya, Kitamura & Sinnaga 2014;Henshaw et al 2014;Jiménez-Serra et al 2014;Li et al 2014) have revealed nonthermal line broadening. Arzoumanian et al (2013) presented molecular line measurements of the internal velocity dispersions in 46 Herschel identified filaments.…”

“…Arzoumanian et al (2013) presented molecular line measurements of the internal velocity dispersions in 46 Herschel identified filaments. Noting the thermal sound speed of ∼0.2 km s −1 for T=10 K, they found velocity dispersions in the range 0.2-0.4 km s −1 for thermally subcritical and nearly critical filaments, implying the level of "turbulent" motions is almost constant and does not dominate over thermal support.…”

The impact of turbulence and magnetic field orientation on star-forming filaments

“…An initial trans-to supersonic turbulent velocity field as well as the subsequent injection of turbulence by accretion (Klessen & Hennebelle 2010;Arzoumanian et al 2013;Heitsch 2013) are not sufficient to stabilize a filament at a width of 0.1 pc. To overcome this problem, turbulence would have to be replenished in a more efficient way, e.g.…”

“…More recently, the advent of the Herschel satellite has uncovered the filamentary structure in molecular clouds and infrared dark clouds in great detail (e.g. André et al 2010;Könyves et al 2010;Arzoumanian et al 2011Arzoumanian et al , 2013Peretto et al 2012;Schneider et al 2012;Palmeirim et al 2013, but see also the review of André et al 2014).…”