Formation and evolution of interstellar filaments

Hints from velocity dispersion measurements^⋆,⋆⋆

¹ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, IRFU/Service d’Astrophysique, C.E.A. Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
e-mail: doris.arzoumanian@ias.u-psud.fr, pandre@cea.fr
² Present address: IAS, CNRS (UMR 8617), Université Paris-Sud, Bâtiment 121, 91400 Orsay, France

Received: 29 November 2012
Accepted: 12 March 2013

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 ¹³CO, C¹⁸O, and N₂H⁺ 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 ∝ Σ₀^0.5) 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.