What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?

¹ Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: soler@mpia.de
² Laboratoire AIM, Paris-Saclay, CEA/IRFU/SAp – CNRS – Université Paris Diderot, 91191 Gif-sur-Yvette Cedex, France

Received: 26 April 2017
Accepted: 4 September 2017

Abstract

We investigate the conditions of ideal magnetohydrodynamic (MHD) turbulence responsible for the relative orientation between density gradients (∇ρ) and magnetic fields (B) in molecular clouds (MCs). For that purpose, we construct an expression for the time evolution of the angle (φ) between ∇ρ and B based on the transport equations of MHD turbulence. Using this expression, we find that the configuration where ∇ρ and B are mostly parallel, cosφ = ± 1, and where ∇ρ and B are mostly perpendicular, cosφ = 0, constitute equilibrium points, that is, the system tends to evolve towards either of these configurations and they are more represented than others. This would explain the predominant alignment or anti-alignment between column density (N_H) structures and the projected magnetic field orientation (⟨B̂_⊥⟩) reported in observations. Additionally, we find that departures from the cosφ = 0 configurations are related to convergent flows, quantified by the divergence of the velocity field (∇·v) in the presence of a relatively strong magnetic field. This would explain the observed change in relative orientation between N_H structures and ⟨B̂_⊥⟩ towards MCs, from mostly parallel at low N_H to mostly perpendicular at the highest N_H, as the result of the gravitational collapse and/or convergence of flows. Finally, we show that the density threshold that marks the observed change in relative orientation towards MCs, from N_H and ⟨B̂_⊥⟩ being mostly parallel at low N_H to mostly perpendicular at the highest N_H, is related to the magnetic field strength and constitutes a crucial piece of information for determining the role of the magnetic field in the dynamics of MCs.