2017/05/01 by Juan D. Soler, Patrick Hennebelle · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.1051/0004-6361/201731049
published as A&A 607, A2 (2017) · 10 pages, 8 figures. Submitted to A&A
arxiv created 2017/05/01 · arxiv updated 2017/11/01
We investigate the conditions of ideal magnetohydrodynamic (MHD) turbulence responsible for the relative orientation between density structures, characterized by their gradient, ∇ρ, and the magnetic field, 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 attractors, that is, the system tends to evolve towards either of these configurations and they are more represented than others. This fact would explain the predominant alignment or anti-alignment between column density, NH, structures and the projected magnetic field orientation, B_⊥, 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 NH-structures and B_⊥ towards MCs, from mostly parallel at low NH to mostly perpendicular at the highest NH, 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 NH and B_⊥ being mostly parallel at low NH to mostly perpendicular at the highest NH, 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.