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标题：What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?
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作者：J. D. Soler ; P. Hennebelle
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2017
卷号：607
页码：1-10
DOI：10.1051/0004-6361/201731049
语种：English
出版社：EDP Sciences
摘要：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 (NH) 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 NH structures and ⟨B̂⊥⟩ 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 ⟨B̂⊥⟩ 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.
关键词：enmagnetohydrodynamics (MHD)turbulenceISM: generalISM: structureISM: magnetic fieldsISM: clouds