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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents



ID: 742440.0, MPI für Astronomie / Publikationen_mpia
What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?
Authors:Soler, J. D.; Hennebelle, P.
Date of Publication (YYYY-MM-DD):2017
Title of Journal:Astronomy and Astrophysics
Volume:607
Start Page:id. A2 (10 pp)
Audience:Not Specified
Abstract / Description: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⟩<!-<alternatives><![CDATA[<\hat{B\perp>}]]><inline-graphic mime-subtype="png" mimetype="image" xlink:href="aa31049-17-eq7.png"/><mml:math id="mml_eq1" type="inline"><mml:mo mathvariant="normal">⟨</mml:mo><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="bolditalic">B</mml:mi></mml:mrow><mml:mo>ˆ</mml:mo></mml:mover><mml:msub><mml:mi/><mml:mo mathvariant="normal">⊥</mml:mo></mml:msub><mml:mo mathvariant="normal">⟩</mml:mo></mml:math></alternatives>->) 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⟩<!-<alternatives><![CDATA[<\hat{B\perp>}]]><inline-graphic mime-subtype="png" mimetype="image" xlink:href="aa31049-17-eq7.png"/><mml:math id="mml_eq2" type="inline"><mml:mo mathvariant="normal">⟨</mml:mo><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="bolditalic">B</mml:mi></mml:mrow><mml:mo>ˆ</mml:mo></mml:mover><mml:msub><mml:mi/><mml:mo mathvariant="normal">⊥</mml:mo></mml:msub><mml:mo mathvariant="normal">⟩</mml:mo></mml:math></alternatives>-> 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⟩<!-<alternatives><![CDATA[<\hat{B\perp>}]]><inline-graphic mime-subtype="png" mimetype="image" xlink:href="aa31049-17-eq7.png"/><mml:math id="mml_eq3" type="inline"><mml:mo mathvariant="normal">⟨</mml:mo><mml:mover accent="true"><mml:mrow><mml:mi mathvariant="bolditalic">B</mml:mi></mml:mrow><mml:mo>ˆ</mml:mo></mml:mover><mml:msub><mml:mi/><mml:mo mathvariant="normal">⊥</mml:mo></mml:msub><mml:mo mathvariant="normal">⟩</mml:mo></mml:math></alternatives>-> 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.
Free Keywords:magnetohydrodynamics (MHD); turbulence; ISM: general; ISM: structure; ISM: magnetic fields; ISM: clouds
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:ISSN:0004-6361
URL:http://adsabs.harvard.edu/abs/2017A%26A...607A...2...
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