Filaments and striations: anisotropies in observed, supersonic, highly magnetized turbulent clouds

Beattie, James A.; Federrath, Christoph

doi:10.1093/mnras/stz3377

Cited by 24 publications

(15 citation statements)

References 88 publications

(118 reference statements)

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“…We show the full derivation of Equation (12) in Appendix B. This is consistent with other studies, where hydrodynamical shocks appear perpendicular to the mean magnetic field, with the compression happening in the parallel direction to the mean magnetic field (Mocz & Burkhart 2018;Beattie & Federrath 2020b;Seifried et al 2020). We will discuss this further in §4.…”

Section: Model Derivationsupporting

confidence: 87%

“…This is the sub-Alfvénic regime. For MA > 1, ρV 2 B 2 the magnetic field plays a lesser role and the turbulent motions set, for example the statistics of the flow (Padoan & Nordlund 2011;Molina et al 2012;Beattie & Federrath 2020b). This is called the super-Alfvénic regime, and the trans-Alfvénic regime, MA ∼ 1, separates the two.…”

Section: Magnetised Turbulencementioning

confidence: 99%

“…Indeed, strong magnetic fields facilitate and help set the structure of the neutral hydrogen clouds (Planck Collaboration et al 2016a,b;Hennebelle & Inutsuka 2019;Krumholz & Federrath 2019;Clark & Hensley 2019). For example, magnetic fields create magnetosonic striations in star-forming clouds (Tritsis & Tassis 2016, 2018Beattie & Federrath 2020b) and in general, contribute to changing the density dispersion of the clouds through magnetic cushioning (Molina et al 2012;Mandal et al 2020). Magnetic fields polarise the density structures in molecular clouds and hydrodynamical shocks preferentially form across magnetic field lines (Clark et al 2015;Planck Collaboration et al 2016a,b;Cox et al 2016;Malinen et al 2016;Soler et al 2017;Soler 2019;Beattie & Federrath 2020b;Clark & Hensley 2019;Seifried et al 2020).…”

Section: Introductionmentioning

confidence: 99%

“…For example, magnetic fields create magnetosonic striations in star-forming clouds (Tritsis & Tassis 2016, 2018Beattie & Federrath 2020b) and in general, contribute to changing the density dispersion of the clouds through magnetic cushioning (Molina et al 2012;Mandal et al 2020). Magnetic fields polarise the density structures in molecular clouds and hydrodynamical shocks preferentially form across magnetic field lines (Clark et al 2015;Planck Collaboration et al 2016a,b;Cox et al 2016;Malinen et al 2016;Soler et al 2017;Soler 2019;Beattie & Federrath 2020b;Clark & Hensley 2019;Seifried et al 2020). Magnetic fields inhibit cloud fragmentation, in turn influencing the initial mass function for new-born stars in the modern star-forming era (Price & Bate 2008;Hennebelle et al 2011;Federrath & Banerjee 2015;Krumholz & Federrath 2019) and the era of first stars (Sharda et al 2020).…”

Section: Introductionmentioning

confidence: 99%

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Magnetic field fluctuations in anisotropic, supersonic turbulence

Beattie,

Federrath,

Seta

2020

Preprint

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The rich structure that we observe in molecular clouds is due to the interplay between strong magnetic fields and supersonic (turbulent) velocity fluctuations. The velocity fluctuations interact with the magnetic field, causing it too to fluctuate. Using numerical simulations, we explore the nature of such magnetic field fluctuations, δB, over a wide range of turbulent Mach numbers, M = 2 − 20 (i.e., from weak to strong compressibility), and Alfvén Mach numbers, M A0 = 0.1 − 100 (i.e., from strong to weak magnetic mean fields, B 0 ). We derive a compressible quasi-static fluctuation model from the magnetohydrodynamical (MHD) equations and show that velocity gradients parallel to the mean magnetic field give rise to compressible modes in sub-Alfvénic flows, which prevents the flow from becoming two-dimensional, as is the case in incompressible MHD turbulence. We then generalise an analytical model for the magnitude of the magnetic fluctuations to include M, and findwhere c s is the sound speed and ρ 0 is the mean density of gas. This new relation fits well in the strong B-field regime. We go on to study the anisotropy between the perpendicular (B ⊥ ) and parallel (B ) fluctuations and the mean-normalised fluctuations, which we find follow universal scaling relations, invariant of M. We provide a detailed analysis of the morphology for the δB ⊥ and δB probability density functions and find that eddies aligned with B 0 cause parallel fluctuations that reduce B in the most anisotropic simulations. We discuss broadly the implications of our fluctuation models for magnetised gases in the interstellar medium.

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Section: Model Derivationsupporting

confidence: 87%

Section: Magnetised Turbulencementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Magnetic field fluctuations in anisotropic, supersonic turbulence

Beattie,

Federrath,

Seta

2020

Preprint

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“…Filaments are characteristic in hot flare plasma on the surface of the Sun and in cold molecular clouds within the Milky Way Galaxy. Men'shchikov et al (2010) defines molecular cloud filaments using the tint fill algorithm from image processing; they are astrophysically explained as outcomes of supersonic magnetohydrodynamic turbulence (Beattie & Federrath 2019).…”

Section: Galaxy Clusteringmentioning

confidence: 99%

21st Century Statistical and Computational Challenges in Astrophysics

Feigelson,

de Souza,

Ishida

et al. 2020

Preprint

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High-accuracy estimation of magnetic field strength in the interstellar medium from dust polarization

Skalidis

Tassis

2021

A&A

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Context. A large-scale magnetic field permeates our Galaxy and is involved in a variety of astrophysical processes, such as star formation and cosmic ray propagation. Dust polarization has been proven to be one of the most powerful observables for studying the field properties in the interstellar medium (ISM). However, it does not provide a direct measurement of its strength. Different methods have been developed that employ both polarization and spectroscopic data in order to infer the field strength. The most widely applied method was developed by Davis (1951, Phys. Rev., 81, 890) and Chandrasekhar & Fermi (1953, ApJ, 118, 1137), hereafter DCF. The DCF method relies on the assumption that isotropic turbulent motions initiate the propagation of Alfvén waves. Observations, however, indicate that turbulence in the ISM is anisotropic and that non-Alfvénic (compressible) modes may be important. Aims. Our goal is to develop a new method for estimating the field strength in the ISM that includes the compressible modes and does not contradict the anisotropic properties of turbulence. Methods. We adopt the following assumptions: (1) gas is perfectly attached to the field lines; (2) field line perturbations propagate in the form of small-amplitude magnetohydrodynamic (MHD) waves; and (3) turbulent kinetic energy is equal to the fluctuating magnetic energy. We use simple energetics arguments that take the compressible modes into account to estimate the strength of the magnetic field. Results. We derive the following equation: B0 = √2πρδv/√δθ, where ρ is the gas density, δv is the rms velocity as derived from the spread of emission lines, and δθ is the dispersion of polarization angles. We produce synthetic observations from 3D MHD simulations, and we assess the accuracy of our method by comparing the true field strength with the estimates derived from our equation. We find a mean relative deviation of 17%. The accuracy of our method does not depend on the turbulence properties of the simulated model. In contrast, the DCF method, even when combined with the Hildebrand et al. (2009, ApJ, 696, 567) and Houde et al. (2009, ApJ, 706, 1504) method, systematically overestimates the field strength. Conclusions. Compressible modes can significantly affect the accuracy of methods that are based solely on Alfvénic modes. The formula that we propose includes compressible modes; however, it is applicable only in regions with no self-gravity. Density inhomogeneities may bias our estimates to lower values.

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Filaments and striations: anisotropies in observed, supersonic, highly magnetized turbulent clouds

Cited by 24 publications

References 88 publications

Magnetic field fluctuations in anisotropic, supersonic turbulence

Magnetic field fluctuations in anisotropic, supersonic turbulence

21st Century Statistical and Computational Challenges in Astrophysics

High-accuracy estimation of magnetic field strength in the interstellar medium from dust polarization

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