Magnetic fields in star formation: a complete compilation of all the DCF estimations

Liu, Junhao; Qiu, Keping; Zhang, Qizhou

doi:10.48550/arxiv.2111.05836

Cited by 5 publications

(7 citation statements)

References 49 publications

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“…Therefore, one should clearly state the magnetisation level of turbulence (sub-or super-Alfvénic) before arguing about the relative contribution of the various terms in the energy balance. Liu et al (2021) further argues that self-gravity may modify this energy balance. We do not include gravity in this study, but it is possible that gravity may collapse locally bound (by self-gravity) regions in the ISM, enhancing and creating strong magnetic fields (Sur et al 2010).…”

Section: Discussion Of Section 4 and Caveatsmentioning

confidence: 99%

“…If we interpret this experiment at face value, this means that if individual clouds are sub-Alfvénic, we should expect correlated turbulent velocities beyond the extent of the entire sub-Alfvénic region, even if the driving scale is not itself larger than the individual clouds. For the ISM in general, which is probably trans-Alfvénic-to-super-Alfvénic and trans-sonic on average (Gaensler et al 2011;Krumholz et al 2020;Liu et al 2021;Seta & Federrath 2021b), we should expect turbulent motions to be correlated out to the driving scale of the largest turbulent motions. This, of course, is a natural repercussion of one of the central tenets of turbulence: the energy cascade from large (galactic, in this context) to small (molecular cloud and smaller, Armstrong et al 1995) scales.…”

Section: The Averaging Scalementioning

confidence: 99%

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Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

Beattie¹,

Krumholz²,

Skalidis³

et al. 2022

Preprint

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Energy equipartition is a powerful theoretical tool for understanding astrophysical plasmas. It is invoked, for example, to measure magnetic fields in the interstellar medium (ISM), as evidence for small-scale turbulent dynamo action, and, in general, to estimate the energy budget of star-forming molecular clouds. In this study we motivate and explore the role of the volume-averaged root-mean-squared (rms) magnetic coupling term between the turbulent, δB and largescale, B 0 fields, (δBV . By considering the second moments of the energy balance equations we show that the rms coupling term is in energy equipartition with the volume-averaged turbulent kinetic energy for turbulence with a sub-Alfvénic large-scale field. Under the assumption of exact energy equipartition between these terms, we derive relations for the magnetic and coupling term fluctuations, which provide excellent, parameter-free agreement with time-averaged data from 280 numerical simulations of compressible MHD turbulence. Furthermore, we explore the relation between the turbulent, mean-field and total Alfvén Mach numbers, and demonstrate that sub-Alfvénic turbulence can only be developed through a strong, large-scale magnetic field, which supports an extremely super-Alfvénic turbulent magnetic field. This means that the magnetic field fluctuations are significantly subdominant to the velocity fluctuations in the sub-Alfvénic large-scale field regime. Throughout our study, we broadly discuss the implications for observations of magnetic fields and understanding the dynamics in the magnetised ISM.

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Section: Discussion Of Section 4 and Caveatsmentioning

confidence: 99%

Section: The Averaging Scalementioning

confidence: 99%

Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

Beattie¹,

Krumholz²,

Skalidis³

et al. 2022

Preprint

View full text Add to dashboard Cite

show abstract

“…We see some suggestion that isolated cores have lower field strengths for a given density than do structures under stellar feedback, suggesting that B 0 , n 0 and κ may vary with environment. Liu et al (2021b) found a best-fit power law for their compilation of DCF results of κ = 0.57 ± 0.03, between the weak-and strong-field values.…”

Section: Magnetic Field Strength -Density Relationshipmentioning

confidence: 96%

“…As the analysis which we can perform in this chapter is very limited, we have made this data set available as a resource 6 . We draw attention to a recent analysis of a compilation of emission DCF measurements by Liu et al (2021b).…”

Section: Compilation Of Literature Dcf Field Strengthsmentioning

confidence: 99%

Magnetic fields in star formation: from clouds to cores

Pattle¹,

Fissel²,

Tahani³

et al. 2022

Preprint

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In this chapter we review recent advances in understanding the roles that magnetic fields play throughout the star formation process, gained through observations and simulations of molecular clouds, the dense, star-forming phase of the magnetised, turbulent interstellar medium (ISM). Recent results broadly support a picture in which the magnetic fields of molecular clouds transition from being gravitationally sub-critical and near equipartition with turbulence in low-density cloud envelopes, to being energetically sub-dominant in dense, gravitationally unstable star-forming cores. Magnetic fields appear to play an important role in the formation of cloud substructure by setting preferred directions for large-scale gas flows in molecular clouds, and can direct the accretion of material onto star-forming filaments and hubs. Low-mass star formation may proceed in environments close to magnetic criticality; high-mass star formation remains less well-understood, but may proceed in more supercritical environments. The interaction between magnetic fields and (proto)stellar feedback may be particularly important in setting star formation efficiency. We also review a range of widely-used techniques for quantifying the dynamic importance of magnetic fields, concluding that better-calibrated diagnostics are required in order to use the spectacular range of forthcoming observations and simulations to quantify our emerging understanding of how magnetic fields influence the outcome of the star formation process.

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“…On these scales, the field is tangled, chaotic (at least for M A 2, , and different regions in space have causally disconnected magnetic fields. For average ISM parameters, M A ≈ 2 (Gaensler et al, 2011;Krumholz et al, 2020;Liu et al, 2021;, 0 ∼ 100 pc (∼ the Galactic disk scale-height; Karlsson et al, 2013;Falceta-Gonc ¸alves et al, 2014;Li et al, 2014;Krumholz and Ting, 2018), then on scales beyond cor ∼ 25 pc streaming cosmic rays, locked to magnetic field lines, take tangled, chaotic paths that resemble a random walk, leading to a spatial dispersion between cosmic rays originating from the same source. We call this process the "macroscopic diffusion" of SCRs.…”

Section: Cosmic Rays and The Streaming Instabilitymentioning

confidence: 99%

Ion Alfvén velocity fluctuations and implications for the diffusion of streaming cosmic rays

Beattie¹,

Krumholz²,

Federrath³

et al. 2022

Preprint

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The interstellar medium (ISM) of star-forming galaxies is magnetized and turbulent. Cosmic rays (CRs) propagate through it, and those with energies from ∼ GeV − TeV are likely subject to the streaming instability, whereby the wave damping processes balances excitation of resonant ionic Alfv én waves by the CRs, reaching an equilibrium in which the propagation speed of the CRs is very close to the local ion Alfv én velocity. The transport of streaming CRs is therefore sensitive to ionic Alfv én velocity fluctuations. In this paper we systematically study these fluctuations using a large ensemble of compressible MHD turbulence simulations. We show that for sub-Alfv énic turbulence, as obtains for a strongly magnetized ISM, the ionic Alfv én velocity probability density function (PDF) is determined solely by the density fluctuations from shocked gas forming parallel to the magnetic field, and we develop analytical models for the ionic Alfv én velocity PDF up to second moments. For super-Alfv énic turbulence, magnetic and density fluctuations are correlated in complex ways, and these correlations as well as contributions from the magnetic fluctuations sets the ionic Alfv én velocity PDF. We discuss the implications of these findings for underlying "macroscopic" diffusion mechanisms in CRs undergoing the streaming instability, including modeling the macroscopic diffusion coefficient for the parallel transport in a sub-Alfv énic plasma. We also describe how, for highly-magnetized turbulent gas, the gas density PDF, and hence column density PDF, can be used to access information about ionic Alfv én velocity structure from observations of the magnetized ISM.

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Magnetic fields in star formation: a complete compilation of all the DCF estimations

Cited by 5 publications

References 49 publications

Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field

Magnetic fields in star formation: from clouds to cores

Ion Alfvén velocity fluctuations and implications for the diffusion of streaming cosmic rays

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