Magnetic fields in circumstellar disks

Brauer, Robert; Wolf, S.; Flock, Mario

doi:10.1051/0004-6361/201731140

Cited by 32 publications

(21 citation statements)

References 55 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The role of magnetic fields in the formation of high-and low-mass disks is less clear due to a small number of observations, and the issue of scattering by dust. Near-future studies targeting the Zeeman and G-K effects may finally be able to access information about the magnetic field in disks (e.g., Brauer et al, 2017). Furthermore, high resolution studies at longer wavelengths in regions that are optically thick (and thus dominated by scattering) in the ALMA data will be made possible by future telescopes such as the Next Generation Very Large Array (ngVLA; Carilli et al 2015).…”

Section: Discussionmentioning

confidence: 99%

Interferometric Observations of Magnetic Fields in Forming Stars

Hull

Zhang

2019

Front. Astron. Space Sci.

114

103

View full text Add to dashboard Cite

The magnetic field is a key ingredient in the recipe of star formation. However, the importance of the magnetic field in the early stages of the formation of low-and high-mass stars is still far from certain. Over the past two decades, the millimeter and submillimeter interferometers BIMA, OVRO, CARMA, SMA, and most recently ALMA have made major strides in unveiling the role of the magnetic field in star formation at progressively smaller spatial scales; ALMA observations have recently achieved spatial resolutions of up to ∼ 100 au and ∼ 1,000 au in nearby low-and high-mass star-forming regions, respectively. From the kiloparsec scale of molecular clouds down to the inner few hundred au immediately surrounding forming stars, the polarization at millimeter and submillimeter wavelengths is dominated by polarized thermal dust emission, where the dust grains are aligned relative to the magnetic field. Interferometric studies have focused on this dust polarization and occasionally on the polarization of spectral-line emission. We review the current state of the field of magnetized star formation, from the first BIMA results through the latest ALMA observations, in the context of several questions that continue to motivate the studies of highand low-mass star formation. By aggregating and analyzing the results from individual studies, we come to several conclusions: (1) Magnetic fields and outflows from low-mass protostellar cores are randomly aligned, suggesting that the magnetic field at ∼ 1000 au scales is not the dominant factor in setting the angular momentum of embedded disks and outflows. (2) Recent measurements of the thermal and dynamic properties in high-mass star-forming regions reveal small virial parameters, challenging the assumption of equilibrium star formation. However, we estimate that a magnetic field strength of a fraction of a mG to several mG in these objects could bring the dense gas close to a state of equilibrium. Finally, (3) We find that the small number of sources with hourglass-shaped magnetic field morphologies at 0.01-0.1 pc scales cannot be explained purely by projection effects, suggesting that while it does occur occasionally, magnetically dominated core collapse is not the predominant mode of low-or high-mass star formation. Magnetic fields in forming stars Polarized molecular-line emissionPolarization of molecular-line emission is another tracer of the magnetic field in star-forming regions. Molecular and atomic lines are sensitive to magnetic fields, which cause their spectral levels to split into magnetic sub-levels. For some molecules, linear polarization can arise when an anisotropy in the radiation and/or velocity field yields a population of magnetic sub-levels that are not in local thermodynamic equilibrium (LTE); this is known as the Goldreich-Kylafis (G-K) effect. Polarization from the G-K effect is most easily detected where the spectral line emission has an optical depth τ ≈ 1, when the ratio of the collision rate to the radiative transition rate (i.e., the spontaneous emi...

show abstract

Section: Discussionmentioning

confidence: 99%

Interferometric Observations of Magnetic Fields in Forming Stars

Hull

Zhang

2019

Front. Astron. Space Sci.

114

103

View full text Add to dashboard Cite

show abstract

“…Observationally, there is some (contested) evidence of disordered fields lying primarily in the disk plane from dust polarimetry, most notably in the cases of HL Tau and the class 0 object, I16293B (Stephens et al 2014;Rao et al 2013). Future observations of Zeeman splitting of CN lines by ALMA may provide further information about these in situ fields (Brauer et al 2017;Vlemmings et al 2019). Given the prominence of non-ideal MHD in T-Tauri disks, it is natural to ask how magnetic fields alter, and become altered by, the turbulence generated by GI, even if at the present time the ionisation profile of young disks is poorly constrained.…”

Section: Introductionmentioning

confidence: 99%

Global Simulations of Self-gravitating Magnetized Protoplanetary Disks

Deng

Mayer²,

Latter

2020

ApJ

View full text Add to dashboard Cite

In the early stages of a protoplanetary disk, when its mass is a significant fraction of its star's, turbulence generated by gravitational instability (GI) should feature significantly in the disk's evolution. At the same time, the disk may be sufficiently ionised for magnetic fields to play some role in the dynamics. Though usually neglected, the impact of magnetism on the GI may be critical, with consequences for several processes: the efficiency of accretion, spiral structure formation, fragmentation, and the dynamics of solids. In this paper, we report on global three-dimensional magnetohydrodynamical simulations of a self-gravitating protoplanetary disk using the meshless finite mass (MFM) Lagrangian technique. We confirm that GI spiral waves trigger a dynamo that amplifies an initial magnetic field to nearly thermal amplitudes (plasma β < 10), an order of magnitude greater than that generated by the magnetorotational instability alone. We also determine the dynamo's nonlinear back reaction on the gravitoturbulent flow: the saturated state is substantially hotter, with an associated larger Toomre parameter and weaker, more 'flocculent' spirals. But perhaps of greater import is the dynamo's boosting of accretion via a significant Maxwell stress; mass accretion is enhanced by factors of several relative to either pure GI or pure MRI. Our simulations use ideal MHD, an admittedly poor approximation in protoplanetary disks, and thus future studies should explore the full gamut of non-ideal MHD. In preparation for that, we exhibit a small number of Ohmic runs that reveal that the dynamo, if anything, is stronger in a non-ideal environment. This work confirms that magnetic fields are a potentially critical ingredient in gravitoturbulent young disks, possibly controlling their evolution, especially via their enhancement of (potentially episodic) accretion.

show abstract

“…The total magnetic field can be derived by combining Zeeman observations with complimentary observations of linear polarization (Heiles & Haverkorn 2012). However, the requirements for spatially resolved Zeeman observations cannot be met with current instruments (Brauer et al 2017). Only spatially unresolved observations are predicted to be feasible under certain circumstances.…”

Section: Introductionmentioning

confidence: 99%

Characterization of mid-infrared polarization due to scattering in protoplanetary disks

Heese

Wolf

Brauer

2020

A&A

Self Cite

View full text Add to dashboard Cite

Context. It is generally assumed that magnetic fields play an important role in the formation and evolution of protoplanetary disks. One way of observationally constraining magnetic fields is to measure polarized emission and absorption produced by magnetically aligned elongated dust grains. The fact that radiation also becomes linearly polarized by light scattering at optical to millimeter wavelengths complicates magnetic field studies. Aims. We characterize the linear polarization of mid-infrared radiation due to scattering of the stellar radiation and dust thermal re-emission radiation (self-scattering). Methods. We computed the radial polarization profiles at wavelengths across the N and Q bands for a broad range of circumstellar disk configurations. These simulations served as a basis to analyze the correlations between selected disk parameters and the resulting linear polarization.Results. We find that the thermal re-emission radiation is stronger than the scattered stellar radiation for disks with inner holes smaller than ∼ 10 au within the considered parameter range. The mid-infrared polarization due to scattering shows several clear trends: For scattered stellar radiation only, the linear polarization degree decreases slightly with increasing radial distance, while it increases with radial distance for thermal re-emission radiation only and for a combination of scattered stellar radiation and thermal re-emission radiation. The linear polarization degree decreases with increasing disk flaring and luminosity of the central star. An increasing inner radius shifts the increase of the linear polarization degree further outside, while a larger scale height increases the linear polarization degree for small radial distances and decreases this degree further outside. For longer wavelengths, i.e., toward the Q band in our study, the linear polarization degree converges more slowly. Conclusions. We found several clear trends for polarization due to scattering. These trends are the basis to distinguish polarization due to scattering from polarization due to dichroic emission and absorption.

show abstract

Magnetic fields in circumstellar disks

Cited by 32 publications

References 55 publications

Interferometric Observations of Magnetic Fields in Forming Stars

Interferometric Observations of Magnetic Fields in Forming Stars

Global Simulations of Self-gravitating Magnetized Protoplanetary Disks

Characterization of mid-infrared polarization due to scattering in protoplanetary disks

Contact Info

Product

Resources

About