scholarly journals Magnetic Field and Early Evolution of Circumstellar Disks

2016 ◽  
Vol 33 ◽  
Author(s):  
Yusuke Tsukamoto
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Gravitational Collapse ◽  
Central Region ◽  
Early Phase ◽  
Circumstellar Disks ◽  
Early Evolution ◽  
Disk Formation ◽  
Magnetic Braking ◽  
The Magnetic Field

AbstractThe magnetic field plays a central role in the formation and evolution of circumstellar disks. The magnetic field connects the rapidly rotating central region with the outer envelope and extracts angular momentum from the central region during gravitational collapse of the cloud core. This process is known as magnetic braking. Both analytical and multidimensional simulations have shown that disk formation is strongly suppressed by magnetic braking in moderately magnetised cloud cores in the ideal magnetohydrodynamic limit. On the other hand, recent observations have provided growing evidence of a relatively large disk several tens of astronomical units in size existing in some Class 0 young stellar objects. This introduces a serious discrepancy between the theoretical study and observations. Various physical mechanisms have been proposed to solve the problem of catastrophic magnetic braking, such as misalignment between the magnetic field and the rotation axis, turbulence, and non-ideal effect. In this paper, we review the mechanism of magnetic braking, its effect on disk formation and early evolution, and the mechanisms that resolve the magnetic braking problem. In particular, we emphasise the importance of non-ideal effects. The combination of magnetic diffusion and thermal evolution during gravitational collapse provides a robust formation process for the circumstellar disk at the very early phase of protostar formation. The rotation induced by the Hall effect can supply a sufficient amount of angular momentum for typical circumstellar disks around T Tauri stars. By examining the combination of the suggested mechanisms, we conclude that the circumstellar disks commonly form in the very early phase of protostar formation.

scholarly journals Magnetic braking of supermassive stars through winds

2019 ◽  
Vol 623 ◽  
pp. L7 ◽  
Author(s):  
L. Haemmerlé ◽  
G. Meynet
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Formation Process ◽  
Magnetic Coupling ◽  
Rotation Velocity ◽  
Stellar Surface ◽  
Stellar Winds ◽  
Stellar Rotation ◽  
Magnetic Braking ◽  
The Magnetic Field

Context. Supermassive stars (SMSs) are candidates for being progenitors of supermassive quasars at high redshifts. However, their formation process requires strong mechanisms that would be able to extract the angular momentum of the gas that the SMSs accrete. Aims. We investigate under which conditions the magnetic coupling between an accreting SMS and its winds can remove enough angular momentum for accretion to proceed from a Keplerian disc. Methods. We numerically computed the rotational properties of accreting SMSs that rotate at the ΩΓ-limit and estimated the magnetic field that is required to maintain the rotation velocity at this limit using prescriptions from magnetohydrodynamical simulations of stellar winds. Results. We find that a magnetic field of 10 kG at the stellar surface is required to satisfy the constraints on stellar rotation from the ΩΓ-limit. Conclusions. Magnetic coupling between the envelope of SMSs and their winds could allow for SMS formation by accretion from a Keplerian disc, provided the magnetic field is at the upper end of present-day observed stellar fields. Such fields are consistent with primordial origins.

scholarly journals The Early Era: How do protostellar discs form?

2013 ◽  
Vol 8 (S299) ◽  
pp. 163-164
Author(s):  
Marc Joos ◽  
Patrick Hennebelle ◽  
Andrea Ciardi ◽  
Sébastien Fromang
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Central Region ◽  
Turbulent Diffusion ◽  
Rotation Axis ◽  
Core Collapse ◽  
Turbulent Environment ◽  
Protostellar Discs ◽  
The Magnetic Field ◽  
Disc Formation

AbstractDiscs are a key element in star and planet formation; however, magnetic fields can efficiently transport angular momentum away from the central region of the collapsing core during the dense core collapse, preventing disc formation. We perform numerical simulations of magnetically supercritical collapsing cores with a misalignment between the rotation axis and the magnetic field (Joos et al. 2012) and in a turbulent environment (Joos et al. 2013). The early formation of massive discs can take place at moderate magnetic intensities if the rotation axis is tilted or in a turbulent environment, because of misalignment and turbulent diffusion.

scholarly journals Formation and evolution of protostellar accretion discs – I. Angular-momentum budget, gravitational self-regulation, and numerical convergence

2021 ◽  
Vol 502 (4) ◽  
pp. 4911-4929
Author(s):  
Wenrui Xu ◽  
Matthew W Kunz
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Gravitational Instability ◽  
Self Regulation ◽  
Computational Cost ◽  
Ambipolar Diffusion ◽  
Ohmic Dissipation ◽  
Numerical Convergence ◽  
Magnetic Braking ◽  
The Magnetic Field

ABSTRACT We investigate the formation and early evolution of a protostellar disc from a magnetized prestellar core using non-ideal magnetohydrodynamic (MHD) simulations including ambipolar diffusion and Ohmic dissipation. The dynamical contraction of the prestellar core ultimately leads to the formation of a first hydrostatic core, after ambipolar diffusion decouples the magnetic field from the predominantly neutral gas. The hydrostatic core accumulates angular momentum from the infalling material, evolving into a rotationally supported torus; this ‘first hydrostatic torus’ then forms an accreting protostar and a rotationally supported disc. The disc spreads out by gravitational instability, reaching ∼30 au in diameter at ∼3 kyr after protostar formation. The total mass and angular momentum of the protostar–disc system are determined mainly by accretion of gas from an infalling pseudo-disc, which has low specific angular momentum because of magnetic braking; their removal from the protostar–disc system by outflow and disc magnetic braking are negligible, in part because the magnetic field is poorly coupled there. The redistribution of angular momentum within the protostar–disc system is facilitated mainly by gravitational instability; this allows formation of relatively large discs even when the specific angular momentum of infalling material is low. We argue that such discs should remain marginally unstable as they grow (with Toomre Q ∼ 1–2), an idea that is broadly consistent with recent observational estimates for Class 0/I discs. We discuss the numerical convergence of our results, and show that properly treating the inner boundary condition is crucial for achieving convergence at an acceptable computational cost.

Why only a small fraction of quasars are radio loud?

2018 ◽  
Vol 14 (S342) ◽  
pp. 201-204
Author(s):  
Xinwu Cao
Keyword(s):  
Magnetic Field ◽  
Black Hole ◽  
Angular Momentum ◽  
Angular Velocity ◽  
Strong Magnetic Field ◽  
Weak Magnetic Field ◽  
Thin Disk ◽  
Relativistic Jets ◽  
Jet Formation ◽  
The Magnetic Field

AbstractIt is still a mystery why only a small fraction of quasars contain relativistic jets. A strong magnetic field is a necessary ingredient for jet formation. Gas falls from the Bondi radius RB nearly freely to the circularization radius Rc, and a thin accretion disk is formed within Rc We suggest that the external weak magnetic field threading interstellar medium is substantially enhanced in this region, and the magnetic field at Rc can be sufficiently strong to drive outflows from the disk if the angular velocity of the gas is low at RB. In this case, the magnetic field is efficiently dragged in the disk, because most angular momentum of the disk is removed by the outflows that lead to a significantly high radial velocity. The strong magnetic field formed in this way may accelerate jets in the region near the black hole, either by the Blandford-Payne or/and Blandford-Znajek mechanisms. If the angular velocity of the circumnuclear gas is low, the field advection in the thin disk is inefficient, and it will appear as a radio-quiet (RQ) quasar.

Kinematics of neutral and ionzied gas in the candidate protostar with efficient magnetic braking B335

2018 ◽  
Vol 14 (A30) ◽  
pp. 120-120
Author(s):  
Hsi-Wei Yen ◽  
Bo Zhao ◽  
Patrick M. Koch
Keyword(s):  
Magnetic Field ◽  
Ambipolar Diffusion ◽  
Neutral Gas ◽  
Upper Limit ◽  
Hall Parameter ◽  
Neutral Material ◽  
Magnetic Braking ◽  
Velocity Drift ◽  
The Magnetic Field ◽  
Inner Envelope

AbstractAmbipolar diffusion can cause a velocity drift between ions and neutrals. This is one of the non-ideal MHD effects proposed to enable the formation of large Keplerian disks with sizes of tens of au (Zhao et al. 2018). To observationally study ambipolar diffusion in collapsing protostellar envelopes, we analyzed the ALMA H13CO+ (3–2) and C18O (2–1) data of the protostar B335, which is a candidate source with efficient magnetic braking (Yen et al. 2015). We constructed kinematical models to fit the velocity structures observed in H13CO+ and C18O. With our kinematical models, the infalling velocities in H13CO+ and C18O are both measured to be 0.85 ± 0.2 km s−1 at a radius of 100 au, suggesting that the velocity drift between the ionized and neutral gas is at most 0.3 km s−1 at a radius of 100 au in B335. The Hall parameter for H13CO+ is estimated to be ≫1 on a 100 au scale in B335, so that H13CO+ is expected to be attached to the magnetic field. Our non-detection or upper limit of the velocity drift between the ionized and neutral gas could suggest that the magnetic field remains rather well coupled to the bulk neutral material on a 100 au scale in B335, and that any significant field-matter decoupling, if present, likely occurs only on a smaller scale, leading to an accumulation of magnetic flux and thus efficient magnetic braking in the inner envelope in B335.

scholarly journals 8.4. Properties of synchrotron emission and magnetic fields in the central region of M31

1998 ◽  
Vol 184 ◽  
pp. 351-352 ◽  
Author(s):  
P. Hoernes ◽  
R. Beck ◽  
E.M. Berkhuijsen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Spectral Index ◽  
Central Region ◽  
Relativistic Electrons ◽  
Synchrotron Emission ◽  
The Magnetic Field

At the centre of M31 the nonthermal spectral index between λ20 cm and λ6 cm is −0.2. It slowly decreases along the southern arm and the northern filaments visible in Hα, but perpendicular to these features it increases much faster. The magnetic field runs along the arm and the filaments. These phenomena suggest the existence of a mono-energetic source of relativistic electrons in the nucleus.

scholarly journals Magnetorotational supernovae with different equations of state

2011 ◽  
Vol 7 (S279) ◽  
pp. 357-358
Author(s):  
Sergey G. Moiseenko ◽  
Gennady S. Bisnovatyi-Kogan
Keyword(s):  
Magnetic Field ◽  
Shock Wave ◽  
Angular Momentum ◽  
Differential Rotation ◽  
Equations Of State ◽  
Supernova Explosion ◽  
Explosion Energy ◽  
Iron Core ◽  
Wave Forms ◽  
The Magnetic Field

AbstractWe present results of the simulation of a magneto-rotational supernova explosion. We show that, due to the differential rotation of the collapsing iron core, the magnetic field increases with time. The magnetic field transfers angular momentum and a MHD shock wave forms. This shock wave produces the supernova explosion. The explosion energy computed in our simulations is 0.5-2.5 ċ 1051erg. We used two different equations of state for the simulations. The results are rather similar.

scholarly journals Theory of Protostellar Disk Formation

2015 ◽  
Vol 11 (S315) ◽  
pp. 118-125
Author(s):  
Zhi-Yun Li ◽  
Ruben Krasnopolsky ◽  
Hsien Shang
Keyword(s):  
Magnetic Field ◽  
Rotation Axis ◽  
Misalignment Angle ◽  
Disk Formation ◽  
Magnetic Field Rotation ◽  
Ideal Mhd ◽  
Magnetic Braking ◽  
Field Rotation ◽  
Field Lines ◽  
Protostellar Collapse

AbstractHow large, 100-AU scale, rotationally supported disks form around protostars remains unsettled, both observationally and theoretically. In this contribution, we discuss the theoretical difficulties with disk formation in the presence of a dynamically significant magnetic field and their possible resolutions. These difficulties are caused by the concentration of magnetic field lines close to the forming star by protostellar collapse, and the strong magnetic braking associated with the concentrated field. Possible resolutions include magnetic field-rotation axis misalignment, non-ideal MHD effects, and turbulence. The field-rotation misalignment has been shown to promote disk formation, especially when the field is relatively weak and the misalignment angle is relatively large. Non-ideal MHD effects can enable the formation of small disks at early times. How such disks grow at later times remains to be fully quantified. Turbulence has been found to enable disk formation in a number of simulations, but the exact reason for its beneficial effect is debated.

scholarly journals Magnetic Fields in OH Maser Clouds

1974 ◽  
Vol 60 ◽  
pp. 275-292 ◽  
Author(s):  
R. D. Davies
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Magnetic Fields ◽  
Magnetic Flux ◽  
Field Data ◽  
Zeeman Splitting ◽  
Galactic Rotation ◽  
Magnetic Field Data ◽  
The Magnetic Field ◽  
Maser Sources

Observations of Class I OH maser sources show a range of features which are predicted on the basis of Zeeman splitting in a source magnetic field. Magnetic field strengths of 2 to 7 mG are derived for eight OH maser sources. The fields in all the clouds are directed in the sense of galactic rotation. A model of W3 OH is proposed which incorporates the magnetic field data. It is shown that no large amount of magnetic flux or angular momentum has been lost since the condensation from the interstellar medium began.

scholarly journals Does Misalignment between Magnetic Field and Angular Momentum Enhance or Suppress Circumstellar Disk Formation?

2018 ◽  
Vol 868 (1) ◽  
pp. 22 ◽  
Author(s):  
Y. Tsukamoto ◽  
S. Okuzumi ◽  
K. Iwasaki ◽  
M. N. Machida ◽  
S. Inutsuka
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Circumstellar Disk ◽  
Disk Formation
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