Stability of accretion discs threaded by a strong magnetic field

Stehle, R.; Spruit, H. C.

doi:10.1046/j.1365-8711.2001.04186.x

Cited by 37 publications

(50 citation statements)

References 27 publications

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“…The magnetic field is, therefore, the essential feature of the model and the equations do not change to standard α-disk equations in the low field limit. The existence of magnetic-induced accretion is also confirmed by the results of Stehle & Spruit (2001). Our results actually demonstrate the close relation between magnetic field dragging and the vertical structure of thin accretion disks.…”

Section: Discussionsupporting

confidence: 83%

The magnetic geometry of magnetic-dominated thin accretion disks

Rüdiger

Shalybkov

2002

A&A

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Abstract. The presence of an imposed external magnetic field drastically influences the structure of thin accretion disks. The magnetic field energy is assumed to be in balance with the thermal energy of the accretion flow, and a diffusion approximation simulates the (vertical) energy transport. Our main result is that inside the corotation radius the resulting radial inclination i of the magnetic field lines from the rotation axis easily exceeds the critical value 30• (required to launch cold jets) even for magnetic Prandtl numbers of order unity. The self-consistent consideration of both magnetic field and accretion flow demonstrates only a weak dependence of the inclination angle on the magnetic Prandtl number. The surface values of the toroidal magnetic fields necessary to induce considerably high values for the radial inclination are much smaller than expected. As the innermost part of the disk produces the largest B tor , the largest radial inclination can also be expected there. The idea is therefore supported that the cold jets are basically launched in the central disk area.

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Section: Discussionsupporting

confidence: 83%

The magnetic geometry of magnetic-dominated thin accretion disks

Rüdiger

Shalybkov

2002

A&A

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“…In such a subcritical cloud, force balance along the field lines is achieved during the quasi-static phase of evolution and is well maintained even during the subsequent dynamic collapse (e.g., Fiedler & Mouschovias 1993;Nakamura, Hanawa, & Nakano 1995). Thus, we adopt the standard thin-disk approximation in which exact force balance along the vertical direction is assumed during the entire evolution (e.g., Ciolek & Mouschovias 1993;Basu & Mouschovias 1994;Li & Shu 1996;Nakamura & Hanawa 1997;Stehle & Spruit 2001;Li 1999). The MHD equations are integrated in the vertical direction, and the cloud evolution is followed in the (x-y) disk plane of a Cartesian coordinate system ðx; y; zÞ with a two-dimensional MHD code.…”

Section: Formulation Of the Problemmentioning

confidence: 99%

Binary and Multiple Star Formation in Magnetic Clouds: Bar Growth and Fragmentation

Nakamura

2003

ApJ

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In the standard scenario of isolated low-mass star formation, strongly magnetized molecular clouds are envisioned to condense gradually into dense cores, driven by ambipolar diffusion. Once the cores become magnetically supercritical, they collapse to form stars. Previous studies based on this scenario are limited to axisymmetric calculations leading to single supercritical core formation. The assumption of axisymmetry has precluded a detailed investigation of cloud fragmentation, generally thought to be a necessary step in the formation of binary and multiple stars. In a series of papers, we studied the nonaxisymmetric evolution of initially magnetically subcritical clouds, using a two-dimensional magnetohydrodynamic code based on the physically motivated thin-disk approximation. We found that such clouds become unstable to nonaxisymmetric perturbations after the supercritical cores are formed, because of ambipolar diffusion. In this paper, we focus on the evolution of clouds perturbed by an m ¼ 2 mode of a modest fractional amplitude of 5%, with an eye on binary and multiple star formation. We show that for a wide range of initial cloud parameters, the m ¼ 2 mode grows nonlinearly into a bar during the isothermal collapse after the supercritical core formation. The instability is driven by the domination of the magnetically diluted gravity over the combined thermal and magnetic pressure gradient in the supercritical cores. Such gravity-dominated cores can break up into fragments during or after the isothermal phase of cloud evolution. The outcome of fragmentation depends on the initial cloud conditions, such as the magnetic field strength, rotation rate, amount of cloud mass (relative to thermal Jeans mass), and mass distribution. It is classified into three different types: (1) '' separate-core formation,'' in which the bar (m ¼ 2) mode breaks up into two separate cores during the isothermal collapse, with a core separation of order 10 4 AU, (2) '' bar fragmentation,'' in which the m ¼ 2 mode evolves into a needle-like, opaque '' first bar '' (at a density ne10 12 cm À3 ), which breaks up into multiple fragments with initial masses of order 10 À2 M and separations of order 10 2 -10 3 AU, and (3) '' disk fragmentation,'' in which the bar growth remains slow during the isothermal collapse and the central region evolves into a rapidly rotating, opaque '' first disk,'' which breaks up into several self-gravitating blobs with separations less than the disk size (d10 2 AU). These three types of fragmentation loosely correspond to the empirical classification of embedded binary and multiple systems of Looney, Mundy, & Welch, based on millimeter dust continuum observations. The well-studied starless core L1544 appears to belong to the bar fragmentation type. We expect it to produce a highly elongated, opaque bar at the center in the future, which should break up into fragments of initial masses in the substellar regime.

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“…Though the origin of such fields is not as clear as the turbulent fields that evolve from initially weak seed fields, they are by far the most logical configurations for producing systematic outflows from accretion disks. Spruit et al (1995), Stehle (1997) and Stehle & Spruit (2001) have shown that even surprisingly strong poloidal fields, approaching (a fraction of) equipartition with orbital kinetic energy, can still be stably anchored in a disk. These analyses did not include instabilities associated with magnetically driven outflows however, which are the subject of the present study.…”

Section: Modelmentioning

confidence: 99%

Instability of an accretion disk with a magnetically driven wind

Cao

Spruit

2002

A&A

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Abstract. We present a linear analysis of the stability of accretion disks in which angular momentum is removed by the magnetic torque exerted by a centrifugally driven wind. The effects of the dependence of the wind torque on field strength and inclination, the sub-Keplerian rotation due to magnetic forces, and the compression of the disk by the field are included. A WKB dispersion relation is derived for the stability problem. We find that the disk is always unstable if the wind torque is strong. The growth time scale of the instability can be as short as the orbital time scale. The instability is mainly the result of the sensitivity of the mass flux to changes in the inclination of the field at the disk surface. Magnetic diffusion in the disk stabilizes if the wind torque is small.

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Stability of accretion discs threaded by a strong magnetic field

Cited by 37 publications

References 27 publications

The magnetic geometry of magnetic-dominated thin accretion disks

The magnetic geometry of magnetic-dominated thin accretion disks

Binary and Multiple Star Formation in Magnetic Clouds: Bar Growth and Fragmentation

Instability of an accretion disk with a magnetically driven wind

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