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Accretion, Outflows, and Winds of Magnetized Stars

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Abstract

Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars are radiatively driven. In all of these cases, the magnetic field influences matter flow from the stars and determines many observational properties. In this chapter we review recent studies of accretion, outflows, and winds of magnetized stars with a focus on three main topics: (1) accretion onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and (3) winds from isolated massive magnetized stars. We show results obtained from global magnetohydrodynamic simulations and, in a number of cases compare global simulations with observations.

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Notes

  1. We further discuss the disk-magnetosphere interaction and the magnetospheric radius in Sect. 2.2.

  2. In these initial conditions the corona above the disk rotates with the angular velocity of the disk, so that the magnetic field lines do not experience discontinuity at the disk-magnetosphere boundary, and there is no initial magnetic breaking. There is also an initial balance between the gravitation, pressure and centrifugal forces in each point of the simulation region.

  3. Sometimes, the condition \(\beta=8\pi p/B^{2}=1\) is used to find the magnetospheric radius (e.g., Bessolaz et al. 2008). This condition, however, yields somewhat a larger radius at which the matter flows from the disk to the funnel stream.

  4. Simulations of accretion onto stars with larger magnetosphere require much longer computing time. Test simulations of such accretion are described in Sect. 2.5. However, to obtain a formula for magnetospheric radius, multiple simulations are needed.

  5. The compression of the magnetosphere is probably connected with the ram pressure in the radial direction and so may depend on the value of the radial velocity (which is proportional to \(\alpha\)-parameter of viscosity). The above described simulations were performed for \(\alpha=0.02\). The possible dependence of the compression on \(\alpha\)-parameter should be studied in a separate set of simulations.

  6. We should note that simulations of other groups also show a strong compression of the magnetosphere (e.g., Bessolaz et al. 2008; Zanni and Ferreira 2013).

  7. This phenomenon can be possibly explained by the beat-frequency model (e.g., Miller et al. 1998).

  8. Simulations also show that if a star rotates more slowly than the inner disk, then bending waves are excited at larger distances from the star.

  9. Many axisymmetric simulations also show strong spikes which are connected with very low (only small numerical) diffusivity in the code, and episodic accumulation of matter at the disk-magnetosphere boundary.

  10. This is the main restriction of the current 3D simulations, where the main computing power is used for resolving the low-density closed magnetosphere and the disk, while the density in the corona is relatively high, and matter pressure dominates the magnetic pressure, suppressing magnetic or magneto-centrifugally driven outflows.

  11. This angular momentum depends on the coronal density: it can be larger in case of young stars, which can have strong stellar winds, and smaller in cases of neutron stars.

  12. The effective \(\alpha\)-parameter can be estimated from comparisons of observations with models of accretion. For example, Bisnovatyi-Kogan et al. (2014) derived the value \(\alpha=0.1\mbox{--}0.3\) for outburst of accretion in X-ray transient A0535+26/HDE245770.

  13. (1) In the X-wind model, one of the necessary requirements is the condition \(r_{m} = r_{\mathrm{cor}}\), that is the inner disk should rotate with the angular velocity of the magnetosphere. In conical winds model, there is no such restriction: a star may rotate much more slowly than the inner disk, with \(r_{m} \ll r_{\mathrm{cor}}\). (2) In the X-wind model matter is driven by the centrifugal force and overall situation is closer to the weak propeller regime; in conical winds model the driving force is mainly the magnetic pressure arising from the winding of the magnetic field lines above the inner parts of the disk.

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Acknowledgement

We thank the organizers of ISSI Workshop “The Strongest Magnetic Fields in the Universe” for excellent meeting and hospitality. Resources supporting this work were provided by the NASA High-End Computing (HEC). MMR acknowledges support by NASA grant NNX14AP30G and NSF grant AST-1211318, and contributions of different collaborators, particularly A.V. Koldoba, G.V. Ustyugova, R. Kurosawa, A.A. Blinova and R.V.E. Lovelace. MMR thanks M. Comins for editing the manuscript. SPO acknowledges support by NASA ATP Grants NNX11AC40G and NNX12AC72G, respectively to University of Delaware and University of Wisconsin, and extensive contributions of collaborators in the MiMeS project, particularly D. Cohen, V. Petit, J. Sundqvist, R. Townsend, A. ud-Doula, and G. Wade.

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  1. Department of Astronomy and CRSR, Cornell University, Ithaca, NY, 14853, USA

    Marina M. Romanova

  2. Department of Physics & Astronomy, University of Delaware, Newark, DE, 19716, USA

    Stanley P. Owocki

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  1. Marina M. Romanova
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  2. Stanley P. Owocki
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Correspondence to Marina M. Romanova.

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Romanova, M.M., Owocki, S.P. Accretion, Outflows, and Winds of Magnetized Stars. Space Sci Rev 191, 339–389 (2015). https://doi.org/10.1007/s11214-015-0200-9

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  • DOI: https://doi.org/10.1007/s11214-015-0200-9

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