Adams, Fred C. and Gregory, Scott G. (2012) Magnetically Controlled Accretion Flows onto Young Stellar Objects. Astrophysical Journal, 744 (1). Art. No. 55. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120120-111022937
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Accretion from disks onto young stars is thought to follow magnetic field lines from the inner disk edge to the stellar surface. The accretion flow thus depends on the geometry of the magnetic field. This paper extends previous work by constructing a collection of orthogonal coordinate systems, including the corresponding differential operators, where one coordinate traces the magnetic field lines. This formalism allows for an (essentially) analytic description of the geometry and the conditions required for the flow to pass through sonic points. Using this approach, we revisit the problem of magnetically controlled accretion flow in a dipole geometry, and then generalize the treatment to consider magnetic fields with multiple components, including dipole, octupole, and split monopole contributions. This approach can be generalized further to consider more complex magnetic field configurations. Observations indicate that accreting young stars have substantial dipole and octupole components, and that accretion flow is transonic. If the effective equation of state for the fluid is too stiff, however, the flow cannot pass smoothly through the sonic points in steady state. For a multipole field of order ℓ, we derive a general constraint on the polytropic index, n > ℓ + 3/2, required for steady transonic flow to reach free-fall velocities. For octupole fields, inferred on surfaces of T Tauri stars, the index n > 9/2, so that the flow must be close to isothermal. The inclusion of octupole field components produces higher densities at the stellar surface and smaller areas for the hot spots, which occur at higher latitudes; the magnetic truncation radius is smaller (larger) for octupole components that are aligned (anti-aligned) with the stellar dipole. This contribution thus increases our understanding of magnetically controlled accretion for young stellar objects and can be applied to a variety of additional astrophysical problems.
|Additional Information:||© 2012 The American Astronomical Society. Received 2011 August 11; accepted 2011 September 27; published 2011 December 13. This paper benefited from discussions with many colleagues, especially Daniele Galli, Lynne Hillenbrand, and Susana Lizano. This project was initiated during a sabbatical visit by F.C.A. to the California Institute of Technology, and we are grateful for the generous hospitality of the CalTech Astronomy Department. This work was supported at the University of Michigan through theMichigan Center for Theoretical Physics. F.C.A. is supported by NASA through the Origins of Solar Systems program (grant NNX11AK87G), and by NSF through the Division of Applied Mathematics (grant DMS-0806756). S.G.G. is supported by NASA grant HST-GO-11616.07-A.|
|Subject Keywords:||magnetohydrodynamics (MHD); protoplanetary disks; stars: formation; stars: magnetic field; stars: pre-main sequence|
|Official Citation:||Magnetically Controlled Accretion Flows onto Young Stellar Objects Fred C. Adams and Scott G. Gregory 2012 ApJ 744 55|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||20 Jan 2012 19:27|
|Last Modified:||26 Dec 2012 14:43|
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