Journal Article

Stochastic wobble of accretion discs and jets from turbulent rocket torques

Ryan Pettibone and Eric G. Blackman

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 3, pages 1783-1788
Published in print July 2009 | ISSN: 0035-8711
Published online June 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14863.x
Stochastic wobble of accretion discs and jets from turbulent rocket torques

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Models of accretion discs and their associated outflows often incorporate assumptions of axisymmetry and symmetry across the disc plane. However, for turbulent discs these symmetries only apply to averaged quantities and do not apply locally. The local asymmetries can induce local imbalances in outflow power across the disc mid–plane, which can in turn induce local tilting torques. Here we calculate the effect of the resulting stochastic torques on disc annuli that are a consequence of standard mean field accretion disc models. The torques induce a random walk of the vector perpendicular to the plane of each averaged annulus. This random walk is characterized by a radially dependent diffusion coefficient which we calculate for small angle tilt. We use the coefficient to calculate a radially dependent time–scale for annular tilt and associated jet wobble. The wobble time depends on the square of the wander angle so the age of a given system determines the maximum wobble angle. We apply this to examples of blazars, young stellar objects and binary engines of pre–planetary nebulae and microquasars. It is noteworthy that for an averaging time tw∼ 3 d, we estimate a wobble angle for jets in SS 433 of θ∼ 0.8°, not inconsistent with observational data. In general the non–periodic nature of the stochastic wobble could distinguish it from faster periodic jet precession.

Keywords: accretion, accretion discs; turbulence; binaries: general; ISM: jets and outflows; planetary nebulae: general; X–rays: binaries

Journal Article.  3706 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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