Paper: astro-ph/0510014
Date: Sat, 1 Oct 2005 05:10:34 GMT (364kb)

Title: The Origin of Episodic Accretion Bursts in the Early Stages of Star
Formation

Authors: E. I. Vorobyov (1 and 2) and Shantanu Basu (1) ((1) University of
Western Ontario, Canada, (2) Rostov Institute of Physics, Russia)

Categories: astro-ph

Comments: 5 pages, 2 figures, accepted for publication in ApJL
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We study numerically the evolution of rotating cloud cores, from the collapse
of a magnetically supercritical core to the formation of a protostar and the
development of a protostellar disk during the main accretion phase. We find
that the disk quickly becomes unstable to the development of a spiral structure
similar to that observed recently in AB Aurigae. A continuous infall of matter
from the protostellar envelope makes the protostellar disk unstable, leading to
spiral arms and the formation of dense protostellar/protoplanetary clumps
within them. The growing strength of spiral arms and ensuing redistribution of
mass and angular momentum creates a strong centrifugal disbalance in the disk
and triggers bursts of mass accretion during which the dense
protostellar/protoplanetary clumps fall onto the central protostar. These
episodes of clump infall may manifest themselves as episodes of vigorous
accretion rate (\ge 10^{-4} M_sun/yr) as is observed in FU Orionis variables.
Between these accretion bursts, the protostar is characterized by a low
accretion rate (< 10^{-6} M_sun/yr). During the phase of episodic accretion,
the mass of the protostellar disk remains less than or comparable to the mass
of the protostar.

\\ ( http://arXiv.org/abs/astro-ph/0510014 , 364kb)

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Paper: astro-ph/0510015
Date: Sat, 1 Oct 2005 08:43:23 GMT (373kb)

Title: Turbulence and its parameterization in accretion discs

Authors: Axel Brandenburg

Categories: astro-ph

Comments: 11 pages, 11 figures

Report-no: NORDITA-2005-59

Journal-ref: Astron. Nachr. 326, 787-797 (2005)

DOI: 10.1002/asna.200510414
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Accretion disc turbulence is investigated in the framework of the shearing
box approximation. The turbulence is either driven by the magneto-rotational
instability or, in the non-magnetic case, by an explicit and artificial forcing
term in the momentum equation. Unlike the magnetic case, where most of the
dissipation occurs in the disc corona, in the forced hydrodynamic case most of
the dissipation occurs near the midplane. In the hydrodynamic case evidence is
presented for the stochastic excitation of epicycles. When the vertical and
radial epicyclic frequencies are different (modeling the properties around
rotating black holes), the beat frequency between these two frequencies appear
to show up as a peak in the temporal power spectrum in some cases. Finally, the
full turbulent resistivity tensor is determined and it is found that, if the
turbulence is driven by a forcing term, the signs of its off-diagonal
components are such that this effect would not be capable of dynamo action by
the shear--current effect.

\\ ( http://arXiv.org/abs/astro-ph/0510015 , 373kb)