309 research outputs found
Accretion, Outflows, and Winds of Magnetized Stars
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.Comment: 60 pages, 44 figure
Global bifurcations to subcritical magnetorotational dynamo action in Keplerian shear flow
Magnetorotational dynamo action in Keplerian shear flow is a three-dimensional, non-linear magnetohydrodynamic process whose study is relevant to the understanding of accretion processes and magnetic field generation in astrophysics. Transition to this form of dynamo action is subcritical and shares many characteristics of transition to turbulence in non-rotating hydrodynamic shear flows. This suggests that these different fluid systems become active through similar generic bifurcation mechanisms, which in both cases have eluded detailed understanding so far. In this paper, we build on recent work on the two problems to investigate numerically the bifurcation mechanisms at work in the incompressible Keplerian magnetorotational dynamo problem in the shearing box framework. Using numerical techniques imported from dynamical systems research, we show that the onset of chaotic dynamo action at magnetic Prandtl numbers larger than unity is primarily associated with global homoclinic and heteroclinic bifurcations of nonlinear magnetorotational dynamo cycles. These global bifurcations are found to be supplemented by local bifurcations of cycles marking the beginning of period-doubling cascades. The results suggest that nonlinear magnetorotational dynamo cycles provide the pathway to turbulent injection of both kinetic and magnetic energy in incompressible magnetohydrodynamic Keplerian shear flow in the absence of an externally imposed magnetic field. Studying the nonlinear physics and bifurcations of these cycles in different regimes and configurations may subsequently help to better understand the physical conditions of excitation of magnetohydrodynamic turbulence and instability-driven dynamos in a variety of astrophysical systems and laboratory experiments. The detailed characterization of global bifurcations provided for this three-dimensional subcritical fluid dynamics problem may also prove useful for the problem of transition to turbulence in hydrodynamic shear flows
Turbulent Magnetic Field Amplification from Spiral SASI Modes: Implications for Core-Collapse Supernovae and Proto-Neutron Star Magnetization
We extend our investigation of magnetic field evolution in three-dimensional
flows driven by the stationary accretion shock instability (SASI) with a suite
of higher-resolution idealized models of the post-bounce core-collapse
supernova environment. Our magnetohydrodynamic simulations vary in initial
magnetic field strength, rotation rate, and grid resolution. Vigorous
SASI-driven turbulence inside the shock amplifies magnetic fields
exponentially; but while the amplified fields reduce the kinetic energy of
small-scale flows, they do not seem to affect the global shock dynamics. The
growth rate and final magnitude of the magnetic energy are very sensitive to
grid resolution, and both are underestimated by the simulations. Nevertheless
our simulations suggest that neutron star magnetic fields exceeding G
can result from dynamics driven by the SASI, \emph{even for non-rotating
progenitors}.Comment: 28 pages, 17 figures, accepted for publication in the Ap
Super-Eddington accretion; flow regimes and conditions in high-z galaxies
We review and discuss theoretical studies addressing the possibility of gas
accretion onto black holes occurring at rates exceeding the Eddington limit.
Our focus is on the applications to the growth of black hole seeds at high
redshift. We first present the general notion of Super-Eddington accretion, and
then summarize the different models and numerical simulations developed to
study such regime. We consider optically thick flows in accretion disks as well
as in spherically symmetric envelopes, and devote particular attention to the
widely adopted model based on the SLIM disk solution. While attractive for its
simplicity, the SLIM disk solution is challenged by the latest generation of
three-dimensional radiation (magneto)-hydrodynamical simulations, in which
radiative losses can be an order of magnitude higher, and the mechanisms of
radiation transport is more complex than straight advection as it takes place
in a complex turbulent regime. We then discuss the gas supply rate to the
sub-pc scale accretion disk or envelope from larger scales, revisiting gas
inflow rates in protogalaxies under various conditions. We conclude that in the
dense gaseous nuclei of high-z galaxies the conditions necessary for the onset
of Super Eddington accretion regimes, such as a high optical depth and high gas
supply rates from large scales, should be naturally met. Feedback from the
growing BH seed should not alter significantly such conditions according to the
results of radiation magneto-hydrodynamical simulations of super-critical flows
in accretion disks. Furthermore, based on the required nuclear gas inflow rates
and the tendency of stellar feedback to remove efficiently gas in low mass
halos, we argue that super-critical accretion will be more easily achieved in
relatively sizable halos, with virial masses
M, which become more common at .Comment: Preprint of the chapter "Super-Eddington accretion; flow regimes and
conditions in high-z galaxies", to be published in the review volume
"Formation of the First Black Holes", Latif, M. and Schleicher, D. R. G.,
eds., World Scientific Publishing Company, 2018, pp 195-228 [ see
https://www.worldscientific.com/worldscibooks/10.1142/10652
Feedback first: the surprisingly weak effects of magnetic fields, viscosity, conduction, and metal diffusion on galaxy formation
Using high-resolution simulations with explicit treatment of stellar feedback
physics based on the FIRE (Feedback in Realistic Environments) project, we
study how galaxy formation and the interstellar medium (ISM) are affected by
magnetic fields, anisotropic Spitzer-Braginskii conduction and viscosity, and
sub-grid metal diffusion from unresolved turbulence. We consider controlled
simulations of isolated (non-cosmological) galaxies but also a limited set of
cosmological "zoom-in" simulations. Although simulations have shown significant
effects from these physics with weak or absent stellar feedback, the effects
are much weaker than those of stellar feedback when the latter is modeled
explicitly. The additional physics have no systematic effect on galactic star
formation rates (SFRs) . In contrast, removing stellar feedback leads to SFRs
being over-predicted by factors of . Without feedback, neither
galactic winds nor volume filling hot-phase gas exist, and discs tend to
runaway collapse to ultra-thin scale-heights with unphysically dense clumps
congregating at the galactic center. With stellar feedback, a multi-phase,
turbulent medium with galactic fountains and winds is established. At currently
achievable resolutions and for the investigated halo mass range
, the additional physics investigated here (MHD,
conduction, viscosity, metal diffusion) have only weak (-level)
effects on regulating SFR and altering the balance of phases, outflows, or the
energy in ISM turbulence, consistent with simple equipartition arguments. We
conclude that galactic star formation and the ISM are primarily governed by a
combination of turbulence, gravitational instabilities, and feedback. We add
the caveat that AGN feedback is not included in the present work
MHD simulations of the formation and propagation of protostellar jets to observational length scales
We present 2.5-D global, ideal MHD simulations of magnetically and
rotationally driven protostellar jets from Keplerian accretion discs, wherein
only the initial magnetic field strength at the inner radius of the disc,
, is varied. Using the AMR-MHD code AZEUS, we self-consistently
follow the jet evolution into the observational regime ()
with a spatial dynamic range of . The simulations reveal a
three-component outflow: 1) A hot, dense, super-fast and highly magnetised 'jet
core'; 2) a cold, rarefied, trans-fast and highly magnetised 'sheath'
surrounding the jet core and extending to a tangential discontinuity; and 3) a
warm, dense, trans-slow and weakly magnetised shocked ambient medium entrained
by the advancing bow shock. The simulations reveal power-law relationships
between and the jet advance speed, , the average jet
rotation speed, , as well as fluxes of mass,
momentum, and kinetic energy. Quantities that do not depend on
include the plasma- of the transported material which, in all cases,
seems to asymptote to order unity. Jets are launched by a combination of the
'magnetic tower' and 'bead-on-a-wire' mechanisms, with the former accounting
for most of the jet acceleration---even for strong fields---and continuing well
beyond the fast magnetosonic point. At no time does the leading bow shock leave
the domain and, as such, these simulations generate large-scale jets that
reproduce many of the observed properties of protostellar jets including their
characteristic speeds and transported fluxes.Comment: 26 pages, 16 figures. Accepted for publication in MNRA
Turbulence in the interstellar medium
Turbulence is ubiquitous in the insterstellar medium and
plays a major role in several processes such as the formation of dense
structures and stars, the stability of molecular clouds, the amplification of
magnetic fields, and the re-acceleration and diffusion of cosmic rays.
Despite its importance, interstellar turbulence, like turbulence in general,
is far from being fully understood. In this review we present the basics of
turbulence physics, focusing on the statistics of its structure and energy
cascade. We explore the physics of compressible and incompressible turbulent
flows, as well as magnetised cases. The most relevant observational
techniques that provide quantitative insights into interstellar turbulence
are also presented. We also discuss the main difficulties in developing a
three-dimensional view of interstellar turbulence from these observations.
Finally, we briefly present what the main sources of turbulence in the
interstellar medium could be
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