4,088 research outputs found
MHD disc winds
This is a doctorate level lecture on the physics of accretion discs driving
magnetically self-confined jets, usually referred to in the literature as disc
winds. I will first review the governing magnetohydrodynamic equations and then
discuss their physical content. At that level, necessary conditions to drive
jets from keplerian accretion discs can already be derived. These conditions
are validated with self-similar calculations of accretion-ejection structures.
In a second part, I will critically discuss the biases introduced when using
self-similarity as well as some other questions such as: Are these systems
really unstable? Can a standard accretion disc provide the conditions to launch
jets in its innermost parts? What is the difference between X-winds and
disc-winds? Finally, the magnetic interaction between a protostar and its
circumstellar disc will be discussed with a focus on stellar spin down.Comment: 25 pages, 11 figures to be published in Lecture Notes in Physics,
"Jets from Young Stars: Models and Constraints", J. Ferreira, C. Dougados and
E. Whelan (eds), Springer Verla
Braking down an accreting protostar: disc-locking, disc winds, stellar winds, X-winds and Magnetospheric Ejecta
Classical T Tauri stars are low mass young forming stars that are surrounded
by a circumstellar accretion disc from which they gain mass. Despite this
accretion and their own contraction that should both lead to their spin up,
these stars seem to conserve instead an almost constant rotational period as
long as the disc is maintained. Several scenarios have been proposed in the
literature in order to explain this puzzling "disc-locking" situation: either
deposition in the disc of the stellar angular momentum by the stellar
magnetosphere or its ejection through winds, providing thereby an explanation
of jets from Young Stellar Objects.
In this lecture, these various mechanisms will be critically detailed, from
the physics of the star-disc interaction to the launching of self-confined jets
(disc winds, stellar winds, X-winds, conical winds). It will be shown that no
simple model can account alone for the whole bulk of observational data and
that "disc locking" requires a combination of some of them.Comment: 60 pages, 29 figures Lecture held in Evry Schatzman School 201
On fan-shaped cold MHD winds from Keplerian accretion discs
We investigate under which conditions cold, fan-shaped winds can be steadily
launched from thin (Keplerian) accretion discs. Such winds are
magneto-centrifugal winds launched from a thin annulus in the disc, along open
magnetic field lines that fan out above the disc. In principle, such winds
could be found in two situations: (1) at the interface between an inner Jet
Emitting Disc, which is itself powering magneto-centrifugally driven winds, and
an outer standard accretion disc; (2) at the interface between an inner closed
stellar magnetosphere and the outer standard accretion disc. We refer to
Terminal or T-winds to the former kind and to Magnetospheric or M-winds to the
latter.
The full set of resistive and viscous steady state MHD equations are analyzed
for the disc (the annulus), which allow us to derive general expressions valid
for both configurations. We find that, under the framework of our analysis, the
only source of energy able to power any kind of fan-shaped winds is the viscous
transport of rotational energy coming below the inner radii. Using standard
local prescriptions for the anomalous (turbulent) transport of angular
momentum and magnetic fields in the disc, we derive the strength of the
transport coefficients that are needed to steadily sustain the global
configuration. It turns out that, in order for these winds to be dynamically
relevant and explain observed jets, the disc coefficients must be far much
larger than values expected from current knowledge of turbulence occurring
inside proto-stellar discs.
Either the current view on MHD turbulence must be deeply reconsidered or
steady-state fan-shaped winds are never realized in Nature. The latter
hypothesis seems to be consistent with current numerical simulations.Comment: Among several possibilites, this paper addresses also the case of the
X-wind Accepted for publication in MNRA
Jet launching and field advection in quasi-Keplerian discs
The fact that self-confined jets are observed around black holes, neutron
stars and young forming stars points to a jet launching mechanism independent
of the nature of the central object, namely the surrounding accretion disc. The
properties of Jet Emitting Discs (JEDs) are briefly reviewed. It is argued
that, within an alpha prescription for the turbulence (anomalous viscosity and
diffusivity), the steady-state problem has been solved. Conditions for
launching jets are very stringent and require a large scale magnetic field
close to equipartition with the total (gas and radiation) pressure. The
total power feeding the jets decreases with the disc thickness: fat ADAF-like
structures with cannot drive super-Alfv\'enic jets. However, there
exist also hot, optically thin JED solutions that would be observationally very
similar to ADAFs.
Finally, it is argued that variations in the large scale magnetic field
is the second parameter required to explain hysteresis cycles seen in LMXBs
(the first one would be ).Comment: 4 pages, 1 figure, proceedings of IAU 275 "Jets at all scales"
(Gustavo E. Romero, Rashid A. Sunyaev and Tomaso Belloni, eds
MHD simulations of accretion onto a dipolar magnetosphere. II. Magnetospheric ejections and stellar spin-down
This paper examines the outflows associated with the interaction of a stellar
magnetosphere with an accretion disk. In particular, we investigate the
magnetospheric ejections (MEs) due to the expansion and reconnection of the
field lines connecting the star with the disk. Our aim is to study the
dynamical properties of the outflows and evaluate their impact on the angular
momentum evolution of young protostars. Our models are based on axisymmetric
time-dependent magneto-hydrodynamic simulations of the interaction of the
dipolar magnetosphere of a rotating protostar with a viscous and resistive
disk, using alpha prescriptions for the transport coefficients. Our simulations
are designed in order to model: the accretion process and the formation of
accretion funnels; the periodic inflation/reconnection of the magnetosphere and
the associated MEs; the stellar wind. Similarly to a magnetic slingshot, MEs
can be powered by the rotation of both the disk and the star so that they can
efficiently remove angular momentum from both. Depending on the accretion rate,
MEs can extract a relevant fraction of the accretion torque and, together with
a weak but non-negligible stellar wind torque, can balance the spin-up due to
accretion. When the disk truncation approaches the corotation radius, the
system enters a "propeller" regime, where the torques exerted by the disk and
the MEs can even balance the spin-up due to the stellar contraction. The MEs
spin-down efficiency can be compared to other scenarios, such as the Ghosh &
Lamb, X-wind or stellar wind models. Nevertheless, for all scenarios, an
efficient spin-down torque requires a rather strong dipolar component, which
has been seldom observed in classical T Tauri stars. A better analysis of the
torques acting on the protostar must take into account non-axisymmetric and
multipolar magnetic components consistent with observations.Comment: 21 pages, 16 figures, accepted for publication in Astronomy &
Astrophysic
Inflationary dynamics of kinetically-coupled gauge fields
We investigate the inflationary dynamics of two kinetically-coupled massless
gauge fields with time-varying kinetic-term coefficients. Ensuring that
the system does not have strongly coupled regimes shrinks the parameter space.
Also, we further restrict ourselves to systems that can be quantized using the
standard creation, annihilation operator algebra. This second constraint limits
us to scenarios where the system can be diagonalized into the sum of two
decoupled, massless, vector fields with a varying kinetic-term coefficient.
Such a system might be interesting for magnetogenesis because of how the
strong coupling problem generalizes. We explore this idea by assuming that one
of the gauge fields is the Standard Model field and that the other dark
gauge field has no particles charged under its gauge group. We consider whether
it would be possible to transfer a magnetic field from the dark sector,
generated perhaps before the coupling was turned on, to the visible sector. We
also investigate whether the simple existence of the mixing provides more
opportunities to generate magnetic fields. We find that neither possibility
works efficiently, consistent with the well-known difficulties in inflationary
magnetogenesis.Comment: 17 pages, 0 figures. Matches JCAP versio
Modelling multimodal passenger choices with stated preference data
Redland Shire Council has recently started the implementation of an Integrated Local Transport Plan (ILTP) that aims to reduce the car dependency by enhancing the usage of alternative modes of transport. A multi mode choice model is required that can forecast the travel behaviour across the region in order to achieve the targets set in ILTP. This paper presents the findings of a state-of-the-art literature review done on mode choice modelling and outlines the development and calibration of a model to investigate the travel behaviour of Redlands’ residents. The present study attempts to develop a nested logit model and calibrate it using data obtained from a stated preference (SP) survey to be conducted in the Shire. The model development will consider all the vital attributes of the travelling modes used in the study area including various public transit access modes. The possibility of combining SP and revealed preference (RP) data to calibrate the model using joint-estimation method will be further assessed. It is expected that the outcomes of the research will assist policy makers in the areas of public transport planning and the development of network for public transport access modes including walkways and cycleways
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