10 research outputs found
Disk-Magnetosphere Interaction and Outflows: Conical Winds and Axial Jets
We investigate outflows from the disk-magnetosphere boundary of rotating
magnetized stars in cases where the magnetic field of a star is bunched into an
X-type configuration using axisymmetric and full 3D MHD simulations. Such
configuration appears if viscosity in the disk is larger than diffusivity, or
if the accretion rate in the disk is enhanced. Conical outflows flow from the
inner edge of the disk to a narrow shell with an opening angle 30-45 degrees.
Outflows carry 0.1-0.3 of the disk mass and part of the disk's angular momentum
outward. Conical outflows appear around stars of different periods, however in
case of stars in the "propeller" regime, an additional - much faster component
appears: an axial jet, where matter is accelerated up to very high velocities
at small distances from the star by magnetic pressure force above the surface
of the star. Exploratory 3D simulations show that conical outflows are
symmetric about rotational axis of the disk even if magnetic dipole is
significantly misaligned. Conical outflows and axial jets may appear in
different types of young stars including Class I young stars, classical T Tauri
stars, and EXors.Comment: Invited review, conference proceedings of the meeting "Protostellar
Jets in Context", 7-12 July 2008, island of Rhodes, Greece; editors: profs.
Tom Ray and Kanaris Tsinganos; 10 pages, 10 figures, see animations at
http://www.astro.cornell.edu/~romanova/conical.htm and
http://www.astro.cornell.edu/~romanova/propeller.ht
Stationary relativistic jets
In this paper we describe a simple numerical approach which allows to study the structure of steady-state axisymmetric relativistic jets using one-dimensional time-dependent simulations. It is based on the fact that for narrow jets with vz≈cvz≈c the steady-state equations of relativistic magnetohydrodynamics can be accurately approximated by the one-dimensional time-dependent equations after the substitution z=ctz=ct. Since only the time-dependent codes are now publicly available this is a valuable and efficient alternative to the development of a high-specialised code for the time-independent equations. The approach is also much cheaper and more robust compared to the relaxation method. We tested this technique against numerical and analytical solutions found in literature as well as solutions we obtained using the relaxation method and found it sufficiently accurate. In the process, we discovered the reason for the failure of the self-similar analytical model of the jet reconfinement in relatively flat atmospheres and elucidated the nature of radial oscillations of steady-state jets
Accreting Millisecond X-Ray Pulsars
Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories
without parallel in the study of extreme physics. In this chapter we review the
past fifteen years of discoveries in the field. We summarize the observations
of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength
observations that have been carried out since the discovery of the first AMXP
in 1998. We review accretion torque theory, the pulse formation process, and
how AMXP observations have changed our view on the interaction of plasma and
magnetic fields in strong gravity. We also explain how the AMXPs have deepened
our understanding of the thermonuclear burst process, in particular the
phenomenon of burst oscillations. We conclude with a discussion of the open
problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations
and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer;
[revision with literature updated, several typos removed, 1 new AMXP added
Steady and Time-Dependent MHD Modelling of Jets
International audienceA brief review is given of some results of our work on the construction of (I) steady and (II) time-dependent MHD models for nonrelativistic and relativistic astrophysical outflows and jets, analytically and numerically. The only available exact solutions for MHD outflows are those in separable coordinates, i.e., with the symmetry of radial or meridional self-similarity. Physically accepted solutions pass from the fast magnetosonic separatrix surface in order to satisfy MHD causality. An energetic criterion is outlined for selecting radially expanding winds from cylindrically expanding jets. Numerical simulations of magnetic self-collimation verify the conclusions of analytical steady solutions. We also propose a two-component model consisting of a wind outflow from a central object and a faster rotating outflow launched from a surrounding accretion disk which plays the role of the flow collimator. We also discuss the problem of shock formation during the magnetic collimation of wind-type outflows into jets