Recent papers on astro-ph
The Origin of Jets
The origin of astrophysical jets
Highly-collimated, oppositely directed jets are observed in active galaxies and quasars, and in old compact stars in binaries. Further, well collimated emission line jets are seen in young stellar objects.
Recent work favors models where the twisting of an ordered
magnetic field threading an accretion disk acts to magnetically accelerate the jets.
Our papers Ustyugova et al. (1995), Romanova et al. (1997), and Ustyugova et al. (1999) have to do with the origin of jets in the hydromagnetic regime. The papers Romanova et al. (1998), Ustyugova et al. (2000), Lovelace et al. (2002), Lovelace and Romanova (2003a, 2003b) have to do with the origin of jets in the Poynting regime.
THEORY AND SIMULATIONS OF POINTING JETS
observed to emanate from many compact accreting objects may arise from
the twisting of a magnetic field threading a differentially rotating
accretion disk which acts to magnetically extract angular momentum and
energy from the disk. Two main regimes have been discussed,
hydromagnetic jets, which have a significant mass flux and have energy
and angular momentum carried by both matter and electromagnetic field
and, Poynting jets, where the mass flux is small and energy and angular
momentum are carried predominantly by the electromagnetic field. Our
group has developed the theory of the formation of relativistic Poynting
jets from magnetized accretion disks. Further, we have carried out
relativistic, fully-electromagnetic, particle-in-cell simulations of the
formation of jets from accretion disks. Analog Z-pinch experiments may
Origin of Jets from Accretion Disks",
Model of Double Radio Sources"
MAGNETO-CENTRIFUGALLY DRIVEN WINDS: COMPARISON OF MHD SIMULATIONS WITH THEORY
We have studied MHD outflows from a rotating, conducting accretion disk using axisymmetric simulations. The disk was treated as a boundary condition, and the initial poloidal magnetic field was taken to be a split monopole. The main conclusions of this work are:
In many different runs we observed the formation of stationary MHD outflows from the disk. Close to the disk the main driving force is the centrifugal force. At larger distances the main driving force is the magnetic force ~-(rBf )2. The pressure gradient force is much smaller than these forces, and it has no significant role in driving the outflows.
For the considered conditions, the slow magnetosonic surface lies inside the disk. Above the disk the flow accelerates and passes through the Alfvén and fast magnetosonic surfaces, which are almost parallel to the disk. Within the simulation region, the outflow accelerates from thermal velocity (~cs) to a much larger asymptotic, poloidal flow velocity of the order of 0.5, where M is the mass of the central object and ri is the inner radius of the disk. This asymptotic velocity is much larger than the local escape speed and is larger than fast magnetosonic speed by a factor of ~1.75. The acceleration distance for the outflow, over which the flow accelerates from ~0% to, say, 90% of the asymptotic speed, occurs at a flow distance ~80ri.
The outflow is only slightly collimated within the simulation region. The collimation distance for the outflow, over which the flow becomes collimated (with divergence less than, say, 10°) is much larger than the size of our simulation region. This "poor" collimation is similar to that found in our earlier work (Romanova et al. 1997) using a different initial magnetic field and is qualitatively similar to the very gradual collimation found by Sakurai (1987). MHD simulations using much larger computational regions are needed to determine the collimation of the outflow at large distances. Furthermore, separate simulations are also needed to study the collimating influence of an external medium (Lovelace et al. 1991; Mellema & Frank 1998).
The stationarity of the MHD flows was checked in a number of ways, including comparisons of simulation results with predictions of theory of stationary axisymmetric flows. We found that: (1) fluxes of mass, angular momentum, and energy across the surface z=0.5Zmax become independent of time with high precision at early times of simulations t < 0.1 tout , where tout ~ 2200 ti and ti =2p ri /; (2) integrals of the motion become constants on flux surfaces with accuracy 5% - 15% for t>tout; and (3) vectors of poloidal velocity are parallel to those of the poloidal magnetic field lines to a high accuracy.
Different outer boundary conditions on the toroidal magnetic field Bf were investigated. We analyzed simulation results and found that the collimation of the jet and other characteristics of the flow depend critically on the outer boundary condition on Bf (as well as the shape of the simulation region, as is discussed below). We observed that the outer, "free" boundary condition on Bf leads to an artificial force that can give apparent magnetic collimation of the flow. "Force-free" and "force-balance" outer boundary conditions were also investigated. The "force-free" outer boundary condition was found to give valid flow solutions if the simulation region is not narrow in r-direction (compared with z-direction).
The question of the optimum shape of simulation region was investigated. We have shown that if region is narrow in the r-direction, then an essential part of the Mach cones on the outer boundaries may be directed toward the inside of the computational region. This can lead to the influence of the boundary on the calculated flow and to artificial collimation. This effect is reduced or absent if the computational region is approximately square, if it is elongated in the r-direction, or if it is spherical. In these cases the Mach cones tend to point outside of the computational region.