Active galactic nuclei jet mass loading and truncation by stellar winds

Active galactic nuclei can produce extremely powerful jets. While tightly collimated, the scale of these jets and the stellar density at galactic centres implies that there will be many jet/star interactions, which can mass load the jet through stellar winds. Previous work employed modest wind mass outflow rates, but this does not apply when mass loading is provided by a small number of high mass-loss stars. We construct a framework for jet mass loading by stellar winds for a broader spectrum of wind mass-loss rates than has previously been considered. Given the observed stellar mass distributions in galactic centres, we find that even highly efficient (0.1 Eddington luminosity) jets from supermassive black holes of masses M _BH≲ 10⁴ M_⊙ are rapidly mass loaded and quenched by stellar winds. For  10⁴ M_⊙ < M _BH < 10⁸ M_⊙  , the quenching length of highly efficient jets is independent of the jet's mechanical luminosity. Stellar wind mass loading is unable to quench efficient jets from more massive engines, but can account for the observed truncation of the inefficient M87 jet, and implies a baryon-dominated composition on scales ≳2 kpc therein even if the jet is initially pair plasma dominated.     

Magnetized centrifugal winds

It is argued that for steady, axisymmetric, non-relativistic magneto-centrifugal winds, not only the boundary and criticality conditions but also the current-closure condition are of crucial significance as global conditions in resolving the acceleration-collimation problem. In Sakurai's numerical models, the split-monopole field adopted at the surface of the source provided the most favourable condition for global collimation of the flow, by making the domain of anti -collimating flow with outgoing electric current degenerate into an infinitely thin boundary layer at the equator, and hence suppressing the explicit appearance of the current-closure condition.
For more general or realistic boundary conditions at the source, it is shown that the current-closure condition yields a two-component structure (with the return current at least in part in a volume current, not totally a sheet current) as a natural consequence of the transfield equation in the asymptotic domain. This equation, combined with the Bernoulli (and other) integrals, requires the wind to tend asymptotically to a 'quasi-conical' structure, as a natural consequence of the flow particles' becoming more and more ballistic as a result of the magnetohydrodynamic (MHD) acceleration. This is a result that the Poynting energy flux diminishes to zero along each field line. The criticality problem is solved for magneto-centrifugal winds, to give the eigenvalues of the Alfvénic distance and other quantities at the fast magnetosonic surface, situated somewhere between the subasymptotic and asymptotic domains.     

Dark matter distribution in the Draco dwarf from velocity moments

If the observed relativistic plasma outflows in astrophysical jets are magnetically collimated and a single-component model is adopted, consisting of a wind-type outflow from a central object, then a problem arises with the inefficiency of magnetic self-collimation to collimate a sizeable portion of the mass and magnetic fluxes in the relativistic outflow from the central object. To solve this dilemma, we have applied the mechanism of magnetic collimation to a two-component model consisting of a relativistic wind-type outflow from a central source and a non-relativistic wind from a surrounding disc. By employing a numerical code for a direct numerical solution of the steady-state problem in the zone of super-fast magnetized flow, which allows us to perform a determination of the flow with shocks, it is shown that in this two-component model it is possible to collimate into cylindrical jets all the mass and magnetic fluxes that are available from the central source. In addition, it is shown that the collimation of the plasma in this system is usually accompanied by the formation of oblique shock fronts. The non-relativistic disc-wind not only plays the role of the jet collimator, but it also induces the formation of shocks as it collides with the initially radial inner relativistic wind and also as the outflow is reflected by the system axis. Another interesting feature of this process of magnetic collimation is a sequence of damped oscillations in the width of the jet.     

Collapse and fragmentation of rotating magnetized clouds – II. Binary formation and fragmentation of first cores

Subsequent to Paper I, the evolution and fragmentation of a rotating magnetized cloud are studied with use of three-dimensional magnetohydrodynamic nested grid simulations. After the isothermal runaway collapse, an adiabatic gas forms a protostellar first core at the centre of the cloud. When the isothermal gas is stable for fragmentation in a contracting disc, the adiabatic core often breaks into several fragments. Conditions for fragmentation and binary formation are studied. All the cores which show fragmentation are geometrically thin, as the diameter-to-thickness ratio is larger than 3. Two patterns of fragmentation are found. (1) When a thin disc is supported by centrifugal force, the disc fragments into a ring configuration (ring fragmentation). This is realized in a rapidly rotating adiabatic core as  Ω > 0.2τ⁻¹_ff  , where Ω and  τ_ff  represent the angular rotation speed and the free-fall time of the core, respectively. (2) On the other hand, the disc is deformed to an elongated bar in the isothermal stage for a strongly magnetized or rapidly rotating cloud. The bar breaks into 2–4 fragments (bar fragmentation). Even if a disc is thin, the disc dominated by the magnetic force or thermal pressure is stable and forms a single compact body. In either ring or bar fragmentation mode, the fragments contract and a pair of outflows is ejected from the vicinities of the compact cores. The orbital angular momentum is larger than the spin angular momentum in the ring fragmentation. On the other hand, fragments often quickly merge in the bar fragmentation, since the orbital angular momentum is smaller than the spin angular momentum in this case. Comparison with observations is also shown.     

The motion of wind-driven shells

We present a method for solving problems in which a stellar wind interacts with the surrounding environment through the production of a 'double radiative shock' structure. This condition is generally met in problems involving winds ejected from young stars. We describe a method that can be applied to problems of winds with arbitrary time and angular dependence, interacting with a stationary environment with an arbitrary density distribution. We apply the method to the interaction of: a steady wind (with an instantaneous 'turning-on') with a power-law environmental density stratification, a 'wind plus jet' ejection with a toroidal environmental density stratification, and to the interaction of an isotropic wind with a clumpy environment. These three examples illustrate the wide range of possible applications of the proposed method. We also show a comparison between some of our thin-shell solutions and three-dimensional isothermal gasdynamic simulations of the flows. These comparisons are used as an evaluation of the applicability of our thin-shell solutions to the real flows.     

On the central core in MHD winds and jets

We analytically determine the structure of highly magnetized astrophysical jets at the origin in a region where the flow has been already collimated by an external medium, in both relativistic and non-relativistic regimes. We show that this can be achieved by solving a system of first-order ordinary differential equations that describe the transversal jet structure for a variety of external confining pressure profiles that collimate the jet to a near-cylindrical configuration. We obtain solutions for a central jet surrounded either by a self-similar wind or by an external pressure profile and derive the dependence of the velocity and the magnetic field strength along and across our jets. In particular, we find that the central core in a jet – the part of a flow with a nearly homogeneous magnetic field – must contain a poloidal field which is not much smaller than the critical value B _min. This allows us to determine the magnetic flux in a core which is much smaller than the total magnetic flux. We show that for such a small core flux the solutions with a magnetic field in a core much smaller than B _min are non-physical. For astrophysical objects the value of the critical magnetic field is quite large: 1 G for active galactic nuclei, 10¹⁰ G for gamma-ray bursts and 10⁻¹ G for young stellar objects. In a relativistic case for the core field greater than or of the order of B _min we show analytically that the plasma Lorentz factor must grow linearly with the cylindrical radius. For non-relativistic highly magnetized jets we propose that an oblique shock exists near the base of the jet so that the finite gas pressure plays an important role in force balance.     

L 43: the late stages of a molecular outflow

Our new 21-arcsec resolution CO J  = 2 → 1 map of the L 43 dark cloud shows a poorly collimated molecular outflow, with little evidence for wings at velocities 10 km s⁻¹. The outflow appears not to be currently driven by a jet: its structure can instead be modelled as a slowly expanding shell. The shell may be compressed either by a wide-angled wind catching up with an existing shell (as in the case of planetary nebulæ), or by the thermal pressure of a hot low-emissivity medium interior to the shell. The outflow is most probably in a late stage of evolution, and appears to be in the process of blowing away its molecular cloud. We also present a 45-arcsec resolution CO J  = 1 → 0 map of the whole molecular cloud, showing that the outflow structure is clearly visible even in the integrated intensity of this low excitation line, and suggesting that rapid mapping may prove useful as a way of finding regions of outflow activity. We also examine the immediate surroundings of the driving source with 450 μm imaging: this confirms that the outflow has already evacuated a bay in the vicinity of the young stellar object.     

The origin of the strings in the outer regions of η Carinae

The narrow optical filaments ('strings' or 'spikes') emerging from the Homunculus of η Carinae are modelled as resulting from the passage of ballistic 'bullets' of material through the dense circumstellar environment. In this explanation, the string is the decelerating flow of ablated gas from the bullet. An archive Hubble Space Telescope image and new forbidden-line profiles of the most distinct of the strings are presented and discussed in terms of this simple model.     

GRMHD/RMHD simulations & stability of magnetized spine-sheath relativistic jets

A new general relativistic magnetohydrodynamics (GRMHD) code “RAISHIN” used to simulate jet generation by rotating and non-rotating black holes with a geometrically thin Keplarian accretion disk finds that the jet develops a spine-sheath structure in the rotating black hole case. Spine-sheath structure and strong magnetic fields significantly modify the Kelvin-Helmholtz (KH) velocity shear driven instability. The RAISHIN code has been used in its relativistic magnetohydrodynamic (RMHD) configuration to study the effects of strong magnetic fields and weakly relativistic sheath motion, c/2, on the KH instability associated with a relativistic, γ=2.5, jet spine-sheath interaction. In the simulations sound speeds up to and Alfvén wave speeds up to ∼0.56c are considered. Numerical simulation results are compared to theoretical predictions from a new normal mode analysis of the RMHD equations. Increased stability of a weakly magnetized system resulting from c/2 sheath speeds and stabilization of a strongly magnetized system resulting from c/2 sheath speeds is found.