Driven Disk Winds Research Articles

The effects of X-ray photoionization and heating on the structure of circumstellar discs

We present the results of a theoretical study investigating the effects of photoionization and heating by X‐rays on discs around low‐mass stars. In particular, we address the question of whether or not X‐rays can drive a disc wind. First, we construct a one‐dimensional ‘quasi‐hydrostatic’ model, which solves for the vertical structure introduced by X‐ray heating. We consider uniform X‐ray illumination of the disc, but the X‐ray fluxes required to heat the disc significantly are much greater than those seen by recent observations. When the model is extended to consider heating from a central X‐ray source, we find that the one‐dimensional model is only valid very close to the star. We extend our analysis to consider a simple two‐dimensional model, treating the disc as a two‐layered structure and solving for its density profile self‐consistently. For T Tauri stars, we are able to set a crude upper limit on the mass‐loss rate that can be driven by X‐ray photoevaporation, with a value of ≃10−13 g cm‐2 s−1. Our model is designed to maximize this value, and most likely overestimates it significantly. However, we still find a mass‐loss rate which is less than that found in studies of ultraviolet photoevaporation. We conclude that in the presence of a significant ultraviolet field, X‐ray driven disc winds are unlikely to play a significant role in the evolution of discs around low‐mass stars.

Early Disk Evolution

A variety of processes play a role in the evolution of protostellar disks. Here I focus on the uncertain issue of magnetic field-disk coupling and its implications for magnetically-driven turbulence and disk-driven winds. At present it is clear that the magnetic field plays a crucial role in disk evolution, but detailed conclusions cannot be drawn because the complicated interplay between dynamics and the evolution of the grain population remains to be explored.

Three‐dimensional Simulations of Jets from Keplerian Disks: Self‐regulatory Stability

We present the extension of previous two-dimensional simulations of the time-dependent evolution of nonrelativistic outflows from the surface of Keplerian accretion disks to three dimensions. As in the previous work, we investigate the outflow that arises from a magnetized accretion disk that is initially in hydrostatic balance with its surrounding cold corona. The accretion disk itself is taken to provide a set of fixed boundary conditions for the problem. We find that the mechanism of jet acceleration is identical to what was established from the previous two-dimensional simulations. The three-dimensional results are consistent with the theory of steady, axisymmetric, centrifugally driven disk winds up to the Alfven surface of the outflow. Beyond the Alfven surface, however, the jet in three dimensions becomes unstable to nonaxisymmetric, Kelvin-Helmholtz instabilities. The most important result of our work is that while the jet is unstable at super-Alfvenic speeds, it survives the onset of unstable modes that appear in this physical regime. We show that jets maintain their long-term stability through a self-limiting process wherein the average Alfvenic Mach number within the jet is maintained to the order of unity. This is accomplished in at least two ways. First, the poloidal magnetic field is concentrated along the central axis of the jet forming a backbone in which the Alfven speed is sufficiently high to reduce the average jet Alfvenic Mach number to unity. Second, the onset of higher order Kelvin-Helmholtz flute modes (m ≥ 2) reduces the efficiency with which the jet material is accelerated and transfers kinetic energy of the outflow into the stretched, poloidal field lines of the distorted jet. This too has the effect of increasing the Alfven speed and thereby reducing the Alfvenic Mach number. The jet is able to survive the onset of the more destructive m = 1 mode in this way. Our simulations also show that jets can acquire corkscrew or wobbling types of geometries in this relatively stable end state depending on the nature of the perturbations on them. Finally, we suggest that jets go into alternating periods of low and high activity since the disappearance of unstable modes in the sub-Alfvenic regime enables another cycle of acceleration to super-Alfvenic speeds.

Black Hole Magnetospheres around Thin Disks Driving Inward and Outward Winds

We construct a simple model for stationary, axisymmetric black hole magnetospheres, in which the poloidal magnetic field is generated by a toroidal electric current in a thin disk with an inner edge, by solving the vacuum Maxwell equations in the Schwarzschild background. In this work, to obtain a concise analytical form of the magnetic stream function, we use the approximation that the inner edge is far distant from the event horizon. The global magnetospheric structure with a closed-loop and open field lines threading the inner and outer parts of the disk is explicitly shown, claiming that the model is useful as a starting point to study astrophysical problems involving inward disk-driven winds to a black hole and outward ones to infinity. The asymptotic shape of the field lines at the event horizon becomes nearly cylindrical, while at infinity it becomes conical. The magnetic spot in the disk connected with the black hole through the loop field lines occupies a very narrow region with the ring area roughly equal to the horizon area. By taking account of the existence of a uniform (external) magnetic field, we also obtain a model for collimated open field lines. Then, it is found that the magnetic connection between the black hole and the disk breaks down if the uniform field is strong enough. Considering slow rotation of the magnetosphere and angular momentum transfer by inward winds from the disk, the final discussion is devoted to gradual disruption of the closed loops due to radial accretion of disk plasma toward the black hole.

The structure of black hole magnetospheres -- I. Schwarzschild black holes

We introduce a multipolar scheme for describing the structure of stationary, axisymmetric, force-free black-hole magnetospheres in the ``3+1'' formalism. We focus here on Schwarzschild spacetime, giving a complete classification of the separable solutions of the stream equation. We show a transparent term-by-term analogy of our solutions with the familiar multipoles of flat-space electrodynamics. We discuss electrodynamic processes around disk-fed black holes in which our solutions find natural applications: (a) ``interior'' solutions in studies of the Blandford-Znajek process of extracting the hole's rotational energy, and of the formation of relativistic jets in active galactic nuclei and ``microquasars'', and, (b) ``exterior'' solutions in studies of accretion disk dynamos, disk-driven winds and jets. On the strength of existing numerical studies, we argue that the poloidal field structures found here are also expected to hold with good accuracy for rotating black holes, except for maximum possible rotation rates. We show that the closed-loop exterior solutions found here are not in contradiction with the Macdonald-Thorne theorem, since these solutions, which diverge logarithmically on the hole's horizon $\cal H$, apply only to those regions which exclude $\cal H$.

Eclipse Mapping of the Accretion Disk Wind in the Cataclysmic Variable UX Ursae Majoris

We present the results of an effort to model recent HST eclipse observations of the C IV 1550 A resonance line in the high-inclination nova-like cataclysmic variable UX UMa. Assuming this line to be predominantly wind-formed, we are able to roughly reproduce the shapes and strengths of the observed line profiles (both away from and during eclipse) within the framework of a simple kinematic model in which the outflow is described as a rotating, biconical accretion disk wind. Our preferred model also matches most of the detailed behavior of different parts of the C IV line profile (blue wing, line center, and red wing) as a function of orbital phase. The most important result of our modeling is that it strongly suggests the presence of a high column (NH ~ 1023 cm-2), relatively dense (ne 4 × 1012 cm-3), and only slowly outflowing (vpoloidal vescape) transition region between the disk photosphere and the fast-moving wind. We also find that the outflow from UX UMa is probably only moderately collimated (the outer opening angle of our preferred model is θmax 65°). Finally, the rotational disturbance seen in the data is fitted reasonably well by our model, in which the rotational component of motion in the wind is fixed by conserving the specific angular momentum of the outflowing material. The implications of these results for dynamical models of disk-driven winds are discussed.

Axisymmetric MHD Simulations of Stellar Magnetosphere/Accretion Disk Interaction

AbstractThe evolution of the inner disk region of magnetized accreting systems is studied by means of numerical simulations. Two different initial magnetic field topologies are studied: a pure dipole field which threads the disk continuously and a dipole field excluded from the disk by surface currents. When present, the Balbus-Hawley (BH) instability provides efficient angular momentum transport, and accretion is equatorial. Otherwise, magnetic coupling between the disk and star surfaces as the dominant mechanism for transport and nonequatorial accretion results. In all of the simulations, low density, collimated disk-driven winds lead to mass loss.

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.