Hydromagnetic Winds Research Articles

Molecular outflows from G35.2-0.74N

We present interferometric observations of the massive star-forming region associated with G35.2-0.74N using the Berkeley Illinois Maryland Association Array. With the aim of better understanding the outflow in this region we observed , and the 2.7 mm continuum. The and continuum emission peak close to the sources G35.2-0.74N (G35.2N) and G35MM2 and indicate a mass of ~ of circumstellar material associated with these sources. The 12CO traces a weak filament of emission coincident with the radio and infrared jet from G35.2N but the emission is dominated by an extended outflow with a NE-SW axis which has a total mass of ~13 . Each lobe of this extended outflow has a hollow shell structure and the location of these shells makes the source G35MM2 a more likely candidate for the source driving the outflow than G35.2N. The mass-velocity distribution is calculated for several parts of the outflow. Fitting these distributions with power laws some of the same break-points are seen as previously identified in the emission from the outflow. We conclude this indicates the temperature dependence of emissivity is not responsible for all the break-points seen and molecular dissociation is a more plausible explanation for their origin. We model the molecular outflow using the ZEUS-2D hydrodynamic code which we have augmented so that it can also track the composition of the gas. We find that a general hydromagnetic wind, without an enchanced, on axis, jet-like component, can reproduce the shape of the observed outflow. Models looking at the time evolution of the stellar wind indicate that the structure of the outflow is dominated by the initial wind conditions, rather than its later evolution. The models also show that scaling the density of the wind profile effects the apparent collimation of the resulting outflow. This may help explain some of the apparent differences between outflows from high mass and low mass young stars.

Energy Release and Transport Processes in the Centres of Galaxies

Observations over the past decade have verified, beyond reasonable doubt, that most galactic nuclei contain massive black holes. Hole masses are being measured and firm evidence for spin is being sought. Attention is now returning to the study of how gas flows around black holes and how energy is released both from the accreting gas and from the hole itself in the form of radiation, relativistic jets and non-relativistic, hydromagnetic winds. Some of the different possibilities currently under investigation are briefly reviewed and some recent clues from radio scintillation, polarization and X-ray observations are discussed. It is argued that observations of persistent circular polarisation in Sgr A* support the presence of an ordered disk magnetic field. It is also conjectured that adiabatic, sub- and super-critical accretion flows are demand-limited, not supply-driven and are associated with large mass outflow as appears to be the case in Sgr A*. This principle may have to be modified when a massive black hole forms in a protogalaxy and this modification may account for the proposed hole mass-bulge velocity dispersion relation. The final stages of this process may release sufficient wind energy from the nucleus to prevent disk formation.

Jets from accretion discs

Highly collimated jets are associated with young stellar objects of all masses. They are also prevalent in microquasars and a variety of active galactic nuclei. Common to all of these systems is the presence of accretion discs around central objects, be they young stars, compact objects, or black holes. Current observations of protostellar systems support a model that posits accretion–powered, centrifugally accelerated, hydromagnetic disc winds as the underlying mechanism for protostellar jets. This model also has the attractive possibility of unifying protostellar, microquasar and extragalactic jets within a single conceptual framework. We summarize the observations, theory and time–dependent numerical simulations of disc winds as the origin of protostellar jets. We also address the relevance of these winds for the theory of star formation, as well as the more general issue of a unified theory for astrophysical jets.

Magnetohydrodynamic Origin of Jets from Accretion Disks

A review is made of recent magnetohydrodynamic (MHD) theory and simulations of origin of jets from accretion disks. Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are the extra galactic radio jets of active galaxies and quasars, and old compact stars in binaries, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the B field by the rotation of the disk drives the jets by magnetically extracting matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now allow the study of the origin -the acceleration and collimation - of jets from accretion disks. Simulation studies in the hydromagnetic wind regime indicate that the outflows are accelerated close to their region of origin whereas the collimation occurs at much larger distances.

Hydromagnetic Wind Model for the Broad-Line Region of NGC 5548

AbstractA hydromagnetic wind model from a dusty molecular disk (Emmering, Blandford, & Shlosman 1992) was applied to the BLR in NGC 5548 and produced synthetic C IV line profiles as a function of time. The model C IV profiles are compared to data from the 1993 IUE/HST campaign and properties of the the BLR of NGC 5548 are inferred.

Dynamics of Broad Emission‐Line Region in NGC 5548: Hydromagnetic Wind Model versus Observations

We analyze the results of long-term observations of broad-line region (BLR) in the Sy1 galaxy NGC 5548 and provide a critical comparison with the predictions of a hydromagnetically-driven outflow model of Emmering, Blandford and Shlosman. This model was used to generate a time series of CIV line profiles that have responded to a time varying continuum. The model includes cloud emission anisotropy, cloud obscuration, a CLOUDY-generated emissivity function and a narrow-line component used to generate the line profiles, and is driven with continuum input based on the monitoring campaigns of NGC 5548. The line strengths, profiles and lags are compared with the observations. It reproduces the basic features of CIV line variability in this AGN without varying the model parameters. The best fit model provides the effective size, the dominant geometry, the emissivity distribution and the 3D velocity field of the CIV BLR and constrains the mass of the central BH. The inner part of the wind appears to be responsible for the anisotropically emitted CIV line, while its outer part remains dusty and molecular, thus having similar spectral characteristics to a molecular torus, although its dynamics is fundamentally different. In addition, our model predicts a differential response across the CIV line profile, producing a red-side-first response in the mid-wings followed by the blue mid-wing and by the line core. Based on the comparison of data and model cross-correlation functions and 1D and 2D transfer functions, we find that the rotating outflow model is compatible with observations of the BLR in NGC 5548.

A portrait of the torus as a disk-driven hydromagnetic wind

Centrifugally driven winds originating from the surfaces of magnetized accretion disks in AGNs possess unique observational characteristics owing to their highly stratified structure and their exposure to the central engine's radiation field. The obscuration and radiation properties of such winds may provide the physical basis for the dusty tori invoked in various AGN unification schemes. We discuss the angular-dependent optical and X-ray properties of such winds, their infrared emission, and their capacity to polarize the central source continuum spectrum via electron/dust scattering and dichroic extinction by magnetically aligned grains.

Theory of Accretion Disks I: Angular Momentum Transport Processes

Accretion disk flow is a common phenomenon in astrophysics. It provides the nursery for planetary system formation and the channel for mass transfer in interacting binary stars. Such flows are also associated with the central engine for active galactic nuclei. Mass is redistributed in accretion disks as a consequence of angular momentum transfer. The identification of the dominant process involved is an important task in the development of accretion disk theory. Here, we review recent theoretical investigations on several important physical processes, including: 1. the removal of angular momentum from disks through hydromagnetic winds, 2. the amplification of local viscous stress through the onset of turbulence resulting from possible hydromagnetic, convective, or shear flow instabilities, 3. the transport of angular momentum carried by propagating waves, and 4. torque resulting from the presence of nonaxisymmetric unstable modes in self-gravitating and geometrically thick disks. Because of the technical nature of this subject, we present some of the mathematical formalisms in a pedagogical manner. We focus our attention on the physical discussion of the necessary conditions for each process to operate and the efficiency of angular momentum transfer to be expected. In Part II of this review, we shall present observational evidences and discuss applications of theoretical results in different astrophysical contexts.

Theory of Accretion Disks: Part 1

Accretion disk flow is a common phenomenon in astrophysics. It provides the nursery for planetary system formation and the channel for mass transfer in interacting binary stars. Such flows are also associated with the central engine for active galactic nuclei. Mass is redistributed in accretion disks as a consequence of angular momentum transfer. The identification of the dominant process involved is an important task in the development of accretion disk theory. Here, we review recent theoretical investigations on several important physical processes, including: 1. the removal of angular momentum from disks through hydromagnetic winds, 2. the amplification of local viscous stress through the onset of turbulence resulting from possible hydromagnetic, convective, or shear flow instabilities, 3. the transport of angular momentum carried by propagating waves, and 4. torque resulting from the presence of nonaxisymmetric unstable modes in self-gravitating and geometrically thick disks. Because of the technical nature of this subject, we present some of the mathematical formalisms in a pedagogical manner. We focus our attention on the physical discussion of the necessary conditions for each process to operate and the efficiency of angular momentum transfer to be expected. In Part II of this review, we shall present observational evidences and discuss applications of theoretical results in different astrophysical contexts.

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.

Asymptotic domination of cold relativistic MHD winds by kinetic energy flux

We study the conditions which lead to the conversion of most Poynting flux into kinetic energy flux in cold, relativistic hydromagnetic winds. It is shown that plasma acceleration along a precisely radial flow is extremely inefficient due to the near cancellation of the toroidal magnetic pressure and tension forces. However, if the flux tubes in a flow diverge even slightly faster than radially, the fast magnetosonic point moves inward from infinity to a few times the light cylinder radius. Once the flow becomes supermagnetosonic, further divergence of the flux tubes beyond the fast point can accelerate the flow via the 'magnetic nozzle' effect, thereby further converting Poynting flux to kinetic energy flux. We show that the Grad-Shafranov equation admits a generic family of kinetic energy-dominated asymptotic wind solutions with finite total magnetic flux. The Poynting flux in these solutions vanishes logarithmically with distance. The way in which the flux surfaces are nested within the flow depends only on the ratio of angular velocity to poliodal 4-velocity as a function of magnetic flux. Radial variations in flow structure can be expressed in terms of a pressure boundary condition on the outermost flux surface, provided that no external toriodal field surrounds the flow. For a special case, we show explicitly how the flux surfaces merge gradually to their asymptotes. For flows confined by an external medium of pressure decreasing to zero at infinity we show that, depending on how fast the ambient pressure declines, the final flow state could be either a collimated jet or a wind that fills the entire space. We discuss the astrophysical implications of our results for jets from active galactic nuclei and for free pulsar winds such as that believed to power the Crab Nebula.

Forbidden-Line Formation in Hydromagnetic Disk Winds. II. Line Profiles

Forbidden-Line Formation in Hydromagnetic Disk Winds. II. Line Profiles

Hydromagnetic disk winds in young stellar objects and active galactic nuclei

A general theory is presented for the origin and collimation of centrifugally driven, hydromagnetic winds from the surfaces of Keplerian accretion disks that surround central objects such as young stellar objects (YSOs) or black holes. The theory is fully two-dimensional and makes no assumption about self-similarity of the flows. These winds efficiently extract disk angular momentum and convert the gravitational binding energy released in the accretion process into the mechanical energy of the wind. Solutions under the very general assuption the the magnetic flux in the disk scales as a power law in disk radius are investigated

Magnetic acceleration of broad emission-line clouds in active galactic nuclei

view Abstract Citations (307) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic Acceleration of Broad Emission-Line Clouds in Active Galactic Nuclei Emmering, Robert T. ; Blandford, Roger D. ; Shlosman, Isaac Abstract It is proposed that broad emission lines in active galactic nuclei are formed by dense clouds in a molecular, hydromagnetic wind accelerated radiatively and centrifugally away from an accretion disk orbiting a massive black hole. These clouds are supposed to be photoionized by the UV continuum produced in the innermost radii of the accretion disk where the radiation flux is sufficient to evaporate the dust grains, that is, at a radius ~0.2L^1/2^_ion,46_ pc. If the wind is responsible for extracting most of the disk angular momentum, then the estimated ionizing flux, electron density, cloud filling factor, column density, and velocity are in accord with values for these quantities inferred from studies of the line widths, strengths, and variability. In order to reproduce typical line profiles with cusp shapes, blue asymmetry, and blueshifted peaks, it is found that the volume emissivity must scale roughly as r^-3.3^, where r is the cylindrical radius; in this case each decade of radius contributes nearly equally to the line profile. Simple models of the cloud emissivity approximate this dependence. We consider the possibility that the broad wings of the lines are formed through electron scattering by a ~10^6^ K warm intercloud medium. We find that such a phase can be thermally stable, when magnetically confined, and argue that a Thomson optical depth τ_T_~1 is automatically maintained. It is also found that emission over three decades of radius is necessary to reproduce a typical line profile if electron scattering is unimportant, but that two decades suffice when electron scattering broadens the line. Viewing the outflow from smaller angles with respect to the symmetry axis produces a more symmetrical line profile. Reverberation mapping and polarization observation may provide a test of this model. Publication: The Astrophysical Journal Pub Date: February 1992 DOI: 10.1086/170955 Bibcode: 1992ApJ...385..460E Keywords: Active Galactic Nuclei; Emission Spectra; Line Shape; Line Spectra; Particle Acceleration; Accretion Disks; Black Holes (Astronomy); Interstellar Gas; Astrophysics; ACCELERATION OF PARTICLES; BLACK HOLE PHYSICS; GALAXIES: NUCLEI; LINE: FORMATION full text sources ADS | data products SIMBAD (2)

The effects of radiation drag on radial, relativistic hydromagnetic winds

The effects of drag on an idealized relativistic MHD wind of radial geometry are studied. The astrophysical motivation is to understand the effects of radiation drag on the dynamics of a jet or wind passing through the intense radiation field of an accreting compact object. From a critical point analysis, it is found that a slow magnetosonic point can appear in a dragged flow even in the absence of gravitational force, as a result of a balance between the drag force and the combination of thermal pressure and centrifugal forces. As in the undragged case, the Alfven point does not impose any constraints on the flow. Although it is formally possible for a dragged flow to possess more than one fast magnetosonic point, it is shown that this is unlikely in practice. In the limit of a 'cold', centrifugally driven flow, it is shown that the fast magnetosonic point moves to infinite radius, just as in the drag-free case. For a given mass flux, the total energy output carried to infinity, and the final partition between the kinetic energy and the Poynting flux, are the same for the dragged and the drag-free flows. The main effects of radiation drag are to increase the amount of energy and angular momentum extracted from the source and to redistribute the regions where acceleration occurs in the flow. This is accomplished through the storage and release of magnetic energy, as a result of additional winding and compression of the field caused by the action of the drag. For a relativistic wind, the dissipated energy can exceed the final kinetic energy of the flow and may be comparable to the total flow energy (which is dominated by Poynting flux). The energy lost to radiation drag will appear as a Doppler-boosted beam of scattered radiation, which could dominate the background radiation if the flow is well-collimated.

Asymptotic structure of hydromagnetically driven relativistic winds

A fully relativistic analysis has been performed of the asymptotic structure of stationary axisymmetric hydromagnetic winds. If a flow fills the region containing the rotation axis, then the flux surfaces in the flow must collimate to a set of current-carrying cylindrical surface extending to infinite transverse radius, collimate to a set of cylindrical surfaces extending to a finite radius, or collimate to a current-free paraboloidal field configuration which fills up the entire space. If an asymptotically cylindrical flow carries a finite current at radii well beyond the light cylinder, then the Lorentz factor of the terminal flow speed on a given flux surface is proportional to the total current enclosed within this flux surface. If a flow is of type II paraboloidal, then its asymptotic energy flux is carried entirely by the gas motion rather than the electromagnetic fields.

The acceleration of the nonsymmetric and nonpolytropic MHD solar wind

Abstract A method for obtaining analytically several classes of solutions of the full MHD equations for steady, rotating, nonspherically symmetric and nonpolytropic collimated magnetized outflows from a gravitating central object is outlined. These solutions correspond to a polytropic relationship between pressure and density but with a variable polytropic index, while their topology is governed by several hydromagnetic critical points that select a unique wind-type outflow solution. The deviation of the solutions from the original Parker model is parameterized with the introduction of appropriate parameters, such as the deviation of the density from the spherically symmetric case, ω, the deviation of the pressure from a spherically symmetric pressure distribution, κ, the deviation of the streamlines from the spherically symmetric case, s , the effects of rotation, λ and the effects of the magnetic field, β. If ω = κ = λ = β = s = 0, a dipolar, spherically symmetric in density and pressure, nonrotating and nonmagnetized radial Parker-type wind outflow is obtained. We find that the higher is the deviation of the density distribution from the spherically symmetric case, the larger is the initial acceleration. On the other hand, the more magnetically dominated is the outflow and the larger is the pressure inhomogeneity, the rotation amplitude and the flaring, the smaller is the initial acceleration. These results are at variance with attempts to generalize to nonpolytropic outflows the well established result of polytropic outflows wherein, for example, large flaring is usually associated with higher initial acceleration; in nonpolytropic flows exactly the opposite holds. Such properties should be considered in modelling the initial acceleration of the hydromagnetic solar wind.

Numerical Models of Magnetic Disk Accretion in Star Forming Regions

We report on preliminary numerical MHD simulations of magnetic disk accretion surrounding a compact protostar in which the disk and the hydromagnetic wind interact self-consistently. We show that an outflow is indeed produced, but that its properties are sensitive to the assumed initial poloidal magnetic field structure. In particular, we reconcile the apparently disparate models of Pudritz and Norman (1983,1986) and Uchida and Shibata (1985) concerning the mass ejection mechanism.

A new angular momentum loss mechanism for cataclysmic variables

view Abstract Citations (40) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Angular Momentum Loss Mechanism for Cataclysmic Variables Cannizzo, John K. ; Pudritz, Ralph E. Abstract It is well known that there is mass outflow in cataclysmic variables, since IUE observations show P Cyg profiles of various high-excitation lines of C IV and S IV. It is proposed here that these outflows are hydromagnetic winds from accretion disks. The winds extract angular momentum from these systems and therefore drive secular evolution. Disk winds are preferred to winds from the secondary star because of the lower disk surface gravities. Using values of the wind parameters inferred from observations, values are derived for the (time-averaged) induced rate of mass transfer from the red dwarf secondary which span the range inferred by Warner (1976 and 1987). It is shown that this theory accounts for both the IUE observations and the secular evolution of cataclysmic variables. Publication: The Astrophysical Journal Pub Date: April 1988 DOI: 10.1086/166241 Bibcode: 1988ApJ...327..840C Keywords: Accretion Disks; Angular Momentum; Cataclysmic Variables; Stellar Evolution; Stellar Winds; Carbon; Cygnus Constellation; Gravitational Effects; Iue; Secular Variations; Silicon; Astrophysics; STARS: ACCRETION; STARS: DWARF NOVAE; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (3) MAST (1)

Star Formation and Bipolar Flows

Observations of molecular outflows from regions of star formation show that they cannot be radiatively driven ((Ṁv)mol ≥ 102 – 103 (L./c)). The thrust observed to be associated with the smaller scale ionized outflows is also incapable of driving the molecular gas ((Ṁv)ion ≃ (L./c), Persson et al 1984). The results may be explained if bipolar flows are hydromagnetic winds from molecular disks around protostars. These winds carry off disk rotational energy (observed as the mechanical energy of the outflows) and angular momentum (observed when rotation of the outflowing gas is found), which drives an accretion flow through it and onto the protostellar core (Pudritz 1985, Pudritz and Norman 1983, 1986). Therefore star formation and bipolar outflows occur simultaneously when magnetized, rotating disks are the source of activity.