Efficiency Of Angular Momentum Transfer Research Articles

Larger λR in the disc of isolated active spiral galaxies than in their non-active twins

We present a comparison of the spin parameter λR, measured in a region dominated by the galaxy disc, between 20 pairs of nearby (0.005 < z < 0.03) seemingly isolated twin galaxies differing in nuclear activity. We find that 80−82% of the active galaxies show higher values of λR than their corresponding non-active twin(s), indicating larger rotational support in the active galactic nuclei (AGN) discs. This result is driven by the 11 pairs of unbarred galaxies, for which 100% of the AGN show larger λR than their twins. These results can be explained by a more efficient angular momentum transfer from the inflowing gas to the disc baryonic matter in the case of the active galaxies. This gas inflow could have been induced by disc or bar instabilities, although we cannot rule out minor mergers if these are prevalent in our active galaxies. This result represents the first evidence of galaxy-scale differences between the dynamics of active and non-active isolated spiral galaxies of intermediate stellar masses (1010 < M* < 1011 M⊙) in the Universe.

Stellar bars in counter-rotating dark matter haloes: the role of halo orbit reversals

Abstract Disc galaxies can exchange angular momentum and baryons with their host dark matter (DM) haloes. These haloes possess internal spin, λ, which is insignificant rotationally but does affect interactions between the baryonic and DM components. While statistics of prograde and retrograde spinning haloes in galaxies is not available at present, the existence of such haloes is important for galaxy evolution. In the previous works, we analysed dynamical and secular evolution of stellar bars in prograde spinning haloes and the DM response to the bar perturbation, and found that it is modified by the resonant interactions between the bar and the DM halo orbits. In this work, we follow the evolution of stellar bars in retrograde haloes. We find that this evolution differs substantially from evolution in rigid unresponsive haloes, discussed in the literature. First, we confirm that the bar instability is delayed progressively along the retrograde λ sequence. Secondly, the bar evolution in the retrograde haloes differs also from that in the prograde haloes, in that the bars continue to grow substantially over the simulation time of 10 Gyr. The DM response is also substantially weaker compared to this response in the prograde haloes. Thirdly, using orbital spectral analysis of the DM orbital structure, we find a phenomenon we call the orbit reversal – when retrograde DM orbits interact with the stellar bar, reverse their streaming and precession, and become prograde. This process dominates the inner halo region adjacent to the bar and allows these orbits to be trapped by the bar, thus increasing efficiency of angular momentum transfer by the inner Lindblad resonance. We demonstrate this reversal process explicitly in a number of examples.

IS THERE A MAXIMUM MASS FOR BLACK HOLES IN GALACTIC NUCLEI?

ABSTRACT The largest observed supermassive black holes (SMBHs) have a mass of , nearly independent of redshift, from the local ( ) to the early ( ) universe. We suggest that the growth of SMBHs above a few is prevented by small-scale accretion physics, independent of the properties of their host galaxies or of cosmology. Growing more massive BHs requires a gas supply rate from galactic scales onto a nuclear region as high as . At such a high accretion rate, most of the gas converts to stars at large radii (∼10–100 pc), well before reaching the BH. We adopt a simple model for a star-forming accretion disk and find that the accretion rate in the subparsec nuclear region is reduced to the smaller value of at most a few . This prevents SMBHs from growing above in the age of the universe. Furthermore, once an SMBH reaches a sufficiently high mass, this rate falls below the critical value at which the accretion flow becomes advection dominated. Once this transition occurs, BH feeding can be suppressed by strong outflows and jets from hot gas near the BH. We find that the maximum SMBH mass, given by this transition, is between , depending primarily on the efficiency of angular momentum transfer inside the galactic disk, and not on other properties of the host galaxy.

HYDROMAGNETICS OF ADVECTIVE ACCRETION FLOWS AROUND BLACK HOLES: REMOVAL OF ANGULAR MOMENTUM BY LARGE-SCALE MAGNETIC STRESSES

We show that the removal of angular momentum is possible in the presence of large scale magnetic stresses in geometrically thick, advective, sub-Keplerian accretion flows around black holes in steady-state, in the complete absence of alpha-viscosity. The efficiency of such an angular momentum transfer could be equivalent to that of alpha-viscosity with alpha=0.01-0.08. Nevertheless, required field is well below its equipartition value, leading to a magnetically stable disk flow. This is essentially important in order to describe the hard spectral state of the sources, when the flow is non/sub-Keplerian. We show in our simpler 1.5-dimensional, vertically averaged disk model that larger the vertical-gradient of azimuthal component of magnetic field, stronger the rate of angular momentum transfer is, which in turn may lead to a faster rate of outflowing matter. Finding efficient angular momentum transfer, in black hole disks, via magnetic stresses alone is very interesting, when the generic origin of alpha-viscosity is still being explored.

The accretion rate and minimum spin period of accreting pulsars

We consider combined rotational, magnetic, and thermal evolution of the neutron star during the accretion phase in a binary system. A rapid accretion-driven decay of the magnetic field decreases substantially the efficiency of angular momentum transfer. As a result, the neutron star cannot spin up to periods shorter than some limiting value even if accretion is very long and accretion rate is high. The proposed mechanism can explain a discrepancy between the shortest detected period and minimal possible spin period of neutron stars.

Anti-damping spin transfer torque through epitaxial nickel oxide

We prepare the high quality epitaxial MgO(001)[100]/Pt(001)[100]/NiO(001)[100]/FeNi/SiO2 films to investigate the spin transport in the NiO antiferromagnetic insulator. The ferromagnetic resonance measurements of the FeNi under a spin current injection from the Pt by the spin Hall effect revealed the change of the ferromagnetic resonance linewidth depending on the amount of the spin current injection. The results can be interpreted that there is an angular momentum transfer through the NiO. A high efficient angular momentum transfer we observed in the epitaxial NiO can be attributed to the well-defined orientation of the antiferromagnetic moments and the spin quantization axis of the injected spin current.

Compression of matter in the center of accreting neutron stars

To estimate the feasibility of dense-matter phase transition, we studied the evolution of the central density as well as the baryon chemical potential of accreting neutron stars. We compared the thin-disk accretion with and without the magnetic field torque with the spin-down scenario for a selection of recent equations of state. We compared the prevalent (in the recycled-pulsar context) Keplerian thin-disk model, in which the matter is accreted from the marginally-stable circular orbit, with the recent magnetic-torque model that takes into account the influence of stellar magnetic field on the effective inner boundary of the disk. Calculations were performed using a multi-domain spectral methods code in the framework of General Relativity. We considered three equations of state consistent with the recently measured mass of PSR J1614-2230, 1.97 +- 0.04 M_sun (one of them softened by the appearance of hyperons). If there is no magnetic torque and efficient angular momentum transfer from the disk to the star, substantial central compression is limited to the region of initial stellar masses close to the maximum mass. Outside the maximum mass vicinity, accretion-induced central compression is significant only if the angular momentum transfer is inefficient. Accounting for the magnetic field effectively decreases the efficiency of angular momentum transfer and implies a significant central compression. An efficient angular momentum transfer from a thin disk onto a non-magnetized neutron star does not provide a good mechanism for the central compression and possible phase transition. Substantial central compression is possible for a broad range of masses of slowly-rotating initial configurations for magnetized neutron stars. Accretion-induced central compression is particularly strong for stiff equation of state with a high-density softening.

Spin-down of protostars through gravitational torques

Young protostars embedded in circumstellar discs accrete from an angular momentum-rich mass reservoir. Without some braking mechanism, all stars should be spinning at or near break-up velocity. In this paper, we perform simulations of the self-gravitational collapse of an isothermal cloud using the ORION adaptive mesh refinement code and investigate the role that gravitational torques might play in the spin-down of the dense central object. While magnetic effects likely dominate for low mass stars, high mass and Population III stars might be less well magnetised. We find that gravitational torques alone prevent the central object from spinning up to more than half of its breakup velocity, because higher rotation rates lead to bar-like deformations that enable efficient angular momentum transfer to the surrounding medium. We also find that the long-term spin evolution of the central object is dictated by the properties of the surrounding disc. In particular, spiral modes with azimuthal wavenumber $m=2$ couple more effectively to its spin than the lopsided $m=1$ mode, which was found to inhibit spin evolution. We suggest that even in the absence of magnetic fields, gravitational torques may provide an upper limit on stellar spin, and that moderately massive circumstellar discs can cause long-term spin down.

QUASI-SPHERICAL, TIME-DEPENDENT VISCOUS ACCRETION FLOW: ONE-DIMENSIONAL RESULTS

We investigated the instability of advective accretion flow as a consequence of angular momentum transfer in one-dimensional, quasi-spherical transonic accretion flow around a non-rotating black hole. The code is designed to include the effects of viscosity; the hydrodynamics component preserves angular momentum strictly with Lagrangian and remap method in absence of viscosity, while the viscosity component updates viscous angular momentum transfer through the implicit method. We performed two tests to demonstrate the suitability of the code for accretion study. First, we simulated the inviscid, low angular momentum, transonic accretion flow with shocks around a black hole, and then the subsonic, self-similar ADAF solution around a Newtonian object. Both simulations fitted the corresponding analytical curves extremely well. We then simulated a rotating, viscous, transonic fluid with shocks. We showed that for low viscosity parameter, stable shocks at larger distance are possible. For higher viscosity parameter, more efficient angular momentum transfer in the post-shock disk makes the shock structure oscillatory. Moreover, as the shock drifts to larger distances, a secondary inner shock develops. We showed that the inner shock is the direct consequence of expansion of the outer shock, as well as creation of regions with $\partial l / \partial r < 0$ due to more efficient angular momentum transfer near the inner sonic point. We showed that all disk parameters, including emissivity, oscillate with the same period as that of the shock oscillation. Our simulation may have implication for low frequency QPOs, e.g., GRO J1655-40 and XTE J1550-564.

Maximum spin of black holes driving jets

Unbounded outflows in the form of highly collimated jets and broad winds appear to be a ubiquitous feature of accreting black hole systems. The most powerful jets are thought to derive a significant fraction, if not the majority, of their power from the rotational energy of the black hole. Whatever the precise mechanism that causes them, these jets must therefore exert a braking torque on the black hole. We calculate the spin-up function for an accreting black hole, accounting for this braking torque. We find that the predicted black hole spin-up function depends only on the black hole spin and dimensionless parameters describing the accretion flow. Using recent relativistic magnetohydrodynamical numerical simulation results to calibrate the efficiency of angular momentum transfer in the flow, we find that an ADAF flow will spin a black hole up (or down) to an equilibrium value of about 96% of the maximal spin value in the absence of jets. Combining our ADAF system with a simple model for jet power, we demonstrate that an equilibrium is reached at approximately 93% of the maximal spin value, as found in the numerical simulation studies of the spin-up of accreting black holes, at which point the spin-up of the hole by accreted material is balanced by the braking torque arising from jet production. Our model also yields a relationship between jet efficiency and black hole spin that is in surprisingly good agreement with that seen in the simulation studies, indicating that our simple model is a useful and convenient description of ADAF inflow - jet outflow about a spinning black hole for incorporation in models of the formation and evolution of galaxies, groups and clusters of galaxies.

The response of self-gravitating protostellar discs to slow reduction in cooling time-scale: the fragmentation boundary revisited

A number of previous studies of the fragmentation of self-gravitating protostellar discs have involved suites of simulations in which radiative cooling is modelled in terms of a cooling time-scale (t cool ) which is parametrized as a simple multiple (β cool ) of the local dynamical time-scale. Such studies have delineated the 'fragmentation boundary' in terms of a critical value of β cool(βcrit) such that the disc fragments if β cool < β crit . Such an approach however begs the question of how in reality a disc could ever be assembled in a state with βcool < β crit . Here we adopt the more realistic approach of effecting a gradual reduction in β cool , as might correspond to changes in thermal regime due to secular changes in the disc density profile. We find that the effect of gradually reducing β cool (on a time-scale longer than t cool ) is to stabilize the disc against fragmentation, compared with models in which β cool is reduced rapidly (over less than t cool ). We therefore conclude that the ability of a disc to remain in a self-regulated, self-gravitating state (without fragmentation) is partly dependent on the disc's thermal history, as well as its current cooling rate. Nevertheless, the effect of a slow reduction in t cool appears only to lower the fragmentation boundary by about a factor of 2 in t cool and thus only permits maximum 'a' values (which parametrize the efficiency of angular momentum transfer in the disc) that are about a factor of 2 higher than determined hitherto. Our results therefore do not undermine the notion that there is a fundamental upper limit to the heating rate that can be delivered by gravitational instabilities before the disc is subject to fragmentation. An important implication of this work, therefore, is that self-gravitating discs can enter into the regime of fragmentation via secular evolution and it is not necessary to invoke rapid (impulsive) events to trigger fragmentation.

Secular bar formation in galaxies with a significant amount of dark matter

Using high resolution N-body simulations of stellar disks embedded in cosmologically motivated dark matter halos, we study the evolution of bars and the transfer of angular momentum between halos and bars. We find that dynamical friction results in some transfer of angular momentum to the halo, but the effect is much smaller than previously found in low resolution simulations and is incompatible with early analytical estimates. After 5 Gyrs of evolution the stellar component loses only 5% – 7% of its initial angular momentum. Mass and force resolutions are crucial for the modeling of bar dynamics. In low resolution (300 – 500 pc) simulations we find that the bar slows down and angular momentum is lost relatively fast. In simulations with millions of particles reaching a resolution of 20-40 pc, the pattern speed may not change over billions of years. Our high resolution models produce bars which are fast rotators, where the ratio of the corotation radius to the bar major semi-axis lies in the range R = 1.2 1.7, marginally compatible with observational results. In contrast to many previous simulations, we find that bars are relatively short. As in many observed cases, the bar major semi-axis is close to the exponential length of the disk. The transfer of angular momentum between inner and outer parts of the disk plays a very important role in the secular evolution of the disk and the bar. The bar formation increases the exponential length of the disk by a factor of 1.2 -1.5. The transfer substantially increases the stellar mass in the center of the galaxy and decreases the dark matter-to-baryons ratio. As the result, the central 2 kpc region is always strongly dominated by the baryonic component. At intermediate (3 – 10 kpc) scales the disk is sub-dominant. These models demonstrate that the efficiency of angular momentum transfer to the dark matter has been greatly overestimated. More realistic models produce bar structure in striking agreement with observational results.

Momentum Transfer in Oblique Impacts: Implications for Asteroid Rotations

We calculate the momentum transfer efficiency for small cratering impacts from 59 high-velocity (0.76–4.4 km/s) oblique impact experiments using nylon projectiles and targets made of basalt, aluminum, mild steel, mortar, and nylon. High-speed video frames show an initial luminous stream downrange of impact point for all the target materials. For basaltic and mortar targets there follows axi-symmetric ejection of target material around surface normal at the impact point. Our results suggest that: (1) momentum carried away by the axi-symmetric ejecta would significantly contribute to the normal component of momentum transfer efficiency; and (2) thepenetration depth of the projectile into target could determine the tangential momentum transfer efficiency. We calculated the efficiency of angular momentum transfer from the translational motion of projectiles to the rotation of ellipsoidal asteroids using the efficiencies for the linear momentum. The efficiency is more than four times that for spherical asteroid at their principal axis ratio of 2 : 1.4 : 1. If the experimental results hold for the impact that formed the largest crater on 253 Mathilde, the largest projectile estimated may have despun the asteroid to the present slow rotation by chance.

Angular momentum transfer in oblique impacts: Implications for 1989ML

We conducted 10 shots of high-velocity oblique impact experiments (1.95–3.52 km/s) using nylon projectiles and spherical mortar targets. Large craters were formed, but these targets were not disrupted by the impacts. We then calculated the efficiencies of momentum transfer from the projectile to the post-impact target for each experiment. The efficiencies of angular momentum transfer from the translational motion of the projectiles to the rotation of the post-impact targets were also derived. A representative efficiency of angular momentum transfer was calculated to be 0.17 for random successive collisions. The efficiency was applied to an equation expressing the precession angle of asteroids. It is shown that 1989ML, target of Japan-US asteroid-sample-return-mission (MUSES-C) would be tumbling.

Star/Disk Interaction in the Nuclei of Active Galaxies

The most definitive evidence for an accretion disk is provided by the discovery of megamasers (Claussen et al. 1984) which appear to be located in a molecular torus around the nucleus of the mildly active galaxy NGC 4258. Confirmation of Keplerian rotation speed was obtained from radio interferometer (Nakai et al. 1993) and VLBI observations (Greenhill, this volume). Based on the correlation between the spatial locations and radial velocities of the masers, Moran et al. (1995) deduced that the masers are located in a disk at radii R = 0.13–0.26 pc around a black hole with a mass M ≈ 3.5 × 107M⊙ (Watson &amp; Wallin 1994, Maoz 1995). A limit on the thickness (H &lt; 2.5 × 10−3R) is deduced from the velocity dispersion of the maser sources. The mass-diffusion time scale is estimated to be τd &gt; 1015 α−1 s, where α is the viscosity parameter. An efficient angular-momentum transfer mechanism (α &gt; 0.1) is needed for τd ≈ 108 yr, which is the disk evolution time scale inferred from the correlation between interacting galaxies and intense AGN activities (MacKenty 1989, Hernquist 1989). A relatively large value of a is also required to account for the accretion rate needed to power the X-ray flux of NGC 4258.

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.

Angular momentum transfer in low velocity oblique impacts: Implications for asteroids

The efficiency of angular momentum transfer ζ in low velocity oblique impacts was studied experimentally. ζ is defined as the fraction of incident angular momentum transferred to the rotation of the target. Plaster, mortar, cement, and one granite target were studied. Lead and aluminum projectiles were used. Only cratering impacts were considered. ζ was found to decrease with increasing incidence angle φ (relative to the surface normal). For example, for impacts into cylindrical mortar targets a least-squares fit of the form ζ = A(cos φ) β with A = 0.9 and β = 1.7 was found to match the data points reasonably well. In addition, β decreased from 1.9 to 1.4 as the kinetic energy density ϵ (= kinetic energy of the projectile/projectile volume) increased from 0.5 to 1.8 × 10 9 J m −3. This suggests that more energetic impacts transfer angular momentum more efficiently. ζ decreased as the indentation hardness H of the target increased: at φ ∼ 35°, ζ = 0.07 for granite ( H = 850 kg mm −2 and ζ = 0.7 for plaster ( H = 7.5 kg mm −2). Cement and mortar ( H = 76 kg mm −2) yielded intermediate values, although the values for cement ( ζ ∼ 0.3) were appreciably lower than for mortar ( ζ ∼ 0.6). In all cases where the velocity of the ricochetted projectile was determined, the fraction of angular momentum carried away by ejecta was found to be less than 30%. Finally, the results were only weakly dependent on the material of the projectile. If asteroid spin rates are the result of mutual noncatastrophic collisions and the taxonomic classes are indicative of the bulk properties (C asteroids being weaker than S asteroids and the latter weaker than M objects), then the differences between the corresponding spin rates are predicted to be smaller on the basis of our results than what could be expected from a consideration of the relative strength and density only. This conclusion is based on our finding for semirigid targets and ∼1-g projectiles impacting at <1 km sec −1 that hard targets are harder to spin up, i.e., exhibit smaller values of ζ in small-scale experiments. This would bring the predicted spin rates into a better agreement with observations. Moreover, on the basis of our experiments the equilibrium nutation angles should be smaller than the values previously obtained assuming ζ = 1, especially for strong asteroids for which ζ ⪡ 1 are expected from this work. This would make the detection of freely precessing asteroids even more difficult.

Chapter 8. Efficiency of Angular Momentum Transfer by Shear Instability

Chapter 8. Efficiency of Angular Momentum Transfer by Shear Instability

NMR study of compressed supercritical water

The proton spin–lattice relaxation time T1 in water has been measured as a function of pressure in the temperature range 150 to 700°C. This study focuses on the supercritical region (tc=374°C) where the spin–rotation interaction mechanism dominates the observed proton relaxation rate. Since water is an asymmetric top molecule, the analysis of the experimental data involves a number of simplifying assumptions discussed in detail. The experimental finding that in supercritical water the spin–rotation relaxation time T 1SR is a linear function of density ρ, up to relatively high densities (ρ≃ 1.5 ρc) provides rationale for analysis of the NMR experimental data in terms of a model used for dilute gases. The T 1SR data are analyzed on the basis of the assumption that the collision modulated spin–rotation interactions can be described by a single correlation function which is an exponential function of time. Using this procedure, we find that T 1SR/ρ αT−2, i.e.T 1SR/ρ exhibits a stronger temperature dependence than that found (T 1SR/ρ αT−3/2) for many polar and nonpolar gases. The calculated effective cross sections for the transfer of angular momentum σeff which show strong temperature dependence (σeff αT−1.5) are several times larger than the kinetic cross sections. By assuming applicability of expressions derived for isotropic reorientation of spherical-top molecules and using the effective spin–rotation interaction constant as obtained from microwave measurements, we are able to calculate the angular momentum correlation time τJ, over the range of temperatures and densities studied. In the supercritical region τJ⩾τΘ, where τΘ is the reorientational correlation time, and the estimated mean angle of reorientation ΔΘ¯ is in the range 50° to 800°. The T 1SR data are also interpreted in terms of the modified rough hard sphere (RHS) model which for ρ&amp;lt;2ρc takes into account the effect of attractive forces. We find that 1/T 1SR is a linear function of the Enskog relaxation time τE. The experimental ratio of τE/τJ reflecting the efficiency of angular momentum transfer shows density and temperature dependence, in agreement with expectation. There is no quantitative agreement between τE/τJ calculated and observed because of the oversimplified treatment used. An empirical relationship between T 1SR and the shear viscosity in the supercritical compressed water is established, namely, the T 1SR is a linear function of the logarithm of shear viscosity. In addition, the relaxation data in the temperature range 150 to 350°C is also reported and analyzed. In this specific temperature range, both the dipolar (intra- and intermolecular) and the spin–rotation interaction mechanisms contribute to the observed proton relaxation rate.