Black hole accretion flows can be divided into two broad classes: cold and hot. Whereas cold accretion flows consist of cool optically thick gas and are found at relatively high mass accretion rates, hot accretion flows, the topic of this review, are virially hot and optically thin, and occur at lower mass accretion rates. They are described by accretion solutions such as the advection-dominated accretion flow and luminous hot accretion flow. Because of energy advection, the radiative efficiency of these flows is in general lower than that of a standard thin accretion disk. Moreover, the efficiency decreases with decreasing mass accretion rate. Observations show that hot accretion flows are associated with jets. In addition, theoretical arguments suggest that hot flows should produce strong winds. Hot accretion flows are believed to be present in low-luminosity active galactic nuclei and in black hole X-ray binaries in the hard and quiescent states. The prototype is Sgr A*, the ultralow-luminosity supermassive black hole at our Galactic center. The jet, wind, and radiation from a supermassive black hole with a hot accretion flow can interact with the external interstellar medium and modify the evolution of the host galaxy.

Hot Accretion Flows Around Black Holes

Spin is a fundamental property of all elementary particles. Classically it can be viewed as a tiny magnetic moment, but a measurement of an electron spin along the direction of an external magnetic field can have only two outcomes: parallel or anti-parallel to the field. This discreteness reflects the quantum mechanical nature of spin. Ensembles of many spins have found diverse applications ranging from magnetic resonance imaging to magneto-electronic devices, while individual spins are considered as carriers for quantum information. Read-out of single spin states has been achieved using optical techniques, and is within reach of magnetic resonance force microscopy. However, electrical read-out of single spins has so far remained elusive. Here we demonstrate electrical single-shot measurement of the state of an individual electron spin in a semiconductor quantum dot. We use spin-to-charge conversion of a single electron confined in the dot, and detect the single-electron charge using a quantum point contact; the spin measurement visibility is ∼65%. Furthermore, we observe very long single-spin energy relaxation times (up to ∼0.85 ms at a magnetic field of 8 T), which are encouraging for the use of electron spins as carriers of quantum information.

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Single-shot read-out of an individual electron spin in a quantum dot

▪ Abstract Observational studies of low-mass stars during their early stages of evolution, from protostars through the zero-age main sequence, show highly elevated levels of magnetic activity. This activity includes strong fields covering much of the stellar surface and powerful magnetic reconnection flares seen in the X-ray and radio bands. The flaring may occur in the stellar magnetosphere, at the star-disk interface, or above the circumstellar disk. Ionization from the resulting high-energy radiation may have important effects on the astrophysics of the disk, such as promotion of accretion and coupling to outflows, and on the surrounding interstellar medium. The bombardment of solids in the solar nebula by flare shocks and energetic particles may account for various properties of meteorites, such as chondrule melting and spallogenic isotopes. X-ray surveys also improve our samples of young stars, particularly in the weak-lined T Tauri phase after disks have dissipated, with implications for our underst...

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High-Energy Processes in Young Stellar Objects

Preface 1. Historical overview 2. Observations of stellar winds 3. Basic concepts: isothermal winds 4. Basic concepts: non-isothermal winds 5. Coronal winds 6. Sound wave driven winds 7. Dust driven winds 8. Line driven winds 9. Magnetic rotator theory 10. Alfven wave driven winds 11. Outflowing disks from rotating stars 12. Winds colliding with the interstellar medium 13. The effects of mass loss on stellar evolution 14. Problems Appendices Bibliography Object index Index.

Introduction to Stellar Winds

Shapes and Shaping of Planetary Nebulae

A time-dependent jet launching and collimating mechanism is presented. Initial results of numerical simulations of the interaction between an aligned dipole rotator and a conducting circumstellar accretion disk that is initially threaded by the dipole field show that differential rotation between the disk and the star leads to the rapid expansion of the magnetic loops that connect the star to the disk. The expansion of these magnetic loops above and below the disk produces a two-component outflow. A hot, well-collimated outflow is generated by the convergent flow of attached plasma toward the rotation axis, while a cool, slower outflow is produced on the disk side of the expanding loop. The expanding loop, which later forms a plasmoid, defines the boundary between the jetlike flow and the disk wind. Episodic magnetic reconnection above and below the disk releases the jet plasma from the system and allows the process to repeat, reinforcing the hot, well-collimated outflow.

https://iopscience.iop.org/article/10.1086/304774/pdf

Time-dependent Accretion by Magnetic Young Stellar Objects as a Launching Mechanism for Stellar Jets

High-resolution numerical magnetohydrodynamic (MHD) simulations of a new model for the formation of jets from magnetic accreting young stellar objects (YSOs) are presented and compared with observations. The simulation results corroborate a previously laid out conceptual mechanism for forming jets, wherein the interaction of the stellar magnetosphere with a surrounding accretion disk leads to an outflow. The high resolution of the numerical simulation allows optical condensations, which form in the region close to the star to be seen. The optical condensations and the episodic behavior of the jet are effects that are inherent to the jet-launching mechanism itself. A disk wind arises as well. The simulated outflow is compared with observations, and it is shown that simulated images in the forbidden lines [S II] λλ6716+6731 have morphology consistent with recent observations of the jet source HH 30. Furthermore, velocity spectra of the simulated outflow in [S II] λλ6716+6731 and mass weighted by n clearly show a two-component outflow, in agreement with observed outflows from T Tauri stars such as DG Tauri. The mechanism produces a highly collimated fast jet and a slower disk wind. While the match between existing observations and the simulated system are not perfect (the time- and size scales of the jet differ from those in HH 30 by an order of magnitude), the morphology associated with both imagery and velocity spectra of the jet are matched well. A companion paper lays out the physics that control the timescale for knot production and defines the controlling parameters of the jet-launching mechanism in general.

Jets from Accreting Magnetic Young Stellar Objects. I. Comparison of Observations and High-Resolution Simulation Results

This paper addresses the physical principles underpinning a new jet-launching mechanism described in a companion paper. In this new jet formation model, magnetic loops that connect the star to the disk become twisted and expand via helicity injection. This expansion drives an outflow, with the axial symmetry of the disk leading to a concentration of outflowing plasma along the rotation axis, forming the jet. In the companion paper, it is found that the radial location of the inner edge of the disk undergoes oscillations. In this paper, the physical causes of the disk oscillations are investigated. This investigation leads to the conclusion that there are three classes of flows that can arise, depending on the role of diffusive instabilities. The most diffusive flows allow the stellar magnetic field to slip through the accretion disk and yield steady accretion flows. Such configurations are unlikely to produce outflows. The flows with intermediate diffusivity have been described by Lovelace, Romanova, & Bisnovatyi-Kogan and represent conditions in which the field is effectively frozen into the accretion disk azimuthally but slips radially. In the absence of magnetic reconnection, such configurations are predicted to produce steady flows with logarithmically collimated disk winds. The least diffusive flows, in which the bulk radial disk velocities are greater at times than the speeds with which magnetic field lines can diffuse into the disks, lead to the formation of the collimated unsteady jets described in the companion paper and are the primary interest of this paper. The jet velocity is also addressed.

Jets from Accreting Magnetic Young Stellar Objects. II. Mechanism Physics

We present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km/s) outflow (10^{-5} M_sun/yr) from an AGB star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of Planetary Nebulae.

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Disk Formation by AGB Winds in Dipole Magnetic Fields

We present a simple, robust mechanism by which an isolated star can produce an equatorial disk. The mechanism requires that the star have a simple dipole magnetic field on the surface and an isotropic wind acceleration mechanism. The wind couples to the field, stretching it until the field lines become mostly radial and oppositely directed above and below the magnetic equator, as occurs in the solar wind. The interaction between the wind plasma and magnetic field near the star produces a steady outflow in which magnetic forces direct plasma toward the equator, constructing a disk. In the context of a slow (10 km s-1) outflow (10-5 M☉ yr-1) from an asymptotic giant branch star, MHD simulations demonstrate that a dense equatorial disk will be produced for dipole field strengths of only a few Gauss on the surface of the star. A disk formed by this model can be dynamically important for the shaping of planetary nebulae.

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Anthony P. Goodson

Papers

Time-dependent Accretion by Magnetic Young Stellar Objects as a Launching Mechanism for Stellar Jets

Jets from Accreting Magnetic Young Stellar Objects. I. Comparison of Observations and High-Resolution Simulation Results

Jets from Accreting Magnetic Young Stellar Objects. II. Mechanism Physics

Disk Formation by AGB Winds in Dipole Magnetic Fields

Disk Formation by Asymptotic Giant Branch Winds in Dipole Magnetic Fields