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Search Results: 1 - 10 of 302586 matches for " Philip J. Armitage "
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The stream-disk interaction
Philip J. Armitage
Physics , 1997,
Abstract: I review theoretical aspects of the interaction between the accretion stream and the disk in interacting binary systems, concentrating on recent hydrodynamic calculations. At low accretion rates, cooling is expected to be efficient, and the interaction leads to a nearly ballistic stream overflowing the disk rim towards smaller radii. If cooling is ineffectual, the shocked gas produces a bulge on the disk rim, and there is no coherent stream inward of the disk edge. Results are presented for the mass fraction and velocity structure of the overflowing component, and the implications for X-ray observations of `dips', and doppler tomography of cataclysmic variables are briefly discussed.
A reduced efficiency of terrestrial planet formation following giant planet migration
Philip J. Armitage
Physics , 2002, DOI: 10.1086/346198
Abstract: Substantial orbital migration of massive planets may occur in most extrasolar planetary systems. Since migration is likely to occur after a significant fraction of the dust has been locked up into planetesimals, ubiquitous migration could reduce the probability of forming terrestrial planets at radii of the order of 1 au. Using a simple time dependent model for the evolution of gas and solids in the disk, I show that replenishment of solid material in the inner disk, following the inward passage of a giant planet, is generally inefficient. Unless the timescale for diffusion of dust is much shorter than the viscous timescale, or planetesimal formation is surprisingly slow, the surface density of planetesimals at 1 au will typically be depleted by one to two orders of magnitude following giant planet migration. Conceivably, terrestrial planets may exist only in a modest fraction of systems where a single generation of massive planets formed and did not migrate significantly.
Suppression of giant planet formation in stellar clusters
Philip J. Armitage
Physics , 2000,
Abstract: Photoevaporation driven by the ultraviolet radiation from massive stars severely limits the lifetime of protoplanetary discs around stars formed within stellar clusters. I investigate the resulting influence of clustered environments on the probability of giant planet formation, and show that for clusters as rich, or richer than, Orion, the time available for planet formation is likely to be limited to the length of any delay between low mass and high mass star formation. Under popular models for the formation of massive planets, the fraction of stars with giant planets in rich clusters is expected to be substantially suppressed as compared to less clustered star formation environments.
Ab initio simulations of accretion disc boundary layers
Philip J. Armitage
Physics , 2001,
Abstract: I discuss the results of simplified three dimensional magnetohydrodynamic simulations of the boundary layer between a disc and a non-rotating, unmagnetized star. Strong magnetic fields, possibly approaching equipartition with the thermal energy, occur in the boundary layer due to the shearing of disc-generated fields. The mean boundary layer magnetic field, which is highly variable on an orbital timescale, is estimated to exceed 50 kG for a CV with an accretion rate of 10^(-9) solar masses per year. However, these fields do not drive efficient angular momentum transport within the boundary layer. As a consequence the radial velocity in the boundary layer is low, and the density high.
Magnetic activity in accretion disc boundary layers
Philip J. Armitage
Physics , 2001, DOI: 10.1046/j.1365-8711.2002.05152.x
Abstract: We use three dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle (1989), the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission, and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere.
Dynamics of Protoplanetary Disks
Philip J. Armitage
Physics , 2010, DOI: 10.1146/annurev-astro-081710-102521
Abstract: Protoplanetary disks are quasi-steady structures whose evolution and dispersal determine the environment for planet formation. I review the theory of protoplanetary disk evolution and its connection to observations. Substantial progress has been made in elucidating the physics of potential angular momentum transport processes - including self-gravity, magnetorotational instability, baroclinic instabilities, and magnetic braking - and in developing testable models for disk dispersal via photoevaporation. The relative importance of these processes depends upon the initial mass, size and magnetization of the disk, and subsequently on its opacity, ionization state, and external irradiation. Disk dynamics is therefore coupled to star formation, pre-main-sequence stellar evolution, and dust coagulation during the early stages of planet formation, and may vary dramatically from star to star. The importance of validating theoretical models is emphasized, with the key observations being those that probe disk structure on the scales, between 1 AU and 10 AU, where theory is most uncertain.
Turbulence and angular momentum transport in a global accretion disk simulation
Philip J. Armitage
Physics , 1998, DOI: 10.1086/311463
Abstract: The global development of magnetohydrodynamic turbulence in an accretion disk is studied within a simplified disk model that omits vertical stratification. Starting with a weak vertical seed field, a saturated state is obtained after a few tens of orbits in which the energy in the predominantly toroidal magnetic field is still subthermal. The efficiency of angular momentum transport, parameterized by the Shakura-Sunyaev alpha parameter, is of the order of 0.1. The dominant contribution to alpha comes from magnetic stresses, which are enhanced by the presence of weak net vertical fields. The power spectra of the magnetic fields are flat or decline only slowly towards the largest scales accessible in the calculation, suggesting that the viscosity arising from MHD turbulence may not be a locally determined quantity. I discuss how these results compare with observationally inferred values of alpha, and possible implications for models of jet formation.
Physical processes in protoplanetary disks
Philip J. Armitage
Physics , 2015,
Abstract: This review introduces physical processes in protoplanetary disks relevant to accretion and the initial stages of planet formation. After reprising the elementary theory of disk structure and evolution, I discuss the gas-phase physics of angular momentum transport through turbulence and disk winds, and how this may be related to episodic accretion observed in Young Stellar Objects. Turning to solids, I review the evolution of single particles under aerodynamic forces, and describe the conditions necessary for the development of collective gas-particle instabilities. Observations show that disks are not always radially smooth axisymmetric structures, and I discuss how gas and particle processes can interact to form observable large-scale structure (at ice lines, vortices and in zonal flows). I conclude with disk dispersal.
Hydrodynamics of the stream-disk impact in interacting binaries
Philip J. Armitage,Mario Livio
Physics , 1997, DOI: 10.1086/305149
Abstract: We use hydrodynamic simulations to provide quantitative estimates of the effects of the impact of the accretion stream on disks in interacting binaries. For low accretion rates, efficient radiative cooling of the hotspot region can occur, and the primary consequence of the stream impact is stream overflow toward smaller disk radii. The stream is well described by a ballistic trajectory, but larger masses of gas are swept up and overflow at smaller, but still highly supersonic, velocities. If cooling is inefficient, overflow still occurs, but there is no coherent stream inward of the disk rim. Qualitatively, the resulting structure appears as a bulge extending downstream along the disk rim. We calculate the mass fraction and velocity of the overflowing component as a function of the important system parameters, and discuss the implications of the results for X-ray observations and doppler tomography of cataclysmic variables, low-mass X-ray binaries and supersoft X-ray sources.
Accretion during the merger of supermassive black holes
Philip J. Armitage,Priyamvada Natarajan
Physics , 2002, DOI: 10.1086/339770
Abstract: We study the evolution of disk accretion during the merger of supermassive black hole binaries in galactic nuclei. In hierarchical galaxy formation models, the most common binaries are likely to arise from minor galactic mergers, and have unequal mass black holes. Once such a binary becomes embedded in an accretion disk at a separation of the order of 0.1 pc, the merger proceeds in two distinct phases. During the first phase, the loss of orbital angular momentum to the gaseous disk shrinks the binary on a timescale of ~10 Myr. The accretion rate onto the primary black hole is not increased, and can be substantially reduced, during this disk-driven migration. At smaller separations, gravitational radiation becomes the dominant angular momentum loss process, and any gas trapped inside the orbit of the secondary is driven inwards by the inspiralling black hole. The implied accretion rate just prior to coalescence exceeds the Eddington limit, so the final merger is likely to occur within a common envelope formed from the disrupted inner disk, and be accompanied by high velocity (~10,000 km/s) outflows.
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