Variability of black hole accretion discs: The cool, thermal disc component

We extend the model of King et al. (2004) for variability in black hole accretion discs, by taking proper account of the thermal properties of the disc. Because the degree of variability in the King et al. (2004) model depends sensitively on the ratio of disc thickness to radius, H/R, it is important to follow the time-dependence of the local disc structure as the variability proceeds. In common with previous authors, we develop a one-zone model for the local disc structure. We agree that radial heat advection plays an important role in determining the inner disc structure, and also find limit-cycle behaviour. When the stochastic magnetic dynamo model of King et al. (2004) is added to these models, we find similar variability behaviour to before. We are now better placed to put physical constraints on model parameters. In particular, we find that in order to be consistent with the low degree of variability seen in the thermal disc component of black hole binaries, we need to limit the energy density of the poloidal field that can be produced by local dynamo cells in the disc to less than a few percent of the energy density of the dynamo field within the disc itself.Comment: 18 pages, 17 figures, accepted by MNRA

Effects of Radiation Forces on the Frequency of Gravitomagnetic Precession Near Neutron Stars

Gravitomagnetic precession near neutron stars and black holes has received much recent attention, particularly as a possible explanation of 15--60 Hz quasi-periodic brightness oscillations (QPOs) from accreting neutron stars in low-mass X-ray binaries, and of somewhat higher-frequency QPOs from accreting stellar-mass black holes. Previous analyses of this phenomenon have either ignored radiation forces or assumed for simplicity that the radiation field is isotropic, and in particular that there is no variation of the radiation field with angular distance from the rotational equatorial plane of the compact object. However, in most realistic accretion geometries (e.g., those in which the accretion proceeds via a geometrically thin disk) the radiation field depends on latitude. Here we show that in this case radiation forces typically have an important, even dominant, effect on the precession frequency of test particles in orbits that are tilted with respect to the star's rotational equator. Indeed, we find that even for accretion luminosities only a few percent of the Eddington critical luminosity, the precession frequency near a neutron star can be changed by factors of up to $\sim 10$. Radiation forces must therefore be included in analyses of precession frequencies near compact objects, in such varied contexts as low-frequency QPOs, warp modes of disks, and trapped oscillation modes. We discuss specifically the impact of radiation forces on models of low-frequency QPOs involving gravitomagnetic precession, and show that such models are rendered much less plausible by the effects of radiation forces.Comment: 15 pages LaTeX including three figures, submitted to Ap

Externally-polluted white dwarfs with dust disks

We report Spitzer Space Telescope photometry of eleven externally-polluted white dwarfs. Of the nine stars for which we have IRAC photometry, we find that GD 40, GD 133 and PG 1015+161 each has an infrared excess that can be understood as arising from a flat, opaque, dusty disk. GD 56 also has an infrared excess characteristic of circumstellar dust, but a flat-disk model cannot reproduce the data unless there are grains as warm as 1700 K and perhaps not even then. Our data support the previous suggestion that the metals in the atmosphere of GD 40 are the result of accretion of a tidally-disrupted asteroid with a chondritic composition.Comment: ApJ, in pres

Radiation induced warping of protostellar accretion disks

We examine the consequences of radiatively driven warping of accretion disks surrounding pre-main-sequence stars. These disks are stable against warping if the luminosity arises from a steady accretion flow, but are unstable at late times when the intrinsic luminosity of the star overwhelms that provided by the disk. Warps can be excited for stars with luminosities of around 10 solar luminosities or greater, with larger and more severe warps in the more luminous systems. A twisted inner disk may lead to high extinction towards stars often viewed through their disks. After the disk at all radii becomes optically thin, the warp decays gradually on the local viscous timescale, which is likely to be long. We suggest that radiation induced warping may account for the origin of the warped dust disk seen in Beta Pictoris, if the star is only around 10-20 Myr old, and could lead to non-coplanar planetary systems around higher mass stars.Comment: 12 pages, including 3 figures. ApJ Letters, in pres

The alignment of disk and black hole spins in active galactic nuclei

The inner parts of an accretion disk around a spinning black hole are forced to align with the spin of the hole by the Bardeen-Petterson effect. Assuming that any jet produced by such a system is aligned with the angular momentum of either the hole or the inner disk, this can, in principle provide a mechanism for producing steady jets in AGN whose direction is independent of the angular momentum of the accreted material. However, the torque which aligns the inner disk with the hole, also, by Newton's third law, tends to align the spin of the hole with the outer accretion disk. In this letter, we calculate this alignment timescale for a black hole powering an AGN, and show that it is relatively short. This timescale is typically much less than the derived ages for jets in radio loud AGN, and implies that the jet directions are not in general controlled by the spin of the black hole. We speculate that the jet directions are most likely controlled either by the angular momentum of the accreted material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter

Competitive accretion in embedded stellar cluster

We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi-Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity that results means that the tidal radius is smaller than the Bondi-Hoyle radius in these systems. In contrast, when the stars dominate the potential and are virialised, the Bondi-Hoyle radius is smaller than the tidal radius and thus Bondi-Hoyle accretion is a better fit to the accretion rates.Comment: 11 pages, 11 figures, MNRAS in pres