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

An Investigation into the Geometry of Seyfert Galaxies

We present a new method for the statistical investigation into the distributions of the angle beta between the radio axis and the normal to the galactic disk for a sample of Seyfert galaxies. We discuss how further observations of the sample galaxies can strengthen the conclusions. Our data are consistent with the hypothesis that AGN jets are oriented randomly in space, independent of the position of the plane of the galaxy. By making the simple assumption that the Standard Model of AGN holds, with a universal opening angle of the thick torus of phi_c, we demonstrate a statistical method to obtain an estimate of phi_c. Our data are not consistent with the simple-minded idea that Seyfert 1s and Seyfert 2s are differentiated solely by whether or not our line of sight lies within some fixed angle of the jet axis. Our result is significant on the 2 sigma level and can thus be considered only suggestive, not conclusive. A complete sample of Seyfert galaxies selected on an isotropic property is required to obtain a conclusive result.Comment: 13 pages, Tex, 5 Postscript figures. Accepted Ap

Alignment and Precession of a Black Hole with a Warped Accretion Disc

We consider the shape of an accretion disc whose outer regions are misaligned with the spin axis of a central black hole and calculate the steady state form of the warped disc in the case where the viscosity and surface densities are power laws in the distance from the central black hole. We discuss the shape of the resulting disc in both the frame of the black hole and that of the outer disc. We note that some parts of the disc and also any companion star maybe shadowed from the central regions by the warp. We compute the torque on the black hole caused by the Lense-Thirring precession and hence compute the alignment and precession timescales. We generalise the case with viscosity and hence surface density independent of radius to more realistic density distributions for which the surface density is a decreasing function of radius. We find that the alignment timescale does not change greatly but the precession timescale is more sensitive. We also determine the effect on this timescale if we truncate the disc. For a given truncation radius, the the timescales are less affected for more sharply falling density distributions.Comment: Accepted for publication in MNRA

Investigating Binary Properties with Next-Generation Microlensing Surveys

We explore the usefulness of future gravitational microlensing surveys in the study of binary properties such as the binary fraction and the distributions of binary separation and mass ratio by using the binary sample detectable through a channel of repeating events. For this, we estimate the rate of repeating microlensing eventstoward the Galactic bulge field based on standard models of dynamical and physical distributions of Galactic matter combined with models of binary separation and mass function. From this, we find that the total number of repeating events expected to be detected from $\sim 4$-year space-based surveys will be $\sim 200$--400, that is $\sim 40$--50 times higher than the rate of current surveys. We find that the high detection rate is due to the greatly improved sensitivity to events associated with faint source stars and low-magnification events. We find that the separation range of the binaries to be covered by the repeating events will extend up to 100 AU. Therefore, the future lensing surveys will provide a homogeneous sample that will allow to investigate the statistical properties of Galactic binaries unbiased by brightness of the binary components.Comment: total 6 pages, including 4 figures, ApJ, in pres

The Shape of an Accretion Disc in a Misaligned Black Hole Binary

We model the overall shape of an accretion disc in a semi-detached binary system in which mass is transfered on to a spinning black hole the spin axis of which is misaligned with the orbital rotation axis. We assume the disc is in a steady state. Its outer regions are subject to differential precession caused by tidal torques of the companion star. These tend to align the outer parts of the disc with the orbital plane. Its inner regions are subject to differential precession caused by the Lense-Thirring effect. These tend to align the inner parts of the disc with the spin of the black hole. We give full numerical solutions for the shape of the disc for some particular disc parameters. We then show how an analytic approximation to these solutions can be obtained for the case when the disc surface density varies as a power law with radius. These analytic solutions for the shape of the disc are reasonably accurate even for large misalignments and can be simply applied for general disc parameters. They are particularly useful when the numerical solutions would be slow.Comment: Accepted for publication in MNRA

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