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

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

Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient

We analyse the two-dimensional MHD configurations characterising the steady state of the accretion disk on a highly magnetised neutron star. The model we describe has a local character and represents the extension of the crystalline structure outlined in Coppi (2005), dealing with a local model too, when a specific accretion rate is taken into account. We limit our attention to the linearised MHD formulation of the electromagnetic back-reaction characterising the equilibrium, by fixing the structure of the radial, vertical and azimuthal profiles. Since we deal with toroidal currents only, the consistency of the model is ensured by the presence of a small collisional effect, phenomenologically described by a non-zero constant Nernst coefficient (thermal power of the plasma). Such an effect provides a proper balance of the electron force equation via non zero temperature gradients, related directly to the radial and vertical velocity components. We show that the obtained profile has the typical oscillating feature of the crystalline structure, reconciled with the presence of viscosity, associated to the differential rotation of the disk, and with a net accretion rate. In fact, we provide a direct relation between the electromagnetic reaction of the disk and the (no longer zero) increasing of its mass per unit time. The radial accretion component of the velocity results to be few orders of magnitude below the equatorial sound velocity. Its oscillating-like character does not allow a real matter in-fall to the central object (an effect to be searched into non-linear MHD corrections), but it accounts for the out-coming of steady fluxes, favourable to the ring-like morphology of the disk.Comment: 15 pages, 1 figure, accepted for publication on Modern Physics Letters

Are there brown dwarfs in globular clusters?

We present an analytical method for constraining the substellar initial mass function in globular clusters, based on the observed frequency of transit events. Globular clusters typically have very high stellar densities where close encounters are relatively common, and thus tidal capture can occur to form close binary systems. Encounters between main sequence stars and lower-mass objects can result in tidal capture if the mass ratio is > 0.01. If brown dwarfs exist in significant numbers, they too will be found in close binaries, and some fraction of their number should be revealed as they transit their stellar companions. We calculate the rate of tidal capture of brown dwarfs in both segregated and unsegregated clusters, and find that the tidal capture is more likely to occur over an initial relaxation time before equipartition occurs. The lack of any such transits in recent HST monitoring of 47 Tuc implies an upper limit on the frequency of brown dwarfs (< 15 % relative to stars) which is significantly below that measured in the galactic field and young clusters.Comment: MNRAS in pres

Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations

We model the variability profiles of millisecond period X-ray pulsars. We performed three-dimensional magnetohydrodynamic simulations of disk accretion to millisecond period neutron stars with a misaligned magnetic dipole moment, using the pseudo-Newtonian Paczynski-Wiita potential to model general relativistic effects. We found that the shapes of the resulting funnel streams of accreting matter and the hot spots on the surface of the star are quite similar to those for more slowly rotating stars obtained from earlier simulations using the Newtonian potential. The funnel streams and hot spots rotate approximately with the same angular velocity as the star. The spots are bow-shaped (bar-shaped) for small (large) misalignment angles. We found that the matter falling on the star has a higher Mach number when we use the Paczynski-Wiita potential than in the Newtonian case. Having obtained the surface distribution of the emitted flux, we calculated the variability curves of the star, taking into account general relativistic, Doppler and light-travel-time effects. We found that general relativistic effects decrease the pulse fraction (flatten the light curve), while Doppler and light-travel-time effects increase it and distort the light curve. We also found that the light curves from our hot spots are reproduced reasonably well by spots with a gaussian flux distribution centered at the magnetic poles. We also calculated the observed image of the star in a few cases, and saw that for certain orientations, both the antipodal hot spots are simultaneously visible, as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some typo

On the Relative Surface Density Change of Thermally Unstable Accretion Disks

The relations among the relative changes of surface density, temperature, disk height and vertical integrated pressure in three kinds of thermally unstable accretion disks were quantitatively investigated by assuming local perturbations. The surface density change was found to be very small in the long perturbation wavelength case but can not be ignored in the short wavelength case. It becomes significant in an optically thin, radiative cooling dominated disk when the perturbation wavelength is shorter than 15H (H is the scale height of disk) and in a geometrically thin, optically thick and radiation pressure dominated disk when the perturbation wavelength is shorter than 50H. In an optically thick, advection-dominated disk, which is thermally unstable against short wavelength perturbations, the relative surface density change is much larger. We proved the positive correlation between the changes of surface density and temperature in an optically thick, advection- dominated disk which was previously claimed to be the essential point of its thermal instability. Moreover, we found an anticorrelation between the changes of disk height and temperature in an optically thick, advection-dominated disk. This is the natural result of the absence of appreciable vertical integrated pressure change.Comment: 10 pages AAS LaTex file, 3 figures, accepted for publication in Ap

Massive planet migration: Theoretical predictions and comparison with observations

We quantify the utility of large radial velocity surveys for constraining theoretical models of Type II migration and protoplanetary disk physics. We describe a theoretical model for the expected radial distribution of extrasolar planets that combines an analytic description of migration with an empirically calibrated disk model. The disk model includes viscous evolution and mass loss via photoevaporation. Comparing the predicted distribution to a uniformly selected subsample of planets from the Lick / Keck / AAT planet search programs, we find that a simple model in which planets form in the outer disk at a uniform rate, migrate inward according to a standard Type II prescription, and become stranded when the gas disk is dispersed, is consistent with the radial distribution of planets for orbital radii 0.1 AU < a < 2.5 AU and planet masses greater than 1.65 Jupiter masses. Some variant models are disfavored by existing data, but the significance is limited (~95%) due to the small sample of planets suitable for statistical analysis. We show that the favored model predicts that the planetary mass function should be almost independent of orbital radius at distances where migration dominates the massive planet population. We also study how the radial distribution of planets depends upon the adopted disk model. We find that the distribution can constrain not only changes in the power-law index of the disk viscosity, but also sharp jumps in the efficiency of angular momentum transport that might occur at small radii.Comment: ApJ, in press. References updated to match published versio

The evolution of a warped disc around a Kerr black hole

We consider the evolution of a warped disc around a Kerr black hole, under conditions such that the warp propagates in a wavelike manner. This occurs when the dimensionless effective viscosity, alpha, that damps the warp is less than the characteristic angular semi-thickness, H/R, of the disc. We adopt linearized equations that are valid for warps of sufficiently small amplitude in a Newtonian disc, but also account for the apsidal and nodal precession that occur in the Kerr metric. Through analytical and time-dependent studies, we confirm the results of Demianski & Ivanov, and of Ivanov & Illarionov, that such a disc takes on a characteristic warped shape. The inner part of the disc is not necessarily aligned with the equator of the hole, even in the presence of dissipation. We draw attention to the fact that this might have important implications for the directionality of jets emanating from discs around rotating black holes.Comment: 8 pages, 6 figures, to be published in MNRA