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J. E. Pringle

Publications and source records attributed to J. E. Pringle.

At least 109 records · Page 6Linked to original sources
arXiv2003

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.

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arXiv2003

Active galactic nuclei and the minor merger hypothesis

We have investigated the dynamics of the merging process in the minor merger hypothesis for active galactic nuclei. We find that for a satellite galaxy to be able to merge directly with the nucleus of the host galaxy (for example, to give rise to the compact dust discs which are seen in early type active galaxies) requires the initial orbit of the satellite to be well aimed. For the case of the host galaxy being a disc galaxy, if the initial orbits of the satellites are randomly oriented with respect to the host galaxy, then the orbits of those which reach the host nuclear regions in a reasonable time, are also fairly randomly oriented once they reach the nucleus. We note that this result might be able to provide an explanation of why the jet directions in the nuclei of Seyfert galaxies are apparently unrelated to the plane of the galaxy discs.

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arXiv2003

The Disk-Jet Connection in Microquasars and AGN

We propose a new model for the disk-jet connection in black hole x-ray transients and active galactic nuclei. In our model, the inner part of the accretion disk around the central black hole switches between two states. In one state, the accretion energy is dissipated locally (some within the disk, some within a corona) to produce the observed disk luminosity. In the second state, the accretion energy is deposited into the bulk flow of a relativistic jet. We associate the transition between the two states with the generation of a global, poloidal magnetic field. We show that this model can explain the observed behavior of black hole accretors.

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arXiv2002

Viscous Effects on the Interaction between the Coplanar Decretion Disc and the Neutron Star in Be/X-Ray Binaries

We study the viscous effects on the interaction between the coplanar Be-star disc and the neutron star in Be/X-ray binaries, using a three-dimensional, smoothed particle hydrodynamics code. For simplicity, we assume the Be disc to be isothermal at the temperature of half the stellar effective temperature. In order to mimic the gas ejection process from the Be star, we inject particles with the Keplerian rotation velocity at a radius just outside the star. Both Be star and neutron star are treated as point masses. We find that the Be-star disc is effectively truncated if the Shakura-Sunyaev viscosity parameter alpha_SS >> 1, which confirms the previous semi-analytical result. In the truncated disc, the material decreted from the Be star accumulates, so that the disc becomes denser more rapidly than if around an isolated Be star. The resonant truncation of the Be disc results in a significant reduction of the amount of gas captured by the neutron star and a strong dependence of the mass capture rate on the orbital phase. We also find that an eccentric mode is excited in the Be disc through direct driving due to a one-armed bar potential of the binary. The strength of the mode becomes greater in the case of a smaller viscosity. In a high-resolution simulation with alpha_SS=0.1, the eccentric mode is found to precess in a prograde sense. The mass capture rate by the neutron star modulates as the mode precesses.

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arXiv2002

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.

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arXiv2002

The Orientation of Jets Relative to Dust Disks in Radio Galaxies

We use HST broad band images, VLA and VLBI continuum data to study the three dimensional orientation of jets relative to nuclear dust disks in 20 radio galaxies. The comparison between the position angles of the jets with those of the dust disks major axes shows a wide distribution, suggesting that they are not aligned preferentially perpendicular to each other. We use a statistical technique to determine the 3 dimensional distribution of angles between jets and dust disks rotation axes. This analysis shows that the observations are consistent with jets homogeneously distributed over a large region, extending over polar caps of 55 degrees to 77 degrees, but seem to avoid lying close to the plane of the dust disks. We argue that the lack of close alignment between jets and dust disks axes is not likely to be caused by feeding the nucleus with gas from mergers originated from random directions. We suggest that the misalignment can be due by a warping mechanism in the accretion disk, like self-irradiation instability or the Bardeen-Petterson effect, or that the gravitational potential in the inner regions of the galaxy is misaligned with respect to that of the dust disk.

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arXiv2002

Predictions for the frequency and orbital radii of massive extrasolar planets

We investigate the migration of massive extrasolar planets due to gravitational interaction with a viscous protoplanetary disc. We show that a model in which planets form at 5 AU at a constant rate, before migrating, leads to a predicted distribution of planets that is a steeply rising function of log (a), where a is the orbital radius. Between 1 AU and 3 AU, the expected number of planets per logarithmic interval in orbital radius roughly doubles. We demonstrate that, once selection effects are accounted for, this is consistent with current data, and then extrapolate the observed planet fraction to masses and radii that are inaccessible to current observations. In total, about 15 percent of stars targeted by existing radial velocity searches are predicted to possess planets with masses 0.3 M_Jupiter < M_p sin (i) < 10 M_Jupiter, and radii 0.1 AU < a < 5 AU. A third of these planets (around 5 percent of the target stars) lie at the radii most amenable to detection via microlensing. A further 5-10 percent of stars could have planets at radii of 5 AU < a < 8 AU that have migrated outwards. We discuss the probability of forming a system (akin to the Solar System) in which significant radial migration of the most massive planet does not occur. About 10-15 percent of systems with a surviving massive planet are estimated to fall into this class. Finally, we note that a smaller fraction of low mass planets than high mass planets is expected to survive without being consumed by the star. The initial mass function for planets is thus predicted to rise more steeply towards small masses than the observed mass function.

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arXiv2002

The Unique History of the Globular Cluster Omega Centauri

Using current observational data and simple dynamical modeling, we demonstrate that Omega Cen is not special among the Galactic globular clusters in its ability to produce and retain the heavy elements dispersed in the AGB phase of stellar evolution. The multiple stellar populations observed in Omega Cen cannot be explained if it had formed as an isolated star cluster. The formation within a progenitor galaxy of the Milky Way is more likely, although the unique properties of Omega Cen still remain a mystery.

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arXiv2002

The Excitation, Propagation and Dissipation of Waves in Accretion Discs: The Non-linear Axisymmetric Case

We analyse the non-linear propagation and dissipation of axisymmetric waves in accretion discs using the ZEUS-2D hydrodynamics code. The waves are numerically resolved in the vertical and radial directions. Both vertically isothermal and thermally stratified accretion discs are considered. The waves are generated by means of resonant forcing and several forms of forcing are considered. Compressional motions are taken to be locally adiabatic ($γ= 5/3$). Prior to non-linear dissipation, the numerical results are in excellent agreement with the linear theory of wave channelling in predicting the types of modes that are excited, the energy flux by carried by each mode, and the vertical wave energy distribution as a function of radius. In all cases, waves are excited that propagate on both sides of the resonance (inwards and outwards). For vertically isothermal discs, non-linear dissipation occurs primarily through shocks that result from the classical steepening of acoustic waves. For discs that are substantially thermally stratified, wave channelling is the primary mechanism for shock generation. Wave channelling boosts the Mach number of the wave by vertically confining the wave to a small cool region at the base of the disc atmosphere. In general, outwardly propagating waves with Mach numbers near resonance ${\cal M}_{\rm r} \ga 0.01$ undergo shocks within a distance of order the resonance radius.

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arXiv2001

Testing the Unified Model with an Infrared Selected Sample of Seyferts

We present a series of statistical tests using homogeneous data and measurements for a sample of Seyfert galaxies. These galaxies were selected from mostly isotropic properties, their far infrared fluxes and warm infrared colors, which provide a considerable advantage over the criteria used by most investigators in the past, like ultraviolet excess. Our results provide strong support for a Unified Model in which Seyferts 2's contain a torus seen more edge-on than in Seyferts 1's and show that previous results showing the opposite were most likely due to selection effects.

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arXiv2001

The Formation of Molecular Clouds

In a recent paper, Elmegreen (2000) has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical timescales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular we consider the hypothesis that molecular clouds may form not by {\it in situ} cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium (ISM), much, if not most, of which is already in molecular form.

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arXiv2001

Testing the Unified Model with an Infrared Selected Sample of Seyfert Galaxies

We present a series of statistical tests done to a sample of 29 Seyfert 1 and 59 Seyfert 2 galaxies selected from mostly isotropic properties, their far infrared fluxes and warm infrared colors. Such selection criteria provide a profound advantage over the criteria used by most investigators in the past, such as ultraviolet excess. These tests were done using ground based high resolution VLA A-configuration 3.6 cm radio and optical B and I imaging data. From the relative number of Seyfert 1's and Seyfert 2's we calculate that the torus half opening angle is 48deg. We show that, as seen in previous papers, there is a lack of edge-on Seyfert 1 galaxies, suggesting dust and gas along the host galaxy disk probably play an important role in hiding some nuclei from direct view. We find that there is no statistically significant difference in the distribution of host galaxy morphological types and radio luminosities of Seyfert 1's and Seyfert 2's, suggesting that previous results showing the opposite may have been due to selection effects. The average extension of the radio emission of Seyfert 1's is smaller than that of Seyfert 2's by a factor of ~2-3, as predicted by the Unified Model. A search for galaxies around our Seyferts allows us to put a lower and an upper limit on the possible number of companions around these galaxies of 19% and 28%, respectively, with no significant difference in the number of companion galaxies between Seyfert 1's and Seyfert 2's. We also show that there is no preference for the radio jets to be aligned closer to the host galaxy disk axis in late type Seyferts, unlike results claimed by previous papers. These results, taken together, provide strong support for a Unified Model in which type 2 Seyferts contain a torus seen more edge-on than the torus in type 1 Seyferts.

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arXiv2001

Magnetohydrodynamic turbulence in warped accretion discs

Warped, precessing accretion discs appear in a range of astrophysical systems, for instance the X-ray binary Her X-1 and in the active nucleus of NGC4258. In a warped accretion disc there are horizontal pressure gradients that drive an epicyclic motion. We have studied the interaction of this epicyclic motion with the magnetohydrodynamic turbulence in numerical simulations. We find that the turbulent stress acting on the epicyclic motion is comparable in size to the stress that drives the accretion, however an important ingredient in the damping of the epicyclic motion is its parametric decay into inertial waves.

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arXiv2001

Accretion in stellar clusters and the IMF

We present a simple physical mechanism that can account for the observed stellar mass spectrum for masses $\ms \simgreat 0.5 \solm$. The model depends solely on the competitive accretion that occurs in stellar clusters where each star's accretion rate depends on the local gas density and the square of the accretion radius. In a stellar cluster, there are two different regimes depending on whether the gas or the stars dominate the gravitational potential. When the cluster is dominated by cold gas, the accretion radius is given by a tidal-lobe radius. This occurs as the cluster collapses towards a $ρ\propto R^{-2}$ distribution. Accretion in this regime results in a mass spectrum with an asymptotic limit of $γ=-3/2$ (where Salpeter is $γ=-2.35$). Once the stars dominate the potential and are virialised, which occurs first in the cluster core, the accretion radius is the Bondi-Hoyle radius. The resultant mass spectrum has an asymptotic limit of $γ=-2$ with slightly steeper slopes ($γ\approx-2.5$) if the stars are already mass-segr egated. Simulations of accretion onto clusters containing 1000 stars show that as expected, the low-mass stars accumulate the majority of their masses during the gas dominated phase whereas the high-mass stars accumulate the majority of their massed during the stellar dominated phase. This results in a mass spectrum with a relatively shallow $γ\approx 3/2$ power-law for low-mass stars and a steeper, power-law for high-mass stars $ -2.5\simlessγ\le -2$. This competitive accretion model also results in a mass segregated cluster.

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arXiv2001

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.

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arXiv2001

Episodic accretion in magnetically layered protoplanetary discs

We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 AU such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion onto the protostar occurs in bursts with an accretion rate of 10^{-5} solar masses / yr, separated by quiescent intervals where the accretion rate is 10^{-8} solar masses / yr. Such bursts could lead to repeated episodes of strong mass outflow in Young Stellar Objects. The transition to this episodic mode of accretion occurs at an early epoch (t < 1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at around 1 AU. The predicted rate of low mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1-3 AU.

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arXiv2000

Type Ia Supernovae and Remnant Neutron Stars

On the basis of the current observational evidence, we put forward the case that the merger of two CO white dwarfs produces both a Type Ia supernova explosion and a stellar remnant, the latter in the form of a magnetar. The estimated occurrence rates raise the possibility that many, if not most, SNe Ia might result from white dwarf mergers.

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