Magnetically warped discs in close binaries

We demonstrate that measurable vertical structure can be excited in the accretion disc of a close binary system by a dipolar magnetic field centred on the secondary star. We present the first high resolution hydrodynamic simulations to show the initial development of a uniform warp in a tidally truncated accretion disc. The warp precesses retrogradely with respect to the inertial frame. The amplitude depends on the phase of the warp with respect to the binary frame. A warped disc is the best available explanation for negative superhumps.Comment: 11 pages, 10 figures, MNRAS accepte

Application of a hybrid model to reduce bias and improve precision in population estimates for elk (Cervus elaphus) inhabiting a cold desert ecosystem

AbstractAccurately estimating the size of wildlife populations is critical to wildlife management and conservation of species. Raw counts or “minimum counts” are still used as a basis for wildlife management decisions. Uncorrected raw counts are not only negatively biased due to failure to account for undetected animals, but also provide no estimate of precision on which to judge the utility of counts. We applied a hybrid population estimation technique that combined sightability modeling, radio collar-based mark-resight, and simultaneous double count (double-observer) modeling to estimate the population size of elk in a high elevation desert ecosystem. Combining several models maximizes the strengths of each individual model while minimizing their singular weaknesses. We collected data with aerial helicopter surveys of the elk population in the San Luis Valley and adjacent mountains in Colorado State, USA in 2005 and 2007. We present estimates from 7 alternative analyses: 3 based on different methods for obtaining a raw count and 4 based on different statistical models to correct for sighting probability bias. The most reliable of these approaches is a hybrid double-observer sightability model (model MH), which uses detection patterns of 2 independent observers in a helicopter plus telemetry-based detections of radio collared elk groups. Data were fit to customized mark-resight models with individual sighting covariates. Error estimates were obtained by a bootstrapping procedure. The hybrid method was an improvement over commonly used alternatives, with improved precision compared to sightability modeling and reduced bias compared to double-observer modeling. The resulting population estimate corrected for multiple sources of undercount bias that, if left uncorrected, would have underestimated the true population size by as much as 22.9%. Our comparison of these alternative methods demonstrates how various components of our method contribute to improving the final estimate and demonstrates why each is necessary

Reconnection in a Weakly Stochastic Field

We examine the effect of weak, small scale magnetic field structure on the rate of reconnection in a strongly magnetized plasma. This affects the rate of reconnection by reducing the transverse scale for reconnection flows, and by allowing many independent flux reconnection events to occur simultaneously. Allowing only for the first effect and using Goldreich and Sridhar's model of strong turbulence in a magnetized plasma with negligible intermittency, we find that the lower limit for the reconnection speed is the Alfven speed times the Lundquist number to the power (-3/16). The upper limit on the reconnection speed is typically a large fraction of Alfven speed. We argue that generic reconnection in turbulent plasmas will normally occur at close to this upper limit. The fraction of magnetic energy that goes directly into electron heating scales as Lundquist number to the power (-2/5) and the thickness of the current sheet scales as the Lundquist number to the power (-3/5). A significant fraction of the magnetic energy goes into high frequency Alfven waves. We claim that the qualitative sense of these conclusions, that reconnection is fast even though current sheets are narrow, is almost independent of the local physics of reconnection and the nature of the turbulent cascade. As the consequence of this the Galactic and Solar dynamos are generically fast, i.e. do not depend on the plasma resistivity.Comment: Extended version accepted to ApJ, 44pages, 2 figure

Tidal Decay of Close Planetary Orbits

The 4.2-day orbit of the newly discovered planet around 51~Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamically survived in an orbit with a period as short as $\sim10\,$hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars and we re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun.Comment: AAS LaTeX macros v.4, 14 pages, 2 postscript figures, also available from http://ensor.mit.edu/~rasio/, to appear in Ap

Trapping of magnetic flux by the plunge region of a black hole accretion disk

The existence of the radius of marginal stability means that accretion flows around black holes invariably undergo a transition from a MHD turbulent disk-like flow to an inward plunging flow. We argue that the plunging inflow can greatly enhance the trapping of large scale magnetic field on the black hole, and therefore may increase the importance of the Blandford-Znajek (BZ) effect relative to previous estimates that ignore the plunge region. We support this hypothesis by constructing and analyzing a toy-model of the dragging and trapping of a large scale field by a black hole disk, revealing a strong dependence of this effect on the effective magnetic Prandtl number of the MHD turbulent disk. Furthermore, we show that the enhancement of the BZ effect depends on the geometric thickness of the accretion disk. This may be, at least in part, the physical underpinnings of the empirical relation between the inferred geometric thickness of a black hole disk and the presence of a radio jet.Comment: 18 pages, 3 figures, accepted for publication in the Astrophysical Journal. See http://www.astro.umd.edu/~chris/publications/movies/flux_trapping.html for animation

Galactic dynamos with captured magnetic flux and an accretion flow

We examine the behaviour of an axisymmetric galactic dynamo model with a radial accretion flow in the disc. We also introduce a vertical magnetic flux through the galactic midplane, to simulate the presence of a large scale magnetic field trapped by the galaxy when forming. The trapped vertical flux is conserved and advected towards the disc centre by the radial flow. We confirm that accretion flows of magnitude several km/s through a significant part of the galactic disc can markedly inhibit dynamo action. Moreover, advection of the vertical flux in general results in mixed parity galactic fields. However, the effect is nonlinear and non-additive -- global magnetic field energies are usually significantly smaller that the sum of purely dynamo generated and purely advected field energies. For large inflow speeds, a form of `semi-dynamo' action may occur. We apply our results to the accumulation and redistribution, by a radial inflow, of a vertical magnetic flux captured by the Galactic disc. Taking representative values, it appears difficult to obtain mean vertical fields near the centre of the Milky Way that are much in excess of 10 microgauss, largely because the galactic dynamo and turbulent magnetic diffusion modify the external magnetic field before it can reach the disc centre.Comment: 18 pages, 12 figures, LaTE

Aligning spinning black holes and accretion discs

We consider the alignment torque between a spinning black hole and an accretion disc whose angular momenta are misaligned. This situation must hold initially in almost all gas accretion events on to supermassive black holes, and may occur in binaries where the black hole receives a natal supernova kick. We show that the torque always acts to align the hole's spin with the total angular momentum without changing its magnitude. The torque acts dissipatively on the disc, reducing its angular momentum, and aligning it with the hole if and only if the angle theta between the angular momenta J_d of the disc and J_h of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc counteraligns.Comment: MNRAS, in pres

Hydrodynamic instability in eccentric astrophysical discs

Eccentric Keplerian discs are believed to be unstable to three-dimensional hydrodynamical instabilities driven by the time-dependence of fluid properties around an orbit. These instabilities could lead to small-scale turbulence, and ultimately modify the global disc properties. We use a local model of an eccentric disc, derived in a companion paper, to compute the non-linear vertical (‘breathing mode’) oscillations of the disc. We then analyse their linear stability to locally axisymmetric disturbances for any disc eccentricity and eccentricity gradient using a numerical Floquet method. In the limit of small departures from a circular reference orbit, the instability of an isothermal disc is explained analytically. We also study analytically the small-scale instability of an eccentric neutrally stratified polytropic disc with any polytropic index using a Wentzel–Kramers–Brillouin (WKB) approximation. We find that eccentric discs are generically unstable to the parametric excitation of small-scale inertial waves. The non-linear evolution of these instabilities should be studied in numerical simulations, where we expect them to lead to a decay of the disc eccentricity and eccentricity gradient as well as to induce additional transport and mixing. Our results highlight that it is essential to consider the three-dimensional structure of eccentric discs, and their resulting vertical oscillatory flows, in order to correctly capture their evolution