652 research outputs found
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 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
Three-dimensional Calculations of High and Low-mass Planets Embedded in Protoplanetary Discs
We analyse the non-linear, three-dimensional response of a gaseous, viscous
protoplanetary disc to the presence of a planet of mass ranging from one Earth
mass (1 M) to one Jupiter mass (1 M) by using the ZEUS hydrodynamics
code. We determine the gas flow pattern, and the accretion and migration rates
of the planet. The planet is assumed to be in a fixed circular orbit about the
central star. It is also assumed to be able to accrete gas without expansion on
the scale of its Roche radius. Only planets with masses M \gsim 0.1 M
produce significant perturbations in the disc's surface density. The flow
within the Roche lobe of the planet is fully three-dimensional. Gas streams
generally enter the Roche lobe close to the disc midplane, but produce much
weaker shocks than the streams in two-dimensional models. The streams supply
material to a circumplanetary disc that rotates in the same sense as the
planet's orbit. Much of the mass supply to the circumplanetary disc comes from
non-coplanar flow. The accretion rate peaks with a planet mass of approximately
0.1 M and is highly efficient, occurring at the local viscous rate. The
migration timescales for planets of mass less than 0.1 M, based on torques
from disc material outside the planets' Roche lobes, are in excellent agreement
with the linear theory of Type I (non-gap) migration for three-dimensional
discs. The transition from Type I to Type II (gap) migration is smooth, with
changes in migration times of about a factor of 2. Starting with a core which
can undergo runaway growth, a planet can gain up to a few M with little
migration. Planets with final masses of order 10 M would undergo large
migration, which makes formation and survival difficult.Comment: Accepted by MNRAS, 18 pages, 13 figures (6 degraded resolution).
Paper with high-resolution figures available at
http://www.astro.ex.ac.uk/people/mbate
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
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