Abstract More than 36 yr have passed since the discovery of the infrared excess from circumstellar dust orbiting the white dwarf G29-38, which at 17.5 pc it is the nearest and brightest of its class. ...The precise morphology of the orbiting dust remains only marginally constrained by existing data, subject to model-dependent inferences, and thus fundamental questions of its dynamical origin and evolution persist. This study presents a means to constrain the geometric distribution of the emitting dust using stellar pulsations measured at optical wavelengths as a variable illumination source of the dust, which reradiates primarily in the infrared. By combining optical photometry from the Whole Earth Telescope with 0.7–2.5 μ m spectroscopy obtained with SpeX at NASA’s Infrared Telescope Facility, we detect luminosity variations at all observed wavelengths, with variations at most wavelengths corresponding to the behavior of the pulsating stellar photosphere, but toward the longest wavelengths the light curves probe the corresponding time variability of the circumstellar dust. In addition to developing methodology, we find the pulsation amplitudes decrease with increasing wavelength for principal pulsation modes, yet increase beyond ≈2 μ m for nonlinear combination frequencies. We interpret these results as combination modes derived from the principal modes of identical ℓ values and discuss the implications for the morphology of the warm dust. We also draw attention to some discrepancies between our findings and theoretical expectations for the results of the nonlinearity imposed by the surface convection zone on mode–mode interactions and on the behavior of the first harmonic of the highest-amplitude pulsation mode.
In this paper, we consider the effect of the direct ionizing stellar radiation field on the evolution of protoplanetary discs subject to photoevaporative winds. We suggest that models which combine ...viscous evolution with photoevaporation of the disc incorrectly neglect the direct field after the inner disc has drained, at late times in the evolution. We construct models of the photoevaporative wind produced by the direct field, first using simple analytic arguments and later using detailed numerical hydrodynamics. We find that the wind produced by the direct field at late times is much larger than has previously been assumed, and we show that the mass-loss rate scales as R1/2in (where Rin is the radius of the instantaneous inner disc edge). We suggest that this result has important consequences for theories of disc evolution, and go on to consider the effects of this result on disc evolution in detail in a companion paper.
Aim To determine timing, source and vector for the recent introduction of the European green crab, Carcinus maenas (Linnaeus, 1758), to Newfoundland using multiple lines of evidence. Location ...Founding populations in Placentia Bay, Newfoundland, Canada and potential source populations in the north‐west Atlantic (NWA) and Europe. Methods We analysed mitochondrial and microsatellite genetic data from European and NWA populations sampled during 1999-2002 to determine probable source locations and vectors for the Placentia Bay introduction discovered in 2007. We also analysed Placentia Bay demographic data and shipping records to look for congruent patterns with genetic analyses. Results Demographic data and surveys suggested that C. maenas populations are established and were in Placentia Bay for several years (c. 2002) prior to discovery. Genetic data corroboratively suggested central/western Scotian Shelf populations (e.g., Halifax) as the likely source area for the anthropogenic introduction. These Scotian Shelf populations were within an admixture zone made up of genotypes from both the earlier (early 1800s) and later (late 1900s) introductions of the crab to the NWA from Europe. Placentia Bay also exhibited this mixed ancestry. Probable introduction vectors included vessel traffic and shipping, especially vessels carrying ballast water. Main conclusions Carcinus maenas overcame considerable natural barriers (i.e., coastal and ocean currents) via anthropogenic transport to become established and abundant in Newfoundland. Our study thus demonstrates how non‐native populations can be important secondary sources of introduction especially when aided by human transport. Inference of source populations was possible owing to the existence of an admixture zone in central/western Nova Scotia made up of southern and northern genotypes corresponding with the crab's two historical introductions. Coastal vessel traffic was found to be a likely vector for the crab's spread to Newfoundland. Our study demonstrates that there is considerable risk for continued introduction or reintroduction of C. maenas throughout the NWA.
Tidal warping and precession of Be star decretion discs Martin, Rebecca G.; Pringle, J. E.; Tout, Christopher A. ...
Monthly notices of the Royal Astronomical Society,
October 2011, Letnik:
416, Številka:
4
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Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray ...outbursts are thought to occur when the warped disc interacts with the fast stellar wind. We suggest that tides from a misaligned companion neutron star can cause the observed effects. We make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion. Tidal torques from the neutron star truncate the disc at a radius small enough that the neutron star orbit does not intersect the disc unless the eccentricity or misalignment is very large. A magnetic torque from the Be star that is largest at the equator, where the rotation is fastest, is approximated by an inner boundary condition. There are large oscillations in the mass and inclination of the disc as it moves towards a steady state. These large variations may explain the observed changes from Be star to Be shell star and vice versa and also the Type II X-ray outbursts. We find the tidal time-scale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years. If present, the oscillations in mass and disc inclination occur on a fraction of this time-scale depending on the orbital parameters of the binary. The time-scales associated with the tidal torque for observed Be star binaries suggest that these effects are important in all but the longest period binaries.
Instability of warped discs Doğan, S; Nixon, C J; King, A R ...
Monthly notices of the Royal Astronomical Society,
05/2018, Letnik:
476, Številka:
2
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Abstract
Accretion discs are generally warped. If a warp in a disc is too large, the disc can ‘break’ apart into two or more distinct planes, with only tenuous connections between them. Further, if ...an initially planar disc is subject to a strong differential precession, then it can be torn apart into discrete annuli that precess effectively independently. In previous investigations, torque-balance formulae have been used to predict where and when the disc breaks into distinct parts. In this work, focusing on discs with Keplerian rotation and where the shearing motions driving the radial communication of the warp are damped locally by turbulence (the ‘diffusive’ regime), we investigate the stability of warped discs to determine the precise criterion for an isolated warped disc to break. We find and solve the dispersion relation, which, in general, yields three roots. We provide a comprehensive analysis of this viscous-warp instability and the emergent growth rates and their dependence on disc parameters. The physics of the instability can be understood as a combination of (1) a term that would generally encapsulate the classical Lightman–Eardley instability in planar discs (given by ∂(νΣ)/∂Σ < 0) but is here modified by the warp to include ∂(ν1|ψ|)/∂|ψ| < 0, and (2) a similar condition acting on the diffusion of the warp amplitude given in simplified form by ∂(ν2|ψ|)/∂|ψ| < 0. We discuss our findings in the context of discs with an imposed precession, and comment on the implications for different astrophysical systems.
Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disk. Current disk models, referred to as ..."decretion disks," make use of the zero-torque inner boundary condition typically applied to accretion disks, with the "decretion" modeled by adding mass to the disk at a radius of about 2% larger than the inner disk boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disk are implausibly large. What is required is that the disk has not only a source of mass but also a continuing source of angular momentum. We argue that the disk evolution may be more physically modeled by application of the nonzero torque inner boundary condition of Nixon & Pringle, which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disk cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disk material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disk. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disk inner regions to drive the observed disk evolution.
Abstract
We analyse the results of four simulations of isolated galaxies: two with a rigid spiral potential of fixed pattern speed, but with different degrees of star formation induced feedback, one ...with an axisymmetric galactic potential and one with a 'live' self-gravitating stellar component. Since we use a Lagrangian method, we are able to select gas that lies within giant molecular clouds (GMCs) at a particular time frame, and then study the properties of this gas at earlier and later times. We find that gas which forms GMCs is not typical of the interstellar medium (ISM) at least 50 Myr before the clouds form and reaches mean densities within an order of magnitude of mean cloud densities by around 10 Myr before cloud formation. The gas in GMCs takes at least 50 Myr to return to typical ISM gas after dispersal by stellar feedback, and in some cases the gas is never fully recycled. We also present a study of the 2D, vertically averaged velocity fields within the ISM. We show that the velocity fields corresponding to the shortest time-scales (i.e. those time-scales closest to the immediate formation and dissipation of the clouds) can be readily understood in terms of the various cloud formation and dissipation mechanisms. Properties of the flow patterns can be used to distinguish the processes which drive converging flows (e.g. spiral shocks, supernovae) and thus molecular cloud formation, and we note that such properties may be detectable with future observations of nearby galaxies.
In order to provide an explanation for the unexpected radial brightness distribution of the steady accretion discs seen in nova-like variables, we have recently proposed that the accretion energy is ...redistributed outwards by means of strong, magnetically driven surface flows. In this paper we note that the “powerful, rotating disc winds” observed in the soft states of black hole X-ray binaries, and also in the disc around a magnetised neutron star in Her X–1, have the properties of the outflows we recently postulated to exist in the nova-like variable accretion discs around white dwarfs. The relevant properties are that the flows are not winds, but are instead bound flows (travelling at less than the escape velocity) and that the mass fluxes in the flows are a substantial fraction of the accretion rate in the disc.
Pore space percolation in sea ice single crystals Pringle, D. J.; Miner, J. E.; Eicken, H. ...
Journal of Geophysical Research. B. Solid Earth,
December 2009, Letnik:
114, Številka:
C12
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We have imaged sea ice single crystals with X‐ray computed tomography, and characterized the thermal evolution of the pore space with percolation theory. Between −18°C and −3°C the porosity ranged ...from 2 to 12% and we found arrays of near‐parallel intracrystalline brine layers whose connectivity and complex morphology varied with temperature. We have computed key porosity‐dependent functions of classical percolation theory directly from the thermally driven pore space evolution of an individual sample. This analysis is novel for a natural material and provides the first direct demonstration of a connectivity threshold in the brine microstructure of sea ice. In previous works this critical behavior has been inferred indirectly from bulk property measurements in polycrystalline samples. From a finite‐size scaling analysis we find a vertical critical porosity pc,v = 4.6 ± 0.7%. We find lateral anisotropy with pc,pll = 9 ± 2% parallel to the layers and pc,perp = 14 ± 4% perpendicular to them. Lateral connectivity is established at higher brine volumes by the formation of thin necks between the brine layers. We relate these results to measured anisotropy in the bulk dc conductivity and fluid permeability using a dual porosity conceptual model. Our results shed new light on the complex microstructure of sea ice, highlighting single crystal anisotropy and a step toward a realistic transport property model for sea ice based on percolation theory. We present full experimental details of our imaging and segmentation methodology based on a phase relation formulation more widely applicable to ice‐solute systems.
This paper presents a self-consistent model for the evolution of gas produced in the debris disc of β Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation ...of CO, which is itself released from volatile-rich bodies in the debris disc due to grain–grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionization fraction and population levels of carbon and oxygen are followed with the photodissociation region model cloudy, which is coupled to a dynamical viscous α model. We present new gas observations of β Pic, of C i observed with Atacama Pathfinder EXperiment and O i observed with Herschel, and show that these along with published C ii and CO observations can all be explained with this new model. Our model requires a viscosity α > 0.1, similar to that found in sufficiently ionized discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionized and dilute medium. This new model can be tested from its predictions for high-resolution ALMA observations of C i. We also constrain the water content of the planetesimals in β Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.
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