bluuebunny/crossref_metadata_2025_split huggingface dataset:
Accretion Disks with Convective Viscosity
Authors: Smak J. 
Published: July 2016

AbstractModels of vertical structure are calculated for a range of parameters applicable to dwarf novae with two types of viscosity being included: the standard α-disk viscosity and the additional turbulent viscosity in the convective regions. The resulting surface density (∑) υs. effective temperature (Te) relations, compared to those without convective viscosity, show larger separation between the two critical points of the ∑ – Te relation, ∑min and ∑max. This could suggest that with such a modification the dwarf nova outbursts could be reproduced with a single value of α. (Note that in the standard α approach two different α’s are needed on the hot and cool branch of the ∑ – Te relation, with αcool ≈ αhot/4). It turns out, however, that this is not the case. This is due to the fact that additional convective viscosity makes also ∑max smaller than in the pure α case.
Three-Dimensional Simulations of Accretion Disks
Authors: Hawley John F. , Balbus Steven A. 
Published: July 2016

AbstractThe transport of angular momentum is the central issue in accretion disk dynamics. We review recent three-dimensional simulations that investigate possible transport mechanisms. Purely hydrodynamic local instabilities and turbulence are ruled out; global spiral waves remain a possibility. MHD turbulence, arising from a local MHD instability, has been shown effective in transporting angular momentum at dynamically important rates. These results establish the basic picture of accretion disk transport.
Interaction Of A Stellar Magnetic Field With A Turbulent Accretion Disk
Authors: Bardou Anne , Heyvaerts Jean 
Published: July 2016

AbstractMagnetized objects surrounded by a turbulent and keplerian accretion disk are considered. In such systems, magnetic field lines are embedded into the disk. The magnetic field of the central object is assumed to be dipolar in the absence of the accretion disk. In the presence of a turbulent accretion disk, it is shown that the interaction with the disk stretches magnetic field lines along the disk and that most of the non-magnetospheric magnetic flux is expelled outside the disk.
Disc Accretion Onto Magnetic Stars: Slow Rotator And Propeller
Authors: Li Jianke , Wickramasinghe Dayal T. 
Published: July 2016

AbstractWe study two magnetised accretion – outflow problems: the slow rotator and the propeller. In the former case, we show that the funnel flow does not result in the accretion of a significant amount of matter angular momentum, contrary to what is assumed in the standard Ghosh and Lamb model. Implications of this finding are investigated, particularly in relation to the fastness parameter. We also argue that magnetospheric mass ejection (the magnetospheric streamer model) is a viable mechanism for the ejection of mass and angular momentum from a magnetic star-disc system in the propeller regime.
On the Dramatic Spin-Up/Spin-Down Torque Reversals in BATSE Observations of Accretion Powered Pulsars
Authors: Nelson Robert W. , Vaughan Brian A. , Bildsten Lars , Chakrabarty Deepto , Koh Danny T. , Prince Thomas A. , Finger Mark H. , Wilson Robert B. , Rubin Bradley C. 
Published: July 2016

AbstractX-ray pulsars are the only accreting magnetic stars where rotation torques induced by accretion are large enough to be measured on short timescales ~ days. They are thus unique laboratories for studying the interaction between an accretion disk and a stellar magnetosphere. We describe 5 years of continuous pulsar timing observations by the BATSE instrument on GRO which paint a strikingly different picture of pulsar spin behavior than understood from the previous 20 years of sparse observations. In particular, we find that more than half of the persistent pulsars we observe undergo dramatic torque reversals, switching suddenly between extended periods of steady spin-up and steady spin-down. Moreover, variations in pulsed flux are anticorrelated with torque in at least one system undergoing secular spin-down, GX1+4. This behavior contradicts standard accretion torque theory (Ghosh and Lamb 1979). A simple – albeit unconventional – hypothesis which naturally explains these observations is that the disks in these systems somehow alternate between epochs of prograde and retrograde rotation.
A Model for WZ Sge with “Standard” Values of α
Authors: Hameury J.M. , Huré J.M. , Lasota J.P. 
Published: July 2016

AbstractWe present a model for the dwarf nova WZ Sge which does not imply an unusually – and unexplained – low value of the viscosity during low states. We propose that the inner parts of the accretion disc are disrupted by either a magnetic field or evaporation, so that the disc is stable (or very close to being stable) in quiescence, as the mass transfer rate is very low, and the disc can sit on the cool, lower branch of the thermal equilibrium curve. Outbursts are triggered by an increase of mass transfer, causing a standard disc instability, and are strongly enhanced by illumination effects. Both the recurrence time, and the characteristics of the light curve in outburst can be accounted for.
Radiation-Driven Warping: The Origin of Warps and Precession in Accretion Disks
Authors: Maloney Philip R. , Begelman Mitchell C. 
Published: July 2016

AbstractA geometrically thin, optically thick, warped accretion disk with a central source of luminosity is subject to non-axisymmetric forces due to radiation pressure; the resulting torque acts to modify the warp. Initially planar accretion disks are unstable to warping driven by radiation torque, as shown in a local analysis by Pringle (1996) and a global analysis of the stable and unstable modes by Maloney, Begelman, & Pringle (1996). In general, the warp also precesses.We discuss the nature of this instability, and its possible implications for accretion disks in X-ray binaries and active galactic nuclei. Specifically, we argue that this effect provides a plausible explanation for the misalignment and precession of the accretion disks in X-ray binaries such as SS 433 and Her X–l; the same mechanism explains why the maser disk in NGC 4258 is warped.
On Modelling the Lightcurves of Binary X-ray Sources
Authors: Hessman F.V. , van Teeseling A. , Beuermann K. 
Published: July 2016

AbstractThe lightcurves of non-magnetic X-ray binaries are usually modelled using standard techniques developed for ordinary double stars plus some assumptions about the geometry and temperature distribution of the accretion disk. The analyses of the observed optical lightcurves of systems suggest that the disks must have extremely large rims (H/R ~ 0.3) in order to produce the variable obscuration needed to reproduce the often asymmetric lightcurves. However, even models with large rims have problems explaining the X-ray absorption seen in systems with smaller inclinations. We discuss two phenomena which show that non-standard effects in X-ray binaries can play major roles in the determination of the lightcurves: the “spray” caused by the passage of the accretion stream through the “bow-shock” at the outer rim of the disk; and the effects of soft X-ray illumination of the secondary star.
HST GHRS Observations of the Herbig Ae Star HD104237: First UV Observations of a Hot Disk Wind from a Pre-Main Sequence Star
Authors: Brown Alexander , Djie H.R.E. Tjin A , Blondel P.F.C. , Harper G.M. , Bennett P.D. , Skinner S.L. 
Published: July 2016

AbstractWe have obtained ultraviolet spectra of the Herbig Ae star HD104237 using the Goddard High Resolution Spectrograph (GHRS) on HST. The high temperature emission and absorption lines show remarkable outflow absorption features, which have very similar profiles that are essentially independent of formation temperature. The profiles are not those expected from a spherically-symmetric stellar wind but have optically-thick absorption to −280 km s−1 and a high velocity plateau extending to −375 km s−1. The profile shape is a manifestation of the nonspherical geometry of the flow. The observed UV emission is too strong to be associated with the hot X–ray emitting plasma seen by ASCA and probably is generated by the interaction of the innermost part of an accretion disk with the corotating outermost magnetospheric field. The outflow is almost certainly the inner part of a biconical disk wind.
Do Herbig Ae/Be Stars Have Disks?
Authors: Ray T.P. , Corcoran M. 
Published: July 2016

AbstractThe Herbig Ae/Be stars are optically visible pre-main sequence stars of intermediate mass (M* ≈ 3−8M⊙) and are thought to be the higher mass analogues of the T Tauri stars. While there is no doubt that classical T Tauri stars, i.e. those with EW(Hα) ≳ 10 Å, are surrounded by disks, it remains controversial as to whether this is the case with the equivalent Herbig Ae/Be stars. It has even been questioned whether the powerful winds that are ejected by Herbig Ae/Be stars are driven by accretion. To address these problems we have examined a large sample of these stars with the idea of using their forbidden line emission as an indirect diagnostic for the presence of disks. Striking similarities with the classical T Tauri stars are found. For example we have discovered evidence not only for a strong correlation between near-infrared colours and the equivalent width of the forbidden line emission but also that the forbidden line emission normally arises in a blueshifted outflow component. It has already been shown in the case of the classical T Tauri stars that the correlation of near-infrared colour with forbidden line equivalent width is due to a link between the accretion rate and the outflow rate. The virtually identical relationship seen in the case of the Herbig Ae/Be stars must then also have a similar origin. Our finding that the forbidden line emission in Herbig Ae/Be stars is normally blueshifted shows not only that it arises in an outflow but, as in the classical T Tauri stars, such an asymmetry in the velocity centre of the line must be caused by the obscuring effects of a disk. We find that the correlation seen in the classical T Tauri stars between the mass-loss rate and infrared excess can be extended, when we include the Herbig Ae/Be stars, to cover almost 5 orders of magnitude in stellar luminosity. Our observations therefore broaden the findings of earlier observers for low mass young stars and indicate the presence of circumstellar disks around the majority of Herbig Ae/Be stars with forbidden line emission. A corollary of our results is that the same outflow mechanism must operate in both the classical T Tauri stars and the Herbig Ae/Be stars with forbidden line emission.
HST Observations of the HH 47 and HH 111 Stellar Jets
Authors: Hartigan Patrick 
Published: July 2016

AbstractThe superb spatial resolution made possible by the WFPC2 imager on the Hubble Space Telescope has opened a new era in the study of shocks in stellar jets. This paper summarizes the results of imaging surveys of two of the brightest and best resolved stellar jets, HH 111 and HH 47. The new HST images show that the jets appear quite different in the Ha and [S II] lines; Ha concentrates in distinctive filaments while [S II] tends to be clumpier and more widespread. Filamentary Hα emission should trace the location of the shock fronts if the preshock gas is neutral, because a significant fraction of the incident H gas becomes excited collisionally before becoming ionized in moderate velocity shocks.Both HH 111 and HH 47 exhibit multiple bow shocks along the channel of the jet, and in some cases we observe the classical double shock structure of a bow shock/Mach disk expected within the working surface of a jet. The jet material is considerably denser than that of the ambient gas. Ejection angles vary a few degrees with time, giving some jets a ‘wiggling’ appearance. However, motion along the jet appears to be radial; there is no convincing evidence for azimuthal motion in the radial velocity data. Multiple bow shocks observed along the jets probably arise from unsteady accretion of a protostellar disk onto the central object.
Magnetized Accretion Disks and the Origin of Bipolar Outflows
Authors: Königl Arieh 
Published: July 2016

AbstractCentrifugally driven winds from the surfaces of magnetized accretion disks are an attractive mechanism for removing the angular momentum of the accreted matter and of producing bipolar outflows and jets in compact astronomical objects. In this contribution, I first review steady–state disk–wind models that have been constructed for the different density regimes of circumstellar disks and comment on their expected stability. I then consider several nonsteady effects, including disk formation in molecular cloud-core collapse, magnetic flux transport through the disk, and the role of magnetic fields in the FU Orionis outburst phenomenon. I conclude with a discussion of some of the unique observational properties of disk-driven outflows in young stellar objects and in active galactic nuclei. These characteristics are a consequence of the highly stratified density and velocity structures of centrifugally driven outflows, their large momentum discharges (which result in the efficient uplifting of dust from the disk), and, in the case of molecular disks around lowluminosity objects, their comparatively low initial degrees of ionization (which can lead to rapid heating by ambipolar diffusion).
Magnetically-Driven Winds from Protostellar Disks
Authors: Wardle Mark 
Published: July 2016

AbstractAngular momentum transport in protostellar disks can be achieved by the action of a large scale magnetic field that runs vertically through the disk. The magnetic field centrifugally drives material from the disk surfaces into a wind, initiating a bipolar outflow. One apparent difficulty for this model is that the conductivity of the disk is extremely low in the inner 0.1–10 AU of the disk, where grains are the dominant charge carriers. Near the midplane, charged grains are unable to drift through the neutral gas and there is negligible coupling between the magnetic field and the disk material.However, the removal of angular momentum and acceleration of a wind by a magnetic field can still take place in the surface layers of the disk where the gas conductivity increases dramatically. Solutions to the multifluid MHD equations for the vertical structure of a disk at a particular radius are presented. Most of the disk material sits in hydrostatic equilibrium and does not interact with the magnetic field running vertically through it. Near the disk surfaces, the coupling between the magnetic field and disk material is sufficient to initiate an outflow from the disk surface.
The Dark Mass Concentration at the Galactic Center
Authors: Melia F. 
Published: July 2016

AbstractStellar kinematic studies indicate the presence of a concentrated central mass at just under 2 × 106M⊙, in close agreement with the mass deduced from gas velocities measured with the [Ne II] line. Although this mass is most likely a black hole, it may be dominated by a tightly concentrated cluster of stellar remnants. If Sgr A*, a point radio source coincident with this central mass, is a massive black hole embedded in a region with strong gaseous outflows, as suggested by the observation of He I, Brα and Brγ line emission, it is accreting from its environment via the Bondi-Hoyle process. We discuss the consequences of this activity, including the expected mass and angular momentum accretion rate onto the black hole, and the resulting observable characteristics. The latest infrared images of this region appear to rule out the possibility that this large scale flow settles down into a standard α-disk at small radii. We discuss some possible scenarios that might account for this, including strong advection in the disk or the presence of a massive, fossilized disk. Not all of the gas affected in this way by Sgr A*’s strong gravitational field becomes bound. Some of it is redirected into a focused flow that in turn interacts with other coherent gas structures near the black hole. We suggest that the mini-cavity (to the south-west of Sgr A*) may be formed as a result of this activity, and argue that the characteristics of the mini-cavity lend some observational support for the presence of a concentrated mass near Sgr A*. We show, however, that as far as the mini-cavity is concerned, this concentrated mass need not be in the form of a point mass, but may instead be a highly concentrated cluster of stellar remnants.
Periodical Instabilities in the Blackhole Candidate GX339–4 Accretion Disk
Authors: Durouchoux Philippe 
Published: July 2016

AbstractGX339-4, a black hole candidate, has been monitored since more than 30 years by X and gamma-ray spectrometers on board a dozen of satellites. One of the main characteristics of this source is its bimodal behaviour (high and low states). These states, apparently driven by accretion of matter from a companion onto a compact object, appear to be periodic, with a 14.5 month repetitivity, and an approximative 4 month duration. We discuss, in this paper, on periodic instabilities in the accretion disk surrounding GX339–4 to explain this bimodal behaviour.
Nonlinear Outcome of Gravitational Instability in Optically Thick Disks
Authors: Gammie Charles F. 
Published: July 2016

AbstractIn the outer parts of disks in active galactic nuclei and around young stellar objects (YSOs) the internal sources of heat may be too weak to prevent gravitational instability. We are thus motivated to examine the nonlinear outcome of gravitational instability in an optically thick disk. We use height-integrated, two-dimensional numerical simulations that include pressure (the gas has a polytropic equation of state), self-gravity, and cooling. The cooling function is calculated using a one-zone model and is characterized by a single dimensionless parameter τcool, which is the ratio of the cooling time to the orbital period. The simulations are done in the context of a local model of the disk (the “shearing sheet”). We also include the possibility of an effective “viscosity” characterized by a dimensionless parameter α.We find that the gravitational instability in thin, Keplerian disks leads to sustained angular momentum transport that is essentially local in character. Instability drives Q toward 1.7, and provides an angular momentum flux αeff ≃ 0.4/τcool. The disk breaks up when the cooling time is shorter than the dynamical time, which is equivalent to saying that αeff < 1. We conclude that gravitational instability can raise the angular momentum transport rate in the outer parts of AGN and YSO disks (although YSO disks may be too thick for our local approach to be strictly applicable). This can occur over a factor of a few in radius. The gravitationally dominated region is bounded on the inside by that radius where other heating processes (e.g. MHD turbulence driven by the Balbus-Hawley instability) make Q > 1. It is bounded on the outside by that radius where the cooling time is shorter than the dynamical time, or the disk is optically thin so that the equation of state is soft. At larger radius the disk will be clumpy.
Periodic Changes of the Accretion Geometry in the Nearly-Synchronous Polar RX J1940.1–1025
Authors: Geckeler Ralf D. , Staubert Rüdiger 
Published: July 2016

AbstractRX J1940.1-1025 is a newly discovered magnetic Cataclysmic Variable and one of only three known polars with a slightly non-synchronous (< 3 %) rotation of the white dwarf primary. The accretion geometry between the infalling accretion stream and the magnetic field of the WD thus changes periodically with the beat period of 50.0 d between the orbital and the spin period of the WD. The accretion thus occurs along different field lines, depending on the beat phase.For the first time we have observed this effect as a periodic shift in the timings of the absorption troughs in the optical and X-ray pulse profiles of RX J1940.1-1025 with phase of the beat period. There is evidence for a secular decrease of the spin period of RX J1940.1-1025 with a synchronization time scale of the order of 102y.
The Intrinsic UV/Soft X-Ray Spectrum of Quasars
Authors: Haardt Francesco , Madau Piero 
Published: July 2016

AbstractThe detection of a Hell absorption trough in the spectra of three high-redshift quasars provides new constraints on the spectral shape of the UV extragalactic background. The relative strengths of the observed flux decrements at the rest-frame wavelengths of 1216 and 304 Å require a relatively soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic flux is dominated by the integrated light from QSOs, and the Hell reionization of the universe was completed well before z ~ 3, then the UV/soft X-ray spectrum of individual quasars at these epochs must, once the cosmological “filtering” through material along the line of sight is taken into account, satisfy similar constraints on the average. We model the propagation of AGN-like ionizing radiation through the intergalactic medium using CUBA, a numerical code developed in our prior work. We show that, in order to explain the reported Hell absorption, any thermal component responsible for the “soft X-ray excess” observed in the spectra of AGNs at z ≲ 0.3 must, at z ~ 3, have a typical temperature ≳ 80 eV, and a luminosity not exceeding ≈ 20% of that of the “UV bump”.
The Atmospheric Variations of the Peculiar B[e] Star HD 45677 (FS CMa)
Authors: Israelian G. , Friedjung M. , Graham J. , Muratorio G. , Rossi C. , de Winter D. 
Published: July 2016

AbstractWe have studied spectra of the peculiar B[e] star HD 45677. Examination of the Balmer wings enabled us to determine a value of log g = 3.9 indicating a possibly luminosity class V. The weak Heɪ lines togetherwith the Si ɪɪ 4128 and 4130 Å doublet indicate a low rotation velocity in the order of 70 km s−1, which is much lower than the previously claimed value of 200 km s−1 by Swings & Allen (1971). We have examined high-resolution profiles of the strong He I line at 5876 Å and found on one hand that we can explain the variation of the profile by the motions of clouds, some of which accreted. The Balmer lines on the other hand show the presence of an accelerated wind plus absorption by a disk seen edge-on
Southern Hemisphere VLBI Observations of the Expansion of GRO J1655–40
Authors: King E.A. , Jauncey D.L. , Reynolds J.E. , Tzioumis A.K. , McKay D.J. , Kesteven M.J. , Gough R. , Migenes V. , Sinclair M.W. , Tingay S.J. , Preston R.A. , Meier D.L. , Murphy D.W. , Jones D.L. , Lovell J.E.J. , Ellingsen S.P. , McCulloch P.M. , Campbell-Wilson D. , Hunstead R.W. , Quick J. , Smits D. 
Published: July 2016

AbstractWe present high-resolution radio observations of the second Galactic superluminal radio source GRO1655-40, which was detected as an X-ray transient on 1994 July 27. Our radio radio images reveal two components moving away from each other at an angular speed of 65±5 mas/day, corresponding to superluminal motion (υ/c = 1.4 ± 0.4) at the estimated distance of 3–5 kpc. The 12-day delay between the X-ray and radio outbursts suggests that the ejection of material at relativistic speeds occurs during a stable phase of accretion onto a black hole, which follows an unstable phase with a high accretion rate. A complete description and discussion of these observations can be found in Tingay et al 1995 (Nature, 374, pp 141–143).
“Red Blazars”: Evidence Against A Synchrotron Origin
Authors: Masci Frank J. , Webster Rachel L. 
Published: July 2016

AbstractRecently, a substantial population of Blazar-like sources exhibiting sharp near-infrared cutoffs and hence exceedingly reddened optical to near-infrared colours (Bj – K ≿ 6) have been discovered in a complete sample of flat spectrum radio sources (Drinkwater et al. 1996). We argue that such “red” colours cannot be due to a synchrotron radiation mechanism alone.