Edge Of Disk Research Articles

VARIABILITY OF DISK EMISSION IN PRE-MAIN SEQUENCE AND RELATED STARS. III. EXPLORING STRUCTURAL CHANGES IN THE PRE-TRANSITIONAL DISK IN HD 169142

We present near-IR and far-UV observations of the pre-transitional (gapped) disk in HD 169142 using NASA's Infrared Telescope Facility and Hubble Space Telescope. The combination of our data along with existing data sets into the broadband spectral energy distribution reveals variability of up to 45% between ~1.5-10 {\mu}m over a maximum timescale of 10 years. All observations known to us separate into two distinct states corresponding to a high near-IR state in the pre-2000 epoch and a low state in the post-2000 epoch, indicating activity within the <1 AU region of the disk. Through analysis of the Pa {\beta} and Br {\gamma} lines in our data we derive a mass accretion rate in May 2013 of (1.5 - 2.7) x 10^-9 Msun/yr. We present a theoretical modeling analysis of the disk in HD 169142 using Monte-Carlo radiative transfer simulation software to explore the conditions and perhaps signs of planetary formation in our collection of 24 years of observations. We find that shifting the outer edge (r = 0.3 AU) of the inner disk by 0.05 AU toward the star (in simulation of accretion and/or sculpting by forming planets) successfully reproduces the shift in NIR flux. We establish that the ~40-70 AU dark ring imaged in the NIR by Quanz et al. (2013) and Momose et al. (2013) and at 7 mm by Osorio et al. (2014) may be reproduced with a 30% scaled density profile throughout the region, strengthening the link to this structure being dynamically cleared by one or more planetary mass bodies.

PULSE AMPLITUDE DEPENDS ON kHz QPO FREQUENCY IN THE ACCRETING MILLISECOND PULSAR SAX J1808.4-3658

We study the relation between the 300-700 Hz upper kHz quasi-periodic oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi X-ray Timing Explorer. We find that the pulse amplitude systematically changes by a factor of ~2 when the upper kHz QPO frequency passes through 401 Hz: it halves when the QPO moves to above the spin frequency and doubles again on the way back. This establishes for the first time the existence of a direct effect of kHz QPOs on the millisecond pulsations and provides a new clue to the origin of the upper kHz QPO. We discuss several scenarios and conclude that while more complex explanations can not formally be excluded, our result strongly suggests that the QPO is produced by azimuthal motion at the inner edge of the accretion disk, most likely orbital motion. Depending on whether this azimuthal motion is faster or slower than the spin, the plasma then interacts differently with the neutron-star magnetic field. The most straightforward interpretation involves magnetospheric centrifugal inhibition of the accretion flow that sets in when the upper kHz QPO becomes slower than the spin.

CONSTRAINTS FROM ASYMMETRIC HEATING: INVESTIGATING THE EPSILON AURIGAE DISK

Epsilon Aurigae is a long-period eclipsing binary that likely contains an F0Ia star and a circumstellar disk enshrouding a hidden companion, assumed to be a main-sequence B star. High uncertainty in its parallax has kept the evolutionary status of the system in question and, hence, the true nature of each component. This unknown, as well as the absence of solid state spectral features in the infrared, requires an investigation of a wide parameter space by means of both analytic and Monte Carlo radiative transfer (MCRT) methods. The first MCRT models of epsilon Aurigae that include all three system components are presented here. We seek additional system parameter constraints by melding analytic approximations with MCRT outputs (e.g., dust temperatures) on a first-order level. The MCRT models investigate the effects of various parameters on the disk-edge temperatures; these include two distances, three particle size distributions, three compositions, and two disk masses, resulting in 36 independent models. Specifically, the MCRT temperatures permit analytic calculations of effective heating and cooling curves along the disk edge. These are used to calculate representative observed fluxes and corresponding temperatures. This novel application of thermal properties provides the basis for utilization of other binary systems containing disks. We find degeneracies in the model fits for the various parameter sets. However, the results show a preference for a carbon disk with particle size distributions ≥10 μm. Additionally, a linear correlation between the MCRT noon and basal temperatures serves as a tool for effectively eliminating portions of the parameter space.

ALIGNMENT OF PROTOSTARS AND CIRCUMSTELLAR DISKS DURING THE EMBEDDED PHASE

Star formation proceeds via the collapse of a molecular cloud core over multiple dynamical timescales. Turbulence within cores results in a spatially non-uniform angular momentum of the cloud, causing a stochastic variation in orientation of the disk forming from the collapsing material. In the absence of star-disk angular momentum coupling, such disk-tilting would provide a natural mechanism for production of primordial spin-orbit misalignments in the resulting planetary systems. However, owing to high accretion rates in the embedded phase of star formation, the inner edge of the circumstellar disk extends down to the stellar surface, resulting in efficient gravitational and accretional angular momentum transfer between the star and the disk. Here, we demonstrate that the resulting gravitational coupling is sufficient to suppress any significant star-disk misalignment, with accretion playing a secondary role. The joint tilting of the star-disk system leads to a stochastic wandering of star-aligned bipolar outflows. Such wandering widens the effective opening angle of stellar outflows, allowing for more efficient clearing of the remainder of the protostar's gaseous envelope. Accordingly, the processes described in this work provide an additional mechanism responsible for sculpting the stellar Initial Mass Function (IMF).

Potential use of nuclear track membranes in ophthalmology

Potential use of a nuclear track-etched membrane as a semipermeable membrane for fluid transport and maintenance of the cornea in a slightly dehydrated state for its transparency has been studied. The track membrane TOMTREK based on polyethylene terephthalate (PET) with a pore diameter of 0.4 μm and a density of 5 × 106 pore/cm2 has been used in the experiments. The pores have been formed by bombarding the polymer PET with 41.5-MeV 40Ar8+ ions. After bombardment, the membrane was subjected to chemical treatment in an alkali solution. Contact angle θ on the track membrane surface has been measured using the sessile drop method. Disks of a 10 mm diameter have been cut from plasma-treated membrane samples. An appropriate shape has been imparted to the model by incising and sealing the disk edges.

Characteristic signatures in the thermal emission from accreting binary black holes

Abstract We present the results of a calculation of the thermal spectrum from a 2D, moving mesh, high-accuracy, viscous hydrodynamical simulation of an accreting supermassive black hole (SMBHs) binary. We include viscous heating, shock heating, and radiative cooling, evolving for longer than a viscous time so that we reach a quasi-steady accretion state. In agreement with previous work, we find that gas is efficiently stripped from the inner edge of the circumbinary disc and enters the cavity along accretion streams, which feed persistent ‘minidiscs’ surrounding each black hole. We also find that emission from the shock-heated minidiscs and accretion streams prevents any deficit in high-energy emission that may be expected inside the circumbinary cavity, and instead leads to a characteristic brightening of the spectrum beginning in soft X-rays.

Correlation between the radial peripapillary capillaries and the retinal nerve fibre layer in the normal human retina

This study aims to provide evidence of the importance of radial peripapillary capillaries (RPCs) by quantitative study of the relationship between the RPCs and retinal nerve fibre layer (RNFL) in normal human donor eyes. The retinal microvasculature in eleven normal human donor eyes was perfused, fixed and labelled after cannulation of the central retinal artery. The retinas were dissected and whole-mounted for confocal microscopy. Six study regions were taken radially from the edge of the optic disc. RPCs from the optic disc edge to a radial distance up to 2.5 mm were imaged and their diameters, inter-capillary distance and volume occupation measured. These were correlated with the study region as well as thickness of the RNFL. It was found that the pooled average diameter of the RPCs in the first 2.5 mm from the optic disk was 8.9 μm. Significant differences in capillary diameter were present in the six regions, with larger diameter RPCs in the superior, inferior and nasal regions, and significantly smaller diameter in the temporal region. RPCs in the arcuate fibre regions extend the furthest from the optic disc, maintained a close inter-capillary distance for a longer distance than other regions, and have the highest RPCs volume occupancy. The RPCs volume was generally correlated with RNFL thickness. In conclusion, a close correlation between RNFL and RPCs presence has been demonstrated which is supportive of their functional reliance/co-dependence. The significantly smaller temporal RPCs may be a result of the greater presence of RPCs in the two bordering arcuate fibre regions and therefore a richer availability of nutrients diffusing from these two regions.

Symmetry breaking of magnetic vortices before annihilation

Magnetic vortices in nanoscopic ferromagnetic elements can be treated as solitons that exhibit static and dynamic stability even above room temperature, making them promising for technological applications. Although their properties at low energy configurations have been well studied, the properties of solitons undergoing structural breakdown are poorly understood. Here, we destabilize the soliton through Zeeman and demagnetization interactions and investigate the symmetry breaking of the magnetic vortex state. Using high-resolution Lorentz imaging and numerical simulations, we observe phase transitions of the core from the characteristic point-like structure into distinct extended asymmetric states differentiated by the polarity. The deformations start at intermediate displacements and are traced until the point of annihilation at the disc's edge. We discuss the use of these asymmetric states as a method to determine the core polarity for in-plane magnetic imaging techniques.

ACCRETION DISK RADIATION DYNAMICS AND THE COSMIC BATTERY

We investigate the dynamics of radiation in an astrophysical accretion disk around a Kerr black hole. The source of the radiation is the accretion disk itself, and not a central object as in previous studies of the Poynting-Robertson effect. We generate numerical sky maps from photon trajectories that originate on the surface of the disk as seen from the inner edge of the disk at the position of the innermost stable circular orbit (ISCO). We investigate several accretion disk morphologies with a Shakura-Sunyaev surface temperature distribution. Finally, we calculate the electromotive source of the Cosmic Battery mechanism around the inner edge of the accretion disk and obtain characteristic timescales for the generation of astrophysical magnetic fields.

Detailed surface morphology of the ‘lobster louse’ copepod, Nicothoë astaci, a haematophagous gill parasite of the European lobster, Homarus gammarus

The ectoparasitic copepod, Nicothoë astaci (the ‘lobster louse’), infests the gills of the European lobster, Homarus gammarus. There have been limited studies on this haematophagous species; therefore knowledge of this parasite is rudimentary. The current study examines the surface morphology of this parasitic copepod, detached from the host, concentrating on adaptations of the suctorial mouthpart, the oral disc. Cryo-scanning electron microscopy revealed structural adaptations that facilitate attachment of these parasites to the gill filaments of their lobster host. The aperture of the feeding channel, through which host haemolymph is drawn, is only ca. 5μm in diameter. The edge of the oral disc is lined with numerous setae, whilst the surface of the disc is covered with large numbers of small (<1μm in diameter) teeth-like structures, which presumably pierce through, and grip, the cuticle lining of the host’s gill. Overall, these structures are thought to provide a ‘vacuum seal’ to assist in pumping of blood, via peristalsis, into the alimentary canal of the copepod host.

Investigating the mass of the intermediate mass black hole candidate HLX-1 with the slimbh model

In this paper we present a comprehensive study of the mass of the intermediate mass black hole candidate HLX-1 in the galaxy ESO 243-49. We analyse the continuum X-ray spectra collected by Swift, XMM-Newton, and Chandra with the slim disc model, SLIMBH, and estimate the black hole mass for the full range of inclination (inc = 0{\deg} - 85{\deg}) and spin (a* = 0 - 0.998). The relativistic SLIMBH model is particularly suited to study high luminosity disc spectra as it incorporates the effects of advection, such as the shift of the inner disc edge towards smaller radii and the increasing height of the disc photosphere (including relativistic ray-tracing from its proper location rather than the mid-plane of the disc). We find for increasing values of inclination that a zero spin black hole has a mass range of 6,300 - 50,900 M_sun and a maximally spinning black hole has a mass between 16,900 - 191,700 M_sun. This is consistent with previous estimates and reinforces the idea that HLX-1 contains an intermediate mass black hole.

Multilayered optic disc hemorrhages in adolescents.

To describe the clinical characteristics of multilayered optic disc hemorrhages, which are defined as subretinal, superficial retinal, and subhyaloid or vitreous hemorrhages in adolescents. Case records of adolescents with acute multilayered optic disc hemorrhages were identified and evaluated retrospectively from 1994 to 2012. The appearance of the hemorrhages including the size, location, extension, and disc anatomy was recorded. Fluorescein angiography, visual field examination, and spectral-domain optical coherence tomography were performed in select cases. Nine boys and 7 girls were included, with a mean age of 15.0 ± 2.6 years. No precipitating factor, such as involuntary Valsalva maneuver, was identified in the majority of patients (93.75%). All eyes were myopic with an average refraction of -4.64 ± 1.88 diopters. Fifteen (93.75%) of the affected optic discs were crowded and tilted with small cups. Peripapillary subretinal hemorrhages were all crescent in shape and located at the nasal disc. Eight (50%) eyes had marked subretinal blood extension exceeding 1 disc diameter away from the disc edge. Superficial flame hemorrhages were predominantly located in the superotemporal part of the disc. Spectral-domain optical coherence tomography of the disc showed vitreopapillary traction and obvious subretinal hemorrhage with increased thickness. All multilayered optic disc hemorrhages resolved spontaneously. Multilayered optic disc hemorrhages in adolescents more commonly affect myopic eyes with crowded and tilted discs. The visual outcome is excellent.

Modulation transfer function measurement of CT images by use of a circular edge method with a logistic curve-fitting technique.

We propose a method for measuring the modulation transfer function (MTF) of a computed tomography (CT) system by use of a circular edge method with a logistic curve-fitting technique. An American College of Radiology (ACR) phantom was scanned by a Philips Brilliance system, and axial images were reconstructed by the filtered back projection algorithm with a standard reconstruction filter. The radial MTF was measured from a disk image of a rod or cylinder in the ACR phantom by use of the circular edge method. In this study, we applied a logistic curve-fitting technique to an edge-spread function (ESF) to eliminate noise because the edge method is very susceptible to noise in the ESF in a CT image. The circular edge method with the logistic curve-fitting technique provided the MTF without fluctuations due to noise for the entire spatial frequency range. The MTF was not affected by the tube current, the slice thickness, or the disk contrast, which were factors related to the amount of noise in the CT image. However, the MTF was affected by the location of the disk and by the disk size, depending on the average distance from the isocenter to the disk edge. Our results indicated that the MTF measured by the circular edge method with the logistic curve-fitting technique was not susceptible to noise in CT images. Therefore, this method is useful for MTF measurement for not only high-contrast objects, but also low-contrast objects with a large amount of noise.

ANOMALOUS ACCRETION ACTIVITY AND THE SPOTTED NATURE OF THE DQ TAU BINARY SYSTEM

We report the detection of an anomalous accretion flare in the tight eccentric pre-main-sequence binary system DQ Tau. In a multi-epoch survey consisting of randomly acquired low to moderate resolution near-infrared spectra obtained over a period of almost ten years, we detect a significant and simultaneous brightening of four standard accretion indicators (CaII infrared triplet, the Paschen and Brackett series HI lines, and HeI 1.083 um), on back-to-back nights (phase = 0.372, 0.433) with the flare increasing in strength as the system approached apastron (phase = 0.5). The mass accretion rate measured for the anomalous flare is nearly an order of magnitude stronger than the average quiescent rate. While previous observations established that frequent, periodic accretion flares phased with periastron passages occur in this system, these data provide evidence that orbitally-modulated accretion flares occur near apastron, when the stars make their closest approach to the circumbinary disk. The timing of the flare suggests that this outburst is due to interactions of the stellar cores (or the highly truncated circumstellar disks) with material in non-axisymmetric structures located at the inner edge of the circumbinary disk. We also explore the optical/infrared spectral type mismatch previously observed for T Tauri stars and successfully model the shape of the spectra from 0.8 to 1.0 um and the strengths of the TiO and FeH bands as manifestations of large cool spots on the surfaces of the stellar companions in DQ Tau. These findings illustrate that a complete model of near-infrared spectra of many T Tauri stars must include parameters for spot filling factors and temperatures.

Retinal vessel diameter changes induced by transient high perfusion pressure.

To investigate the effects of transient high perfusion pressure on the retinal vessel diameter and retinal ganglion cells. The animals were divided into four groups according to different infusion pressure and infusion time (60 mm Hg-3min, 60 mm Hg-5min, 100 mm Hg-3min, 100 mm Hg-5min). Each group consisted of six rabbits. The left eye was used as the experimental eye and the right as a control. Retinal vascular diameters were evaluated before, during infusion, immediately after infusion, 5min, 10min and 30min after infusion based on the fundus photographs. Blood pressure was monitored during infusion. The eyes were removed after 24h. Damage to retinal ganglion cell (RGC) was analyzed by histology. Retina became whiten and papilla optic was pale during perfusion. Measurements showed significant decrease in retinal artery and vein diameter during perfusion in all of the four groups at the proximal of the edge of the optic disc. The changes were significant in the 100 mm Hg-3min group and 100 mm Hg-5min group compared with 60 mm Hg-3min group (P1=0.025, P2=0.000). The diameters in all the groups recovered completely after 30min of reperfusion. The number of RGC showed no significant changes at the IOP in 100 mm Hg with 5min compared with contralateral untreated eye (P>0.05). Transient fluctuations during infusion lead to temporal changes of retinal vessels, which could affect the retinal blood circulation. The RGCs were not affected by this transient fluctuation. Further studies are necessary to evaluate the effect of pressure during real-time phacoemusification on retinal blood circulation.

Microstructures and mechanical properties of pure tantalum processed by high-pressure torsion

A body-centred cubic (BCC) structure metal, tantalum, was processed by high- pressure torsion (HPT) at room temperature with different numbers of rotations. The microstructural evolution was studied by electron backscatter diffraction (EBSD). The grain sizes were significantly refined at the disk edge area in the early stages of deformation (N = 0.5) but tended to attain saturation after the numbers of rotations was increased to N = 5. As the deformation continued, some coarse grains appeared in the disk edge areas and it appeared that there was structural recovery at the expense of grain boundary migration in the tantalum during HPT processing. Microhardness measurements showed the hardness gradually evolved towards a more homogenized level across the disk surfaces as the numbers of rotations increased. The hardness level after N = 10 turns was slightly lower than after N = 5 turns, thereby indicating the occurrence of a recovery process after 5 turns.

Dynamical evolution of an eccentric planet and a less massive debris disc

We investigate the interaction between an eccentric planet and a less massive external debris disc. This scenario could occur after planet-planet scattering or merging events. We characterise the evolution over a wide range of initial conditions, using a suite of n-body integrations combined with theory. Planets near the disc mid-plane remove the inner debris region, and surviving particles form an eccentric disc apsidally aligned with the planet. The inner disc edge is elliptical and lies just beyond the planet's orbit. Moderately inclined planets ($i_{\rm plt} \gtrsim 20^\circ$ for $e_{\rm plt} = 0.8$) may instead sculpt debris into a bell-shaped structure enveloping the planet's orbit. Finally some highly inclined planets ($i_{\rm plt} \sim 90^\circ$) can maintain a disc orthogonal to the planet's plane. In all cases disc particles undergo rapid evolution, whilst the overall structures evolve more slowly. The shapes of these structures and their density profiles are characterised. The width of the chaotic zone around the planet's orbit is derived in the coplanar case using eccentric Hill radius arguments. This zone is cleared within approximately ten secular or diffusion times (whichever is longer), and debris assumes its final shape within a few secular times. We quantify the planet's migration and show it will almost always be small in this mass regime. Our results may be used to characterise unseen eccentric planets using observed debris features.

Improved automated optic cup segmentation based on detection of blood vessel bends in retinal fundus images.

Glaucoma is a leading cause of permanent blindness. Retinal imaging is useful for early detection of glaucoma. In order to evaluate the presence of glaucoma, ophthalmologists may determine the cup and disc areas and diagnose glaucoma using a vertical optic cup-to-disc (C/D) ratio and a rim-to-disc (R/D) ratio. Previously we proposed a method to determine cup edge by analyzing a vertical profile of pixel values, but this method provided a cup edge smaller than that of an ophthalmologist. This paper describes an improved method using the locations of the blood vessel bends. The blood vessels were detected by a concentration feature determined from the density gradient. The blood vessel bends were detected by tracking the blood vessels from the disc edge to the primary cup edge, which was determined by our previous method. Lastly, the vertical C/D ratio and the R/D ratio were calculated. Using forty-four images, including 32 glaucoma images, the AUCs of both the vertical C/D ratio and R/D ratio by this proposed method were 0.966 and 0.936, respectively.

Spectral-domain optical coherence tomography findings in pediatric tilted disc syndrome.

To report a novel spectral-domain optical coherence tomography (SD-OCT) finding in children affected by tilted disc syndrome (TDS), and to correlate it with early visual field defects. Patients between 5 and 17 years old with TDS were enrolled in this study. The diagnosis of TDS was made by stereoscopic fundus photography, when the upper edge of the optic disc protruded anteriorly relative to its lower edge. All eyes were examined with 12 radial SD-OCT B-scans of 12 mm centered on the optic disc; the fundus area encompassing the optic nerve was additionally scanned using several vertical and horizontal scans.. C-scan SD-OCT were acquired using the Macular Cube 512 x 128 to create the en face image. Standard automated perimetry 24-2 tests were performed on all patients. Thirty-eight eyes of 20 pediatric patients with TDS syndrome were enrolled during this 24-months clinical trial. Their mean age was 10.9 ± 2.7 years (range 7-15 years), 12 (60%) were male and eight (40%) were female. The OCT images of the optic discs showed a protrusion of the upper edge of Bruch's membrane and choroid at the nasal edge of the optic disc in 39.5% of the eyes. The retinal nerve fiber tissue appeared to be herniated into this protrusion and bent superiorly in 15 eyes. This severe bending corresponded to early visual field anomalies that were not reduced by corrective lenses in 46.7% of the eyes. Visual field defects that do not improve by increased myopic correction in TDS may be due to the severe bending of the retinal nerve fiber tissue, which would impair axonal flow.

Periodic radio variabilities of the blazar 1156+295: harmonic oscillations

(Quasi)-periodic oscillations of the radio luminosity of a typical blazar 1156+295 were analysed to investigate the dynamic processes associated with the central engine of the active galactic nucleus. Three numerical techniques, the Lomb-Scargle periodogram, the Weighted Wavelet Z-transform and the improved Phase Dispersion Minimization, were used to search for candidate periodicities in the radio light curves of 1156+295 at 14.5, 8.0 and 4.8 GHz. The results of these methods are consistent with the detection of four periodic components with characteristic periods of similar to 1.7 (P-4), similar to 2.4 (P-3), similar to 3.4 (P-2) and similar to 7.5 yr (P-1), which suggest a harmonic relationship in frequency of 4: 3: 2: 1 with f(1) as the fundamental frequency. Except for the fact that P-1 persistently shows in the whole time span, the three others appear intermittent and are only prominent during certain time ranges. The second harmonic mode (P-2) is most powerful at all three observing frequency bands. The multiplicity and harmonic relation of the periodicities in 1156+295 reconfirms that global p-mode oscillations of the accretion disc are coupled to the jet. The oscillation of the disc is likely driven by Kelvin-Helmholtz instabilities in the inner edge of the accretion disc.