Edge Of Disk Research Articles

Lasing: Room Temperature Excitonic Whispering Gallery Mode Lasing from High‐Quality Hexagonal ZnO Microdisks (Adv. Mater. 19/2011)

Ultraviolet excitonic whispering gallery mode (WGM) lasing from individual hexagonal-shaped ZnO microdisks is reported on p. 2199 by Han Dong Sun and co-workers. The inside cover illustrates the five growth processes for the ZnO microdisks and WGM lasing emitted from the edges of such disks. The work provides detailed WGM lasing characteristics for the microdisks, which may lead to promising applications for low-threshold UV lasers and high-sensitivity sensors.

In Vitro Cytotoxic Evaluation of Processed Natural Coral in Human Osteoblasts

This study aimed to evaluate the in vitro cytotoxic effects of locally produced processed natural coral (PNC) using human osteoblasts (HOS). Cytotoxicity was not observed when HOS cells were cultured with PNC, as assessed by (3-(4,5-dimethylthiazol-2-yl)-2-5-diphenyl tetrazolium bromide; MTT) and Neutral Red (NR) assays at concentration up 200 mg/mL for up to 72 hours. Flow cytometry (FCM) analysis showed that PNC (200 mg/mL) did not decrease viability of HOS cells after 48 and 72 hours of treatment. In a cell attachment study, the HOS cells attached to the edge of the PNC disc, and later grew into the pores of the PNC disc. All results from these studies indicate that locally produced PNC material is noncytotoxic and favors the growth of HOS cells.

A dual-frequency sub-arcsecond study of proto-planetary disks at mm wavelengths: first evidence for radial variations of the dust properties

(Abridged) We attempt to characterize the radial distribution of dust in disks around a sample of young stars from an observational point of view, and, when possible, in a model-independent way, by using parametric laws. We used the IRAM PdBI interferometer to provide very high angular resolution (down to 0.4" in some sources) observations of the continuum at 1.3 mm and 3 mm around a sample of T Tauri stars in the Taurus-Auriga region. The sample includes single and multiple systems, with a total of 23 individual disks. We used track-sharing observing mode to minimize the biases. We fitted these data with two kinds of models: a "truncated power law" model and a model presenting an exponential decay at the disk edge ("viscous" model). ect evidence for tidal truncation is found in the multiple systems. The temperature of the mm-emitting dust is constrained in a few systems. Unambiguous evidence for large grains is obtained by resolving out disks with very low values of the dust emissivity index Beta. In most disks that are sufficiently resolved at two different wavelengths, we find a radial dependence of Beta, which appears to increase from low values (as low as 0) at the center to about 1.7 -- 2 at the disk edge. The same behavior could apply to all studied disks. It introduces further ambiguities in interpreting the brightness profile, because the regions with apparent Beta = 0 can also be interpreted as being optically thick when their brightness temperature is high enough. Despite the added uncertainty on the dust absorption coefficient, the characteristic size of the disk appears to increase with a higher estimated star age. These results provide the first direct evidence of the radial dependence of the grain size in proto-planetary disks.

The influence of binarity on dust obscuration events in the planetary nebula M 2-29 and its analogues

The central star of the planetary nebula (CSPN) M 2-29 shows an extraordinary R Coronae Borealis-like fading event in its optical lightcurve. The only other CSPN to show these events are CPD-568032 (Hen 3-1333) and V651 Mon (NGC 2346). Dust cloud formation in the line of sight appears responsible but the exact triggering mechanism is not well understood. Understanding how planetary nebulae (PNe) trigger dust obscuration events may help understand the same process in a wide range of objects including Population-I WC9 stars, symbiotic stars and perhaps Asymptotic Giant Branch (AGB) stars with long secondary periods (LSPs). A binary scenario involving an eccentric, wide companion that triggers dust formation via interaction at periastron is a potential explanation that has been suggested for LSP variables. Model fits to the lightcurves of CPD-568032 and M 2-29 show the dust forms in excess of 70 AU at the inner edge of a dust disk. In the case of CPD-568032 this radius is far too large to coincide with a binary companion trigger, although a binary may have been responsible for the formation of the dust disk. We find no direct evidence to support previous claims of binarity in M 2-29 either from the OGLE lightcurve or deep medium-resolution VLT FLAMES spectroscopy of the CSPN. We classify the CSPN as Of(H) with T_eff=50+-10 kK and log g=4.0+-0.3. We find a mean distance of 7.4+-1.8 kpc to M 2-29 at which the M_V=-0.9 mag CSPN could potentially hide a subgiant luminosity or fainter companion. A companion would help explain the multiple similarities with D'-type symbiotic stars whose outer nebulae are thought to be bona-fide PNe. The 7.4 kpc distance, oxygen abundance of 8.3 dex and Galactic coordinates (l=4.0, b=-3.0) prove that M 2-29 is a Galactic Bulge PN and not a Halo PN as commonly misconceived.

Photoelastic and Finite Element Stress Analysis of the Gap between the L4 and L5 Vertebrae

The purpose of this study was to analyze the stresses on the intervertebral disc between vertebrae L4 and L5 when a compressive load is applied on vertebra L4 using the photoelasticity transmission technique and the finite element method. Nine photoelastic models were used and were divided into three groups. Each group was formed by three models, according to the localization of the sagittal cut on vertebrae L4-L5. Simulation was carried out using a load of 23 N. The fringe orders were assessed by points close to the edge of the intervertebral disc using the Tardy compensation method. The analyses using the photoelasticity technique and the model of the finite elements showed that the stress generated by the vertebrae on the intervertebral disc was higher in the posterolateral region. Thus, this region is more susceptible to pathologies such as hernia and disc degeneration.

The mass-period distribution of close-in exoplanets

The lower limit to the distribution of orbital periods P for the current population of close-in exoplanets shows a distinctive discontinuity located at approximately one Jovian mass. Most smaller planets have orbital periods longer than P~2.5 days, while higher masses are found down to P~1 day. We analyze whether this observed mass-period distribution could be explained in terms of the combined effects of stellar tides and the interactions of planets with an inner cavity in the gaseous disk. We performed a series of hydrodynamical simulations of the evolution of single-planet systems in a gaseous disk with an inner cavity mimicking the inner boundary of the disk. The subsequent tidal evolution is analyzed assuming that orbital eccentricities are small and stellar tides are dominant. We find that most of the close-in exoplanet population is consistent with an inner edge of the protoplanetary disk being located at approximately P>2 days for solar-type stars, in addition to orbital decay having been caused by stellar tides with a specific tidal parameter on the order of Q'*=10^7. The data is broadly consistent with planets more massive than one Jupiter mass undergoing type II migration, crossing the gap, and finally halting at the interior 2/1 mean-motion resonance with the disk edge. Smaller planets do not open a gap in the disk and remain trapped in the cavity edge. CoRoT-7b appears detached from the remaining exoplanet population, apparently requiring additional evolutionary effects to explain its current mass and semimajor axis.

Tidal truncation of circumplanetary discs

We analyse some properties of circumplanetary discs. Flow through such discs may provide most of the mass to gas giant planets, and such discs are likely sites for the formation of regular satellites. We model these discs as accretion discs subject to the tidal forces of the central star. The tidal torques from the star remove the disc angular momentum near the disc outer edge and permit the accreting disc gas to lose angular momentum at the rate appropriate for steady accretion. Circumplanetary discs are truncated near the radius where periodic ballistic orbits cross, where tidal forces on the disc are strong. This radius occurs at approximately 0.4 r_H for the planet Hill radius r_H. During the T Tauri stage of disc accretion, the disc is fairly thick with aspect ratio H/r > 0.2 and the disc edge tapering occurs over a radial scale ~ H ~ 0.1 r_H. For a circular or slightly eccentric orbit planet, no significant resonances lie within the main body of the disc. Tidally driven waves involving resonances nonetheless play an important role in truncating the disc, especially when it is fairly thick. We model the disc structure using one dimensional time-dependent and steady-state models and also two dimensional SPH simulations. The circumplanetary disc structure depends on the variation of the disc turbulent viscosity with radius and is insensitive to the angular distribution of the accreting gas. Dead zones may occur within the circumplanetary disc and result in density structures. If the disc is turbulent throughout, the predicted disc structure near the location of the regular Jovian and Saturnian satellites is smooth with no obvious feature that would favor formation at their current locations.

IMPLICATIONS OF X-RAY LINE VARIATIONS FOR 4U1822–371

4U 1822-371 is one of the proto-type accretion disk coronal sources with an orbital period of about 5.6 hours. The binary is viewed almost edge-on at a high inclination angle of 83 degrees, which makes it a unique candidate to study binary orbital and accretion disk dynamics in high powered X-ray sources. We observed the X-ray source in 4U 1822-371 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for almost nine binary orbits. X-ray eclipse times provide an update of the orbital ephemeris. We find that our result follows the quadratic function implied by previous observations; however, it suggests a flatter trend. Detailed line dynamics also confirm a previous suggestion that the observed photo-ionized line emission originates from a confined region in the outer edge of the accretion disk near the hot spot. Line properties allow us to impose limits on the size of accretion disk, the central corona, and the emission region. The photo-ionized plasma is consistent with ionization parameters of log(xi) > 2, and when combined with disk size and reasonable assumptions for the plasma density, this suggests illuminating disk luminosities which are over an order of magnitude higher than what is actually observed. That is, we do not directly observe the central emitting X-ray source. The spectral continua are best fit by a flat power law with a high energy cut-off and partial covering absorption (N_H ranging from 5.4-6.3x10^{22} cm^{-2}) with a covering fraction of about 50%. We discuss some implications of our findings with respect to the photo-ionized line emission for the basic properties of the X-ray source.

A DETECTION OF AN X-RAY WIND AND AN IONIZED DISK IN THECHANDRAHETGS OBSERVATION OF THE SEYFERT 2 GALAXY IRAS 18325-5926

We analyze the \textit{Chandra} High Energy Transmission Grating Spectrometer (HETGS) observation of the Seyfert $2$ AGN IRAS\,18325-5926. We detect a $v=-360^{+41}_{-66}$~km~s$^{-1}$ blueshifted ionized absorber in the X-ray spectrum, with photoionization parameter $\log\xi=2.0^{+0.1}_{-0.1}$ and hydrogen column density $N_{\rm H}=1.55^{+0.75}_{-0.38}\times10^{21}$~cm$^{-2}$. The absorber may be a photoionized wind originating in the obscuring torus/global covering around the black hole or outer edge of the accretion disk. The estimated mass outflow rate suggests that the supermassive black hole in IRAS\,18325-5926 may significantly affect the large-scale environment of the host galaxy, unless the solid angle subtended by the outflow or the gas filling factor is small. A second warm absorber may be needed to explain the absorption features in the vicinity of the iron K edge, although insufficient counts in the data beyond $7.0$~keV make it difficult to assess the nature of the second absorber. Most plausible is a high ionization ($\log\xi\sim 2.3$ to $2.6$), high column density ($N_{\rm H}\sim10^{23}$~cm$^{-2}$) absorber with $v\sim -3000$ to $0$~km~s$^{-1}$, although these parameters are not well constrained. We also examine the broad Fe K emission line in the spectrum, which is likely due to Fe XXV in a highly ionized accretion disk inclined at $25^\circ$, consistent with the \textit{XMM-Newton} EPIC observations of the emission line. Because we are able to view both the obscuring gas and the accretion disk of IRAS\,18325-5926, the surrounding gas of IRAS\,18325-5926 may be patchy or we are viewing the system at an angle just grazing the obscuring torus.

Gaseous flows in the inner part of the circumbinary disk of the T Tauri star

We present results of 3D numerical simulations of the matter flow in the disk of a binary T Tauri star. It is shown that two bow-shocks caused by the supersonic motion of the binary components in the gas of the disk are formed in the system having parameters typical for T Tauri stars. These bow-shocks significantly change the flow pattern. In particular, for systems with circular orbits they determine the size and shape of the inner gap. We also show that the redistribution of the angular momentum due to the bow-shocks leads to occurrence of two matter flows propagating from the inner edge of the circumbinary disk to the components. Further redistribution of this matter between the components is considered.

YOUNG STARS AT THE EDGE: STELLAR CLUSTERING IN THE OUTER REGIONS OF THE M33 DISK

We investigate the distribution of bright main sequence stars near the northern edge of the M33 disk. Clustering on sub-kpc scales is seen among stars with ages near 10 Myr, and two large star-forming complexes are identified. Similar large-scale grouping is not evident among stars with ages 100 Myr. These stars are also distributed over a much larger area than those with younger ages, and it is argued that random stellar motions alone, as opposed to orderly motions of the type spurred by large scale secular effects, can re-distribute stars out to distances of at least 2 kpc (i.e. one disk scale length) from their birth places on 100 Myr timescales. Such random motions may thus play a significant role in populating the outer regions of the M33 disk. Finally, it is suggested that - to the extent that the ambient properties of the outer disk mirror those in the main body of the disk - stars in this part of M33 may have formed in star clusters with masses 50 - 250 solar, which is substantially lower than the peak of the solar neighborhood initial cluster mass function.

Dust ejection from planetary bodies by temperature gradients: Laboratory experiments

Laboratory experiments show that dusty bodies in a gaseous environment eject dust particles if they are illuminated. We find that even more intense dust eruptions occur when the light source is turned off. We attribute this to a compression of gas by thermal creep in response to the changing temperature gradients in the top dust layers. The effect is studied at a light flux of 13 kW/m 2 and 1 mbar ambient pressure. The effect is applicable to protoplanetary disks and Mars. In the inner part of protoplanetary disks, planetesimals can be eroded especially at the terminator of a rotating body. This leads to the production of dust which can then be transported towards the disk edge or the outer disk regions. The generated dust might constitute a significant fraction of the warm dust observed in extrasolar protoplanetary disks. We estimate erosion rates of about 1 kg s −1 for 100 m parent bodies. The dust might also contribute to subsequent planetary growth in different locations or on existing protoplanets which are large enough not to be susceptible to particle loss by light induced ejection. Due to the ejections, planetesimals and smaller bodies will be accelerated or decelerated and drift outward or inward, respectively. The effect might also explain the entrainment of dust in dust devils on Mars, especially at high altitudes where gas drag alone might not be sufficient.

FORMING HABITABLE PLANETS AROUND DWARF STARS: APPLICATION TO OGLE-06-109L

Dwarf stars are believed to have small protostar disk where planets may grow up. During the planet formation stage, embryos undergoing type I migration are expected to be stalled at inner edge of magnetic inactive disk ($a_{\rm crit} \sim 0.2-0.3 ~$AU). This mechanism makes the location around $a_{\rm crit}$ a sweet spot of forming planets. Especially, $a_{\rm crit}$ of dwarf stars with masses $\sim 0.5 M_\odot$ is roughly inside the habitable zone of the system. In this paper we study the formation of habitable planets due to this mechanism with a model system OGLE-06-109L. It has a $0.51 M_\odot$ dwarf star with two giant planets in 2.3 and 4.6 AU observed by microlensing. We model the embryos undergoing type I migration in the gas disk with a constant disk accretion rate ($\dot M$). Giant planets in outside orbits affect the formation of habitable planets through secular perturbations at the early stage and secular resonance at the later stage. We find that the existence and the masses of the habitable planets in OGLE-06-10L system depend on both $\dot M$ and the speed of type I migration. If planets formed earlier so that $\dot M$ is larger ($\sim 10^{-7} M_\odot $ yr$^{-1}$), terrestrial planet can not be survived unless the type I migration rate is an order of magnitude less. If planets formed later so that $\dot M$ is smaller ($\sim 10^{-8} M_\odot $ yr$^{-1}$), single and high mass terrestrial planets with high water contents ($\sim 5% $) will be formed by inward migration of outer planet cores. A slower speed migration will result in several planets by collisions of embryos, thus their water contents are low ($\sim 2%$). Mean motion resonances or apsidal resonances among planets may be observed if multiple planets survived in the inner system.

Two solutions for the BVP of a rotating variable-thickness solid disk

This paper presents the analytical and numerical solutions for a rotating variable-thickness solid disk. The outer edge of the solid disk is considered to have free boundary conditions. The governing equation is derived from the basic equations of the rotating solid disk and it is solved analytically or numerically using finite difference algorithm. Both analytical and numerical results for the distributions of stress function and stresses of variable-thickness solid disks are obtained. Finally, the distributions of stress function and stresses are presented and the appropriate comparisons and discussions are made at the same angular velocity.

RADIATIVELY INEFFICIENT ACCRETION FLOWS INDUCED BY GRAVITATIONAL-WAVE EMISSION BEFORE MASSIVE BLACK HOLE COALESCENCE

We study an accretion flow during the gravitational-wave driven evolution of binary massive black holes. After the binary orbit decays due to an interaction with a massive circumbinary disk, the binary is decoupled from the circumbinary disk because the orbital-decay timescale due to emission of gravitational wave becomes shorter than the viscous timescale evaluated at the inner edge of circumbinary disk. During the subsequent evolution, the accretion disk, which is truncated at the tidal radius because of the tidal torque, also shrinks as the orbital decay. Assuming that the disk mass changed by this process is all accreted, the disk becomes radiatively inefficient when the semi-major axis is several hundred Schwarzschild radii. The high-energy radiations, in spite of a low bolometric luminosity, are emitted from an accretion disk around each black hole long before the black hole coalescence as well as the gravitational wave signals. The synchrotron process can notably produce potentially observable radio emissions at large distances if there is a strong, dipole magnetic field around each black hole. In unequal mass-ratio binaries, step-like light variations are seen in the observed light curve because the luminosity is higher and its duration time are shorter in the radio emission by the disk around the secondary black hole than those of the primary black hole. Such a precursor would be unique to not a single black hole system but a binary black hole system, and implies that binary black holes finally merge without accretion disks.

Quasi-static Thermal Deflection of a Thin Clamped Hollow Circular Disk Due to Heat Generation

This paper deals with the determination of the thermal deflection in a thin clamped hollow circular disk defined as a ≤ r ≤ b; 0 ≤ z ≤ h under an unsteady temperature field due to internal heat generation within it. A thin hollow circular disk is considered having an arbitrary initial temperature and subjected to heat flux at the outer circular boundary (r = b) where an inner circular boundary (r = a) is at zero heat flux. Also, the upper surface (z = h) and the lower surface (z = 0) of the disk are at zero temperature. The governing heat conduction equation has been solved by using an integral transform technique. The inner and outer edges of the disk are clamped \({\frac{\partial \omega }{\partial r}=0}\) at r = a, r = b. The results are obtained in a series form in terms of Bessel’s functions and are illustrated graphically.

Application of High-Pressure Torsion to Al-Si Alloys with and without Scandium Additions

Al-2 wt. % Si alloys with and without 0.25 wt. % scandium additions were processed by high-pressure torsion up to five turns at room temperature under a pressure of 6.0 GPa. Microstructural examination of the as-cast Al-2Si-0.25Sc alloy revealed the presence of Al3Sc precipitates which refined the Al grain structure, whereas no major changes were observed in the morphology of the Si particles. Processing by HPT of both experimental alloys revealed submicrometer grains with uniformly distributed Si particles. The mechanical properties were obtained using hardness measurements and the ball-indentation technique. The results show the hardness increased in the first turn of HPT and further increased with increasing numbers of turns. In addition, the hardness values were lower at the centers and continuously increased towards the edges of the disks. The difference in hardness values between the centre and the edge decreased with increasing turns, thereby suggesting an increasing homogeneity with increasing processing. The scandium addition and HPT processing of the Al-2Si alloy strongly influences the grain refinement and mechanical properties. The grain size reduction in the Al-2Si alloy was similar to Al whereas the presence of Sc in Al-2Si during HPT processing was responsible for large precipitation networks and a submicrometer grain formation.

TRANSPORT OF THE FIRST ROCKS OF THE SOLAR SYSTEM BY X-WINDS

It has been suggested that chondrules and calcium-aluminum-rich inclusions (CAIs) were formed at the inner edge of the protoplanetary disk and then entrained in magnetocentrifugal X-winds. We study trajectories of such solid bodies with the consideration of the central star gravity, the protoplanetary disk gravity, and the gas drag of the wind. The efficiency of the gas drag depends on a parameter $\eta$, which is the product of the solid body size and density. We find that the gravity of the protoplanetary disk has a non-negligible effect on the trajectories. If a solid body re-enters the flared disk, the re-entering radius depends on the stellar magnetic dipole moment, the disk's gravity, the parameter $\eta$, and the initial launching angle. The disk's gravity can make the re-entering radius lower by up to 30%. We find a threshold $\eta$, denoted as $\eta_t$, for any particular configuration of the X-wind, below which the solid bodies will be expelled from the planetary system. $\eta_t$ sensitively depends on the initial launching angle, and also depends on the mass of the disk. Only the solid bodies with a $\eta$ larger than but very close to $\eta_t$ can be launched to a re-entering radius larger than 1 AU. This size-sorting effect may explain why chondrules come with a narrow range of sizes within each chondritic class. In general, the size distributions of CAIs and chondrules in chondrites can be determined from the initial size distribution as well as the distribution over the initial launching angle.

Stereo fundus photography in the diagnosis of optic disc melanocytoma

Editor, O ptic disc melanocytoma is a benign, dark brown to black tumour that usually occurs on or adjacent to the optic nerve head. Although most tumours are asymptomatic, some patients may experience visual symptoms related to neural or vascular compression or tumour necrosis (Shields 2000). Here, we present the stereo photographic findings in a patient with a small optic disc melanocytoma. A 43-year-old asymptomatic woman was found on routine examination to have a pigmented optic nerve tumour in her left eye. Otherwise, both eyes were normal with a visual acuity of 6 ⁄6. Ophthalmoscopic evaluation of the left eye revealed a slightly elevated, darkly pigmented tumour located eccentrically over the inferior edge of the optic disc (Fig. 1). An anaglyph stereo fundus photograph that can be viewed with standard red ⁄ green glasses, readily available in the examination equipment of most ophthalmic departments, is shown in Fig. 2. This image illustrates the three dimensionality of the tumour and the scattering of fine pigment clumps into the adjacent retinal nerve fibre layer. The pigmented infiltration of the normally transparent retina leads to fibrillated tumour margins partially obscuring the retinal vessels. The tumour was undetectable on ultrasonography. By fluorescein angiography, pooling of dye was seen in a limited area of secondary disc oedema in the temporal part of the lesion. Optical coherence tomography revealed a Fig. 1. Conventional fundus photograph of the left eye showing a small melanocytoma involving the inferior part of the optic disc.

THE LIGHT CURVE OF THE WEAKLY ACCRETING T TAURI BINARY KH 15D FROM 2005-2010: INSIGHTS INTO THE NATURE OF ITS PROTOPLANETARY DISK

Photometry of the unique pre-main sequence binary system KH 15D is presented, spanning the years 2005-2010. This system has exhibited photometric variations and eclipses over the last 50 years caused by a precessing circumbinary disk. Advancement of the occulting edge across the binary orbit has continued and the photospheres of both stars are now completely obscured at all times. The system is now visible only by scattered light, and yet it continues to show a periodic variation on the orbital cycle with an amplitude exceeding two magnitudes. This variation, which depends only on the binary phase, has likely been present in the data since at least 1995. It can, by itself, account for shoulders on the light curve prior to ingress and following egress, obviating the need for components of extant models such as a scattering halo around star A or forward scattering from a fuzzy disk edge. A plausible source for the variable scattering component is reflected light from the far side of a warped occulting disk. We have detected color changes in V-I of several tenths of a magnitude to both the blue and red that occur during times of minima. These may indicate the presence of a third source of light (faint star) within the system, or a change in the reflectance properties of the disk as the portion being illuminated varies with the orbital motion of the stars. The data support a picture of the circumbinary disk as a geometrically thin, optically thick layer of perhaps mm or cm-sized particles that has been sculpted by the binary stars and possibly other components into a decidedly nonplanar configuration. A simple (infinitely sharp) knife-edge model does a good job of accounting for all of the recent (2005-2010) occultation data.