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1
ZHONG, HONGJIE, e CUNBIAO LEE. "PATHS OF FREELY FALLING DISKS". Modern Physics Letters B 23, n. 03 (30 gennaio 2009): 373–76. http://dx.doi.org/10.1142/s0217984909018436.
Testo completoAbstract (sommario):
The free falling motion of round thin disks in quiescent water was investigated experimentally. The mean density of the disks were about 1.05 g/cm3, slightly larger than water, and thickness to diameter ratio is 0.1. In this letter, We reported two cases with different mass distributions, in one case the disk's center of mass coincided with the geometry center and in another case it was deviated. Two CCD cameras were used to photograph the falling disk simultaneously, the disk's six degrees of freedom were obtained via a stereoscopic vision method. In the first case fluttering motion was observed, the disk's oscillatory motion confined in a vertical plane, and the pitching motion occurred about an axis normal to the plane. In the second case, steady helical falling was observed, the disk moved in a helical path at constant speed, the attacking angle is constant.
2
Montgomery, M. M. "Modeling Fluid Flow Effects in Close Binary and Protoplanetary Systems". Proceedings of the International Astronomical Union 7, S282 (luglio 2011): 549–50. http://dx.doi.org/10.1017/s1743921311028365.
Testo completoAbstract (sommario):
AbstractAccretion disks around some white dwarfs in Cataclysmic Variables are thought to tilt around the line of nodes by the lift force acting at the disk's center of pressure. We investigate whether protoplanetary disks can also experience disk tilt. We find that lift may be possible by an asymmetric, net uni-directional, in-falling gas/dust stream overflowing a bluff body (e.g., Class I sources) or inner annuli of young Class II sources if gas/dust is still in-falling and the aspect ratio and disk surface area are large enough. However, inner disks of Class II sources LkCa 15, UX Tau A, and Rox 44 are not large enough, and therefore disk tilt is not likely.
3
Yakhin, Sergey, Il'fat Aliakberov e Adel Vakhitov. "SUBSTANTIATION OF PARAMETERS OF A ROTARY IMPLEMENT FOR SURFACE TREATMENT OF SOIL WITH INCLINED HEXAGONAL DISC". Vestnik of Kazan State Agrarian University 17, n. 4 (27 gennaio 2023): 106–11. http://dx.doi.org/10.12737/2073-0462-2023-106-111.
Testo completoAbstract (sommario):
For surface tillage, rotary implements with disk working bodies are widely used. In recent decades, intensive research has been carried out on the development of rotary tools with polygonal disks. They were implemented in practice in the form of hexagonal disks mounted on a shaft at an angle of 90° and performing the usual rotational movement around its axis. Of interest are polygonal discs, which are fixed obliquely on the shaft. The well-known tillage tool with inclined square discs was not implemented in practice, since it turned out to be technologically unstable and did not provide a uniform depth of tillage. In the developed soil-cultivating tool, these shortcomings are eliminated. As a result of the study, analytical dependences were obtained, which made it possible to substantiate the parameters of a tillage tool containing obliquely fixed hexagonal disks: the width of the tool (module) capture B = 1.8 m; number of rotary batteries - 2; disk diameter D = 0.32…0.48 m; angle of fastening of discs on the shaft > 50°…55°; distance between disc centers S = 0.100…0.105 m; the number of disks in the battery n = 17; the angle of rotation (displacement) of the disks one relative to the other around the axis of the shaft ɛ = 11.25°. A formula was also obtained to reveal the regularity of the change in the angle of entry of the disk into the soil. The calculation showed that this angle during the rotation of the disk is a variable parameter and varies within 72.5° ... 107.5°. This contributes to the self-cleaning of the inter-disk space from adhering soil and plant residues and is used in determining the traction resistance of the implement.
4
Angelo, Isabel, Gaspard Duchene, Karl Stapelfeldt, Zoie Telkamp, François Ménard, Deborah Padgett, Gerrit Van der Plas et al. "Demographics of Protoplanetary Disks: A Simulated Population of Edge-on Systems". Astrophysical Journal 945, n. 2 (1 marzo 2023): 130. http://dx.doi.org/10.3847/1538-4357/acbb01.
Testo completoAbstract (sommario):
Abstract The structure of protoplanetary disks plays an essential role in planet formation. A disk that is highly inclined, or “edge-on,” is of particular interest since its geometry provides a unique opportunity to study the disk’s vertical structure and radial extent. Candidate edge-on protoplanetary disks are typically identified via their unique spectral energy distributions (SEDs) and subsequently confirmed through high-resolution imaging. However, this selection process is likely biased toward the largest, most-massive disks, and the resulting sample may not accurately represent the underlying disk population. To investigate this, we generated a grid of protoplanetary disk models using radiative transfer simulations and determined which sets of disk parameters produce edge-on systems that could be recovered by the aforementioned detection techniques—i.e., identified by their SEDs and confirmed through follow-up imaging with the Hubble Space Telescope. In doing so, we adopt a quantitative working definition of “edge-on disks” (EODs) that is observation driven and agnostic about the disk inclination or other properties. Folding in empirical disk demographics, we predict an occurrence rate of 6.2% for EODs and quantify biases toward highly inclined, massive disks. We also find that EODs are underrepresented in samples of Spitzer-studied young stellar objects, particularly for disks with host masses of M ≲ 0.5 M ⊙. Overall, our analysis suggests that several dozen EODs remain undiscovered in nearby star-forming regions, and provides a universal selection process to identify EODs for consistent, population-level demographic studies.
5
Khaibrakhmanov, Sergey A., Alexander E. Dudorov e Andrey M. Sobolev. "Rising magnetic flux tubes as a source of IR-variability of the accretion disks of young stars". Proceedings of the International Astronomical Union 14, S345 (agosto 2018): 295–96. http://dx.doi.org/10.1017/s1743921319001431.
Testo completoAbstract (sommario):
AbstractWe investigate dynamics of slender magnetic flux tubes (MFT) in the accretion disks of young stars. Simulations show that MFT rise from the disk and can accelerate to 20-30 km/s causing periodic outflows. Magnetic field of the disk counteracts the buoyancy, and the MFT oscillate near the disk’s surface with periods of 10-100 days. We demonstrate that rising and oscillating MFT can cause the IR-variability of the accretion disks of young stars.
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Hincapié Mejía, Gina María, Juan Miguel Marín S., Luis Alberto Rios e Gloria Restrepo. "Evaluating E. coli degradation using a rotatory disk photoreactor". Ingeniería e Investigación 27, n. 3 (1 settembre 2007): 65–69. http://dx.doi.org/10.15446/ing.investig.v27n3.14846.
Testo completoAbstract (sommario):
Degradation of the bacterium E. coli was studied using a rotatory disk photoreactor manufactured in stainless steel to ascertain this photoreaction system’s behavior in different operating conditions. The photoreactor was equipped with 6 black light bulbs (BLB) used as a source of UV light and fitted with 7 removable disks made of common frosted glass impregnated with TiO2 photocatalyzer which were coupled to a rotation system. TiO2 was impregnated on the disks using the sol-gel method varying molar precursor gel ratios. Abrasion resistance, disk adhesion and surface uniformity tests were carried out for selecting the film presenting the best characteristics. Bacterial photodegradation studies were carried out using the following variable parameters: the disks’ rotation speed, radiation intensity and the number of discs which were directly related to the concentration of photocatalyzer in the system. Up to 64.3% degradation was achieved in 4 hours using 7 disks, low intensity and low rotation speed.
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Kormendy, John. "Internal secular evolution in disk galaxies: the growth of pseudobulges". Proceedings of the International Astronomical Union 3, S245 (luglio 2007): 107–12. http://dx.doi.org/10.1017/s1743921308017407.
Testo completoAbstract (sommario):
AbstractObservational and theoretical evidence that internal, slow (“secular”) evolution reshapes galaxy disks is reviewed in Kormendy & Kennicutt (2004). This update has three aims. First, I emphasize that this evolution is very general – it is as fundamental to the evolution of galaxy disks as (e. g.) core collapse is to globular clusters, as the production of hot Jupiters is to the evolution of protoplanetary disks, and as evolution to red giants containing proto-white-dwarfs is to stellar evolution. One consequence for disk galaxies is the buildup of dense central components that get mistaken for classical (i. e., merger-built) bulges but that were grown out of disk stars and gas. We call these pseudobulges. Second, I review new results on pseudobulge star formation and structure and on the distinction between boxy and disky pseudobulges. Finally, I highlight how these results make a galaxy formation problem more acute. How can hierarchical clustering produce so many pure disk galaxies with no evidence for merger-built bulges?
8
Ryzhkov, A. V., A. V. Machkarin e K. V. Kazakov. "Comparative analysis of soil discarding by spherical disks". IOP Conference Series: Earth and Environmental Science 845, n. 1 (1 novembre 2021): 012138. http://dx.doi.org/10.1088/1755-1315/845/1/012138.
Testo completoAbstract (sommario):
Abstract The article presents data on the influence of various types of spherical disks on the discarding of soil in the horizontal and vertical planes during its processing. These studies were conducted in order to optimize the selection of working bodies of disk tillage machines in terms of resistance and processing quality. Three types of disks were used in the comparative analysis. Two types of discs with cutouts and one solid spherical disc. On one type of disk, the cutouts are made in the form of circular arcs, and on the other-in the form of arcs of a logarithmic spiral. The conducted studies have shown that within the working surface of the disk, the trajectories of movement of soil particles under the influence of the three types of disks under study differ little from each other. Although it can be noted that the steeper rise of the trajectory in the vertical plane provides a solid disk, and the smallest rise of the trajectory - at the disk with cutouts in the form of a logarithmic spiral. In the horizontal plane, the longitudinal movement of the soil mass is less in disks with cutouts, especially in a disk with cutouts along the arc of a logarithmic spiral. As a result of these studies, it was revealed that the disk working bodies with cutouts on the cutting edge in the form of arcs of a logarithmic spiral showed the best quality and energy indicators.
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Kamali, F., C. Henkel, S. Koyama, C. Y. Kuo, J. J. Condon, A. Brunthaler, M. J. Reid et al. "Accretion disk versus jet orientation in H2O megamaser galaxies". Astronomy & Astrophysics 624 (aprile 2019): A42. http://dx.doi.org/10.1051/0004-6361/201834600.
Testo completoAbstract (sommario):
Context. An essential part of the paradigm describing active galactic nuclei is the alignment between the radio jet and the associated rotation axis of the sub-pc accretion disks. Because of the small linear and angular scales involved, this alignment has not yet been checked in a sufficient number of low luminosity active galactic nuclei (LLAGNs). Aims. The project examines the validity of this paradigm by measuring the radio continuum on the same physical scale as the accretion disks to investigate any possible connection between these disks and the radio continuum. Methods. We observed a sample of 18 LLAGNs in the 4.8 GHz (6 cm) radio continuum using the Very Long Baseline Array (VLBA) with 3.3–6.5 ms resolution. The sources were selected to show both an edge-on accretion disk revealed by 22 GHz H2O megamaser emission and signatures of a radio jet. Furthermore, the sources were previously detected in 33 GHz radio continuum observations made with the Very Large Array. Results. Five out of 18 galaxies observed were detected at 8σ or higher levels (Mrk 0001, Mrk 1210, Mrk 1419, NGC 2273, and UGC 3193). While these five sources are known to have maser disks, four of them exhibit a maser disk with known orientation. For all four of these sources, the radio continuum is misaligned relative to the rotation axis of the maser disk, but with a 99.1% confidence level, the orientations are not random and are confined to a cone within 32° of the maser disk’s normal. Among the four sources the misalignment of the radio continuum with respect to the normal vector to the maser disk is smaller when the inner radius of the maser disk is larger. Furthermore, a correlation is observed between the 5 GHz VLBA radio continuum and the [OIII] luminosity and also with the H2O maser disk’s inner radius.
10
Kondo, Katsushi, Satoshi Okuzumi e Shoji Mori. "The Roles of Dust Growth in the Temperature Evolution and Snow Line Migration in Magnetically Accreting Protoplanetary Disks". Astrophysical Journal 949, n. 2 (1 giugno 2023): 119. http://dx.doi.org/10.3847/1538-4357/acc840.
Testo completoAbstract (sommario):
Abstract The temperature structure of protoplanetary disks provides an important constraint on where in the disks rocky planets like our own form. Recent nonideal magnetohydrodynamical (MHD) simulations have shown that the internal Joule heating associated with magnetically driven disk accretion is inefficient at heating the disk midplane. A disk temperature model based on the MHD simulations predicts that in a disk around a solar-mass young star, the water snow line can move inside the current Earth’s orbit within 1 Myr after disk formation. However, the efficiency of the internal Joule heating depends on the disk’s ionization and opacity structures, both of which are governed by dust grains. In this study, we investigate these effects by combining the previous temperature model for magnetically accreting disks with a parameterized model for the grain size and vertical distribution. Grain growth enhances the gas ionization fraction and thereby allows Joule heating to occur closer to the midplane. However, growth beyond 10 μm causes a decrease in the disk opacity, leading to a lower midplane temperature. The combination of these two effects results in the midplane temperature being maximized when the grain size is in the range 10–100 μm. Grain growth to millimeter sizes can also delay the snow line’s migration to the 1 au orbit by up to a few million years. We conclude that accounting for dust growth is essential for accurately modeling the snow line evolution and terrestrial planet formation in magnetically accreting protoplanetary disks.
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Nikolayev, V. A. "Geometric, kinematic and dynamic parameters of the disc ripper". Russian Automobile and Highway Industry Journal 18, n. 5 (25 novembre 2021): 476–87. http://dx.doi.org/10.26518/2071-7296-2021-18-5-476-487.
Testo completoAbstract (sommario):
Introduction. The unit of continuous action for the formation of the underlying layer is designed to increase labour productivity in the construction of roads and other objects, for the construction of which it is necessary to remove the upper layer of soil. For loosening of soil in the unit used bit-like working bodies. Often, disc working bodies are used to cut the soil. Therefore, the expediency of using passive discs in road-building technical means, in particular, in the unit of continuous action for the formation of the underlying layer of highways, is of practical interest. Despite the large number of works, a detailed analysis of the operation of passive disks was not made. Therefore, in order to compare the energy costs for cutting the soil with passive discs and chisel-shaped working bodies, it is necessary to make a theoretical analysis of the operation of passive disks. Analysis of the energy costs of the disk ripper cannot be carried out without having the approximate values of its geometric, kinematic and dynamic parameters.The method of research. As part of a continuous unit to form the underlying layer of roads, each disc would be clamped with soil on both sides and carried out clamped cutting. Therefore, a disk ripper is adopted for analysis, aggregated with a separate energy device. On the basis of the constructive layout, rational geometric parameters of the disk ripper are revealed. The method of calculation of its kinematic and dynamic parameters is developed. In particular, the method of determining the weighted average circumferental velocity of the disk, the angular velocity of the disk and the circumferental velocity of the point on the edge of the disk blade is considered. The modes of cutting the soil by various parts of the disk are considered.Results. On the basis of the developed technique, the dependence of the minimum diameter of the disk on the depth of soil development was revealed. The moment of resistance of the soil to the rotation of the disks is calculated. The horizontal and vertical component of soil resistance to the front disc carrying out clamped cutting and subsequent discs carrying out semi-clamped cutting of the soil are determined. The necessary thrust force of the energy device for cutting the soil with a disk ripper and the dependence of the thrust force of the energy device for cutting the soil on the depth of soil development were revealed. The performance of the unit, including the power device and the disk ripper, is calculated.Conclusion. Since as part of the unit of continuous action for the formation of the underlying layer of roads, the disks will carry out clamped cutting of the soil, for preliminary loosening of the soil with disks, it is more expedient to use a separate unit, including an energy device and a disk ripper. On the basis of the theoretical studies carried out, the necessary thrust force of the energy device for cutting the soil and the total traction force necessary to move the disc ripper were revealed. The performance of the unit is calculated. To compare the energy costs for cutting the soil with passive discs and chisel-shaped working bodies, it is necessary to make a theoretical analysis of the energy costs for the operation of passive disks.
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Hamidov, Namik, Zabit Aslanov e Gultakin Imanova. "SOME FEATURES OF THE OPERATING CAPACITY OF THE REDUCER". Scientific works/Elmi eserler 2 (2 aprile 1996): 109–13. http://dx.doi.org/10.58225/sw.2022.2.109-113.
Testo completoAbstract (sommario):
Abstract. The main factor affecting the performance of the clutch is the load. The load on the clutch is sign-variable and uneven. It strongly depends on the state of the antenna system (with ice, without ice) as well as on the strength and direction of the wind. During operation, in places of the highest specific pressure, the discs seize and weld, which leads to deep tearing of the metal with the formation of deep grooves, ledges and depressions. When the disks seize, which is very often the case in operating conditions, the clutch works like a single shaft, and when the multi-ton cabin moves from its place, the disks do not slip relative to each other, a strong starting current occurs in the motor winding, and the protection system turns it off and the station. If the discs slip relative to each other, then the station also does not rotate. The analysis of friction clutches that were in operation revealed: the shape error has the greatest influence on the destruction of the disks; the wear of the disks in thickness is negligible Keywords: analysis of operational factors, clutch performance, disk surface roughness
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Garufi, A., M. Benisty, P. Pinilla, M. Tazzari, C. Dominik, C. Ginski, Th Henning et al. "Evolution of protoplanetary disks from their taxonomy in scattered light: spirals, rings, cavities, and shadows". Astronomy & Astrophysics 620 (dicembre 2018): A94. http://dx.doi.org/10.1051/0004-6361/201833872.
Testo completoAbstract (sommario):
Context. Dozens of protoplanetary disks have been imaged in scattered light during the last decade. Aims. The variety of brightness, extension, and morphology from this census motivates a taxonomical study of protoplanetary disks in polarimetric light to constrain their evolution and establish the current framework of this type of observation. Methods. We classified 58 disks with available polarimetric observations into six major categories (Ring, Spiral, Giant, Rim, Faint, and Small disks) based on their appearance in scattered light. We re-calculated the stellar and disk properties from the newly available Gaia DR2 and related these properties with the disk categories. Results. More than half of our sample shows disk substructures. For the remaining sources, the absence of detected features is due to their faintness, their small size, or the disk geometry. Faint disks are typically found around young stars and typically host no cavity. There is a possible dichotomy in the near-infrared (NIR) excess of sources with spiral-disks (high) and ring-disks (low). Like spirals, shadows are associated with a high NIR excess. If we account for the pre-main sequence evolutionary timescale of stars with different mass, spiral arms are likely associated with old disks. We also found a loose, shallow declining trend for the disk dust mass with time. Conclusions. Protoplanetary disks may form substructures like rings very early in their evolution but their detectability in scattered light is limited to relatively old sources ( ≳5 Myr) where the recurrently detected disk cavities cause the outer disk to be illuminate. The shallow decrease of disk mass with time might be due to a selection effect, where disks observed thus far in scattered light are typically massive, bright transition disks with longer lifetimes than most disks. Our study points toward spirals and shadows being generated by planets of a fraction of a Jupiter mass to a few Jupiter masses in size that leave their (observed) imprint on both the inner disk near the star and the outer disk cavity.
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Bjorkman, J. E. "The Formation and Structure of Circumstellar Disks". International Astronomical Union Colloquium 175 (2000): 422–47. http://dx.doi.org/10.1017/s0252921100056220.
Testo completoAbstract (sommario):
AbstractSeveral theories have been proposed to explain the origin of Be star disks. Among them are Wind-Compressed Disks, accretion disks, decretion disks, and “explosive” ejections. In reviewing these mechanisms, I first concentrate on the current status of the Wind-Compressed Disk model. In particular, I discuss how non-radial forces may prevent disk formation and then discuss various physical effects that may restore the disk. Second, I examine the observational evidence and what it tells us about the structure of the disk. Of particular interest is evidence in favor of Keplerian disks. Finally, I discuss theories for Keplerian disk formation and some of the constraints such theories must satisfy.
15
Espaillat, Catherine. "A Herschel View of Dust Evolution in Protoplanetary Disks". Proceedings of the International Astronomical Union 8, S299 (giugno 2013): 140–44. http://dx.doi.org/10.1017/s1743921313008120.
Testo completoAbstract (sommario):
AbstractThe details of how protoplanetary disks evolve from initially well-mixed distributions of gas and dust to systems composed mostly of rocky planets and gas giants like our own solar system is a fundamental question in astronomy. It is widely accepted that the first step in planet formation is dust grain growth and settling to the disk midplane. This dust evolution in disks can be studied in greater detail with far-infrared and submillimeter wavelength observations, which offer us unique access to the outer disk's deeper layers. Here we present Herschel far-infrared and submillimeter spectra of GM Aur taken with PACS and SPIRE. GM Aur is a transitional disk, whose inner disk hole is proposed to have been cleared by yet unseen planets. By utilizing Herschel data, we can potentially link the properties of dust evolution in the outer disk to dust clearing in the inner disk. In particular, preliminary SED modeling presented here suggests that GM Aur may have a lower gas-to-dust mass ratio than typically assumed for disks, which may be linked to disk clearing by planets. With further study, such Herschel data may provide insight for theoretical modeling of dust evolution and planet formation.
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Shlyannikov, V. N. "Critical Zone Approach for Structural Integrity of Power Engineering Components". Applied Mechanics and Materials 750 (aprile 2015): 89–95. http://dx.doi.org/10.4028/www.scientific.net/amm.750.89.
Testo completoAbstract (sommario):
This study is concerned with assessing the integrity of cracked steam turbine rotors components which operate under cyclic loading conditions. Damage accumulation and growth have occurred on the inner surface of slot fillet of key and in a disk and blade rivet attachment. Full-size stress-strain state analysis of turbine disk was performed for different stage of lifetime under considering loading conditions. As a result accumulated damage in critical zones of turbine disks depending on time of loading is defined. The tensile tests were performed for determination the main mechanical properties of disk’s material after loading history. The smooth and notched specimens were cut out from critical zones of turbine disk with given operating time. The low-cycle fatigue tests were performed with the harmonic test-cycle. Additional tests were performed on special designed program test-cycle, which equivalent to start-stop cycle of turbine disk. An engineering approach to the prediction of residual lifetime of turbine disks which is sensitive to the loading history at maintenance is proposed. Approximate estimations of carrying capacity are presented for the different stress-strain state of steam turbine disks at the operation.
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Daniels, W. A., B. V. Johnson e D. J. Graber. "Aerodynamic and Torque Characteristics of Enclosed Co/Counterrotating Disks". Journal of Turbomachinery 113, n. 1 (1 gennaio 1991): 67–74. http://dx.doi.org/10.1115/1.2927739.
Testo completoAbstract (sommario):
Experiments were conducted to determine the aerodynamic and torque characteristics of adjacent rotating disks enclosed in a shroud. These experiments were performed to obtain an extended data base for advanced turbine designs such as the counterrotating turbine. Torque measurements were obtained on both disks in the rotating frame of reference for corotating, counterrotating, and one-rotating/one-static disk conditions. The disk models used in the experiments included disks with typical smooth turbine geometry, disks with bolts, disks with bolts and partial bolt covers, and flat disks. A windage diaphragm was installed at midcavity for some experiments. The experiments were conducted with various amounts of coolant throughflow injected into the disk cavity from the disk hub or from the disk o.d. with swirl. The experiments were conducted at disk tangential Reynolds number up to 1.6 × 107 with air as the working fluid. The results of this investigation indicated that the static shroud contributes a significant amount to the total friction within the disk system, the torque on counterrotating disks is essentially independent of coolant flow total rate, flow direction, and tangential Reynolds number over the range of conditions tested, and a static windage diaphragm reduces disk friction in counterrotating disk systems.
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Stapper, L. M., M. R. Hogerheijde, E. F. van Dishoeck e R. Mentel. "The mass and size of Herbig disks as seen by ALMA". Astronomy & Astrophysics 658 (febbraio 2022): A112. http://dx.doi.org/10.1051/0004-6361/202142164.
Testo completoAbstract (sommario):
Context. Many population studies have been performed over the past decade with the Atacama Large millimeter/submillimeter Array (ALMA) to understand the bulk properties of protoplanetary disks around young stars. The studied populations have mostly consisted of late spectral type (i.e., G, K & M) stars, with relatively few more massive Herbig stars (spectral types B, A & F). With Gaia-updated distances, now is a good time to use ALMA archival data for a Herbig disk population study and take an important step forward in our understanding of planet formation. Aims. The aim of this work is to determine the masses and sizes of all Herbig dust disks observed with ALMA to date in a volume-limited sample out to 450 pc. These masses and sizes are put in the context of the Lupus and Upper Sco T Tauri disk populations. Methods. ALMA Band 6 and Band 7 archival data of 36 Herbig stars are used, making this work 64% complete out to 225 pc, and 38% complete out to 450 pc also including Orion. Using stellar parameters and distances, the dust masses and sizes of the disks are determined via a curve-of-growth method. Survival analysis is used to obtain cumulative distributions of the dust masses and radii. Results. Herbig disks have a higher dust mass than the T Tauri disk populations of Lupus and Upper Sco by factors of ~3 and ~7 respectively. In addition, Herbig disks are often larger than the typical T Tauri disk. Although the masses and sizes of Herbig disks extend over a similar range to those of T Tauri disks, the distributions of masses and sizes of Herbig disks are significantly skewed toward higher values. Lastly, group I disks are more massive than group II disks. An insufficient number of group II disks are observed at sufficient angular resolution to determine whether or not they are also small in size compared to group I disks. Conclusions. Herbig disks are skewed towards more massive and larger dust disks compared to T Tauri disks. Based on this we speculate that these differences find their origin in an initial disk mass that scales with the stellar mass, and that subsequent disk evolution enlarges the observable differences, especially if (sub)millimeter continuum optical depth plays a role. Moreover, the larger disk masses and sizes of Herbig stars could be linked to the increasing prevalence of giant planets with host star mass.
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Davies, Claire L., e Jane S. Greaves. "Angular momentum evolution during star and planetary system formation". Proceedings of the International Astronomical Union 8, S299 (giugno 2013): 210–11. http://dx.doi.org/10.1017/s1743921313008351.
Testo completoAbstract (sommario):
AbstractWe focused on analysing the role played by protoplanetary disks in the evolution of angular momentum during star formation. If all the angular momentum contained within collapsing pre-stellar cores was conserved during their formation, proto-stars would reach rotation rates exceeding their break-up velocities before they reached the main sequence (Bodenheimer 1995). In order to avoid this occuring, methods by which proto-stars can lose angular momentum must exist. Angular momentum can be transferred from star to disk via stellar magnetic field lines through a process called magnetic braking (Camenzind 1990; Königl 1991). Alternatively, the stellar angular momentum can be lost from the star-disk system entirely via stellar- or disk-winds (e.g. Pelletier & Pudritz 1992; Matt & Pudritz 2005).The proportion of lost stellar angular momentum retained within the protoplanetary disk is important to studies of planetary system formation. If the bulk motion within the disk remains Keplerian, any increase of angular momentum in the disk causes an outward migration of disk material and an expansion of the disk. Therefore, an increase in disk angular momentum may cause a reduction in the disk surface density, often used to indicate the disk's ability to form planets.We made use of multi-wavelength data available in the literature to directly calculate the stellar and disk angular momenta for two nearby regions of star formation. Namely, these were the densely populated and highly irradiated Orion Nebula Cluster (ONC) and the comparitively sparse Taurus-Auriga region. Due to the limited size of the ONC dataset, we produced an average surface density profile for the region. We modelled the stars as solid body rotators due to their fully convective nature (Krishnamurthi et al. 1997) and assumed the disks are flat and undergo Keplerian rotation about the same rotation axis as the star.We observed the older disks within each of the two star forming regions to be preferentially larger and less massive, consistent with viscous accretion theories and disk dispersal. However, when drawing comparisons between the two regions, the ONC sample appeared to have less massive disks than the Taurus-Auriga sample, even though the population of Taurus-Auriga is older. This may suggest an influence of the birth cloud environment on protoplanetary disk evolution.Finally, the older stars within the ONC were observed to harbour disks that contained more angular momentum than their younger counterparts whereas, in the Taurus-Auriga sample, the amount of angular momentum contained in the older and younger samples was consistent. We suggest that the missing disk angular momentum in the older Taurus-Auriga disks may be contained within yet-undetected planets.
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Avramenko, R., S. Wolf, T. F. Illenseer e S. Rehberg. "Selected aspects of the analysis of molecular line observations of edge-on circumbinary disks". Astronomy & Astrophysics 642 (ottobre 2020): A127. http://dx.doi.org/10.1051/0004-6361/201935610.
Testo completoAbstract (sommario):
Context. Inner cavities, accretion arms, and density waves are characteristic structures in the density distribution of circumbinary disks. They are the result of the tidal interaction of the non-axisymmetric gravitational forces of the central binary with the surrounding disk and are most prominent in the inner region, where the asymmetry is most pronounced. Aims. The goal of this study is to test the feasibility of reconstructing the gas density distribution and quantifying properties of structures in the inner regions of edge-on circumbinary disks using multiple molecular line observations. Methods. The density distribution in circumbinary disks is calculated with 2D hydrodynamic simulations. Subsequently, molecular line emission maps are generated with 3D radiative transfer simulations. Based on these, we investigate the observability of characteristic circumbinary structures located in the innermost region for spatially resolved and unresolved disks. Results. We find that it is possible to reconstruct the inner cavity, accretion arms, and density waves from spatially resolved multi-wavelength molecular line observations of circumbinary disks seen edge-on. For the spatially unresolved observations only, an estimate can be derived for the density gradient in the transition area between the cavity and the disk’s inner rim.
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Rilinger, Anneliese M., Catherine C. Espaillat, Zihua Xin, Álvaro Ribas, Enrique Macías e Sarah Luettgen. "Determining Dust Properties in Protoplanetary Disks: SED-derived Masses and Settling with ALMA". Astrophysical Journal 944, n. 1 (1 febbraio 2023): 66. http://dx.doi.org/10.3847/1538-4357/aca905.
Testo completoAbstract (sommario):
Abstract We present spectral energy distribution (SED) modeling of 338 disks around T Tauri stars from 11 star-forming regions, ranging from ∼0.5 to 10 Myr old. The disk masses we infer from our SED models are typically greater than those reported from (sub)millimeter surveys by a factor of 1.5–5, with the discrepancy being generally higher for the more massive disks. Masses derived from (sub)millimeter fluxes rely on the assumption that the disks are optically thin at all millimeter wavelengths, which may cause the disk masses to be underestimated since the observed flux is not sensitive to the whole mass in the disk; SED models do not make this assumption and thus yield higher masses. Disks with more absorbing material should be optically thicker at a given wavelength, which could lead to a larger discrepancy for disks around massive stars when the disk temperature is scaled by the stellar luminosity. We also compare the disk masses and degree of dust settling across the different star-forming regions and find that disks in younger regions are more massive than disks in older regions, but with a similar degree of dust settling. Together, these results offer potential partial solutions to the “missing” mass problem: disks around T Tauri stars may indeed have enough material to form planetary systems, though previous studies have underestimated the mass by assuming the disks to be optically thin; these planetary systems may also form earlier than previously theorized since significant dust evolution (i.e., settling) is already apparent in young disks.
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Gárate, Matías, Timmy N. Delage, Jochen Stadler, Paola Pinilla, Til Birnstiel, Sebastian Markus Stammler, Giovanni Picogna, Barbara Ercolano, Raphael Franz e Christian Lenz. "Large gaps and high accretion rates in photoevaporative transition disks with a dead zone". Astronomy & Astrophysics 655 (novembre 2021): A18. http://dx.doi.org/10.1051/0004-6361/202141444.
Testo completoAbstract (sommario):
Context. Observations of young stars hosting transition disks show that several of them have high accretion rates, despite their disks presenting extended cavities in their dust component. This represents a challenge for theoretical models, which struggle to reproduce both features simultaneously. Aims. We aim to explore if a disk evolution model, including a dead zone and disk dispersal by X-ray photoevaporation, can explain the high accretion rates and large gaps (or cavities) measured in transition disks. Methods. We implemented a dead zone turbulence profile and a photoevaporative mass-loss profile into numerical simulations of gas and dust. We performed a population synthesis study of the gas component and obtained synthetic images and SEDs of the dust component through radiative transfer calculations. Results. This model results in long-lived inner disks and fast dispersing outer disks that can reproduce both the accretion rates and gap sizes observed in transition disks. For a dead zone of turbulence αdz = 10−4 and an extent rdz = 10 AU, our population synthesis study shows that 63% of our transition disks are still accreting with Ṁg ≥ 10−11 M⊙ yr−1 after opening a gap. Among those accreting transition disks, half display accretion rates higher than 5.0 × 10−10 M⊙ yr−1. The dust component in these disks is distributed in two regions: in a compact inner disk inside the dead zone, and in a ring at the outer edge of the photoevaporative gap, which can be located between 20 and 100 AU. Our radiative transfer calculations show that the disk displays an inner disk and an outer ring in the millimeter continuum, a feature that resembles some of the observed transition disks. Conclusions. A disk model considering X-ray photoevaporative dispersal in combination with dead zones can explain several of the observed properties in transition disks, including the high accretion rates, the large gaps, and a long-lived inner disk at millimeter emission.
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Liu, Xia, Lin Xiao e Li Zhang. "Long-lived Prptoplanetary disk Formation". Journal of Physics: Conference Series 2457, n. 1 (1 marzo 2023): 012040. http://dx.doi.org/10.1088/1742-6596/2457/1/012040.
Testo completoAbstract (sommario):
Abstract Recently, protoplanetary disks around M dwarf stars have had longer ages than that typical protoplanetary disks. These long-lived protoplanetary disks imply that some protoplanetary disks suffer from a low-radiation. On the other hand, the host star’s masses around these protoplanetary disks are very low. We do not know whether long-lived protoplanetary disks can survive around stars with higher mass. In this work, we explore how protoplanetary disks lose their masses and how protoplanetary disk lifetimes dextral photoevaporation on host star mass. We find that only low-mass stars (< 0.5 M*) can host lone-lived protoplanetary disks (∼45 Myr). When star masses > 0.7 M*, the protoplanetary disk lifetimes drop below 45 Myr quickly because of the strong accretion and photoevaporation. The host stars with ∼2 M* may have protoplanetary disks with a lifetime of up to only 14 Myr. According to observations and our simulations, we consider that long-lived protoplanetary disks can only survive around M dwarf stars.
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Kluska, J., H. Van Winckel, Q. Coppée, G. M. Oomen, K. Dsilva, D. Kamath, V. Bujarrabal e M. Min. "A population of transition disks around evolved stars: Fingerprints of planets". Astronomy & Astrophysics 658 (febbraio 2022): A36. http://dx.doi.org/10.1051/0004-6361/202141690.
Testo completoAbstract (sommario):
Context. Post-asymptotic giant branch (post-AGB) binaries are surrounded by massive disks of gas and dust that are similar to the protoplanetary disks that are known to surround young stars. Aims. We assembled a catalog of all known Galactic post-AGB binaries featuring disks. We explore the correlations between the different observables with the aim of learning more about potential disk-binary interactions. Methods. We compiled spectral energy distributions of 85 Galactic post-AGB binary systems. We built a color-color diagram to differentiate between the different disk morphologies traced by the characteristics of the infrared excess. We categorized the different disk types and searched for correlations with other observational characteristics of these systems. Results. Between 8 and 12% of our targets are surrounded by transition disks, that is, disks having no or low near-infrared excess. We find a strong link between these transition disks and the depletion of refractory elements seen on the surface of the post-AGB star. We interpret this correlation as evidence of the presence of a mechanism that stimulates the dust and gas separation within the disk and that also produces the transition disk structure. We propose that such a mechanism is likely to be due to a giant planet carving a hole in the disk, effectively trapping the dust in the outer disk parts. We propose two disk evolutionary scenarios, depending on the actual presence of such a giant planet in the disk. Conclusions. We advocate that giant planets can successfully explain the correlation between the transition disks and the depletion of refractory materials observed in post-AGB binaries. If the planetary scenario is confirmed, disks around post-AGB binaries could be a unique laboratory for testing planet-disk interactions and their influence on the late evolution of binary stars. The question of whether such planets are first- or second-generation bodies also remains to be considered. We argue that these disks are ideal for studying planet formation scenarios in an unprecedented parameter space.
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Валиев, Айрат, e Ayrat Valiev. "INVESTIGATION OF SOIL MOTION ALONG THE WORKING SURFACE OF A DISK CULTIVATOR". Vestnik of Kazan State Agrarian University 12, n. 3 (11 gennaio 2018): 54–60. http://dx.doi.org/10.12737/article_5a1d9587a0d852.23012684.
Testo completoAbstract (sommario):
The results of theoretical studies of a disk cultivator are presented, the working units of which consist of two flat discs, inclined to each other in the horizontal and vertical planes. The distance between the upper leading edges of the discs is greater than between the lower rear edges. When the discs move in the soil, the layer is jammed between them, breaks off from the bottom of the furrow, rises slightly upwards and is laid back into the furrow in the loosened state. The main constructive and adjusting parameters of the working unit are the inclination angles of discs γ, the distance between the discs d and the radius of the disks R. The purpose of this work is a theoretical study of the soil movement process in the inter-disk space and the rationale for the design and adjusting parameters of the developed disc cultivator. Based on the methods of theoretical mechanics, the terms for separating the soil particles from the main monolith and their motion along the working surface of the disks are investigated. Equations, describing the trajectories of soil particles with allowance for specific soil conditions, characterized by the basic mechanical parameters of the soil and also depending on the parameters of the working organ and the depth of soil cultivation are obtained. Based on the results of solving these equations, it is established that in the process of working at a depth of up to 16 cm, with a distance between the disks d = 0.15 ... 0.20 m, disc angles γ = 6 ... 90 and disk radius R = 0.25 m, the soil will be guaranteed to tear off from the monolith, rise to a height above the horizontal axis of the disk working units and leave the interdisk space. At the same time, on the one hand, the limiting value of the relative deformation of the soil is reached in the layer being processed, its fracture and crumbling are ensured, and on the other hand, the soil particles are thrown forward and the working organs of the disk cultivator are blocked.
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van Terwisga, S. E., E. F. van Dishoeck, R. K. Mann, J. Di Francesco, N. van der Marel, M. Meyer, S. M. Andrews et al. "Protoplanetary disk masses in NGC 2024: Evidence for two populations". Astronomy & Astrophysics 640 (agosto 2020): A27. http://dx.doi.org/10.1051/0004-6361/201937403.
Testo completoAbstract (sommario):
Context. Protoplanetary disks in dense, massive star-forming regions are strongly affected by their environment. How this environmental impact changes over time is an important constraint on disk evolution and external photoevaporation models. Aims. We characterize the dust emission from 179 disks in the core of the young (0.5 Myr) NGC 2024 cluster. By studying how the disk mass varies within the cluster, and comparing these disks to those in other regions, we aim to determine how external photoevaporation influences disk properties over time. Methods. Using the Atacama Large Millimeter/submillimeter Array, a 2.9′× 2.9′ mosaic centered on NGC 2024 FIR 3 was observed at 225 GHz with a resolution of 0.25″, or ~100 AU. The imaged region contains 179 disks identified at IR wavelengths, seven new disk candidates, and several protostars. Results. The overall detection rate of disks is 32 ± 4%. Few of the disks are resolved, with the exception of a giant (R = 300 AU) transition disk. Serendipitously, we observe a millimeter flare from an X-ray bright young stellar object (YSO), and resolve continuum emission from a Class 0 YSO in the FIR 3 core. Two distinct disk populations are present: a more massive one in the east, along the dense molecular ridge hosting the FIR 1-5 YSOs, with a detection rate of 45 ± 7%. In the western population, towards IRS 1, only 15 ± 4% of disks are detected. Conclusions. NGC 2024 hosts two distinct disk populations. Disks along the dense molecular ridge are young (0.2–0.5 Myr) and partly shielded from the far ultraviolet radiation of IRS 2b; their masses are similar to isolated 1–3 Myr old SFRs. The western population is older and at lower extinctions, and may be affected by external photoevaporation from both IRS 1 and IRS 2b. However, it is possible these disks had lower masses to begin with.
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Zakhozhay, O., C. del Burgo e V. Zakhozhay. "Geometry of highly inclined protoplanetary disks". Advances in Astronomy and Space Physics 5, n. 1 (2015): 33–38. http://dx.doi.org/10.17721/2227-1481.5.33-38.
Testo completoAbstract (sommario):
We present a geometric model for the modelling of spectral energy distribution of inclined protoplanetary disks. We investigate peculiarities in the geometry of nearly edge-on disks with an inner hole and a central object. In the investigation we consider two cases: that of geometrically thin disks (where the star is larger than the rim of the inner edge of the disk) and that of geometrically thick disks (when the star is smaller than the inner rim of the disk). Our model is appropriate for modelling substellar objects with primordial gas-rich disks, as activity (such as accretion or outflows) in such disks has low amplitude and can be ignored even when modelling early evolution stages. Furthermore, it can also be used to model any symmetric system with a disk and a spherical central body (star, brown dwarf or giant planet).
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Kuffmeier, M., C. P. Dullemond, S. Reissl e F. G. Goicovic. "Misaligned disks induced by infall". Astronomy & Astrophysics 656 (dicembre 2021): A161. http://dx.doi.org/10.1051/0004-6361/202039614.
Testo completoAbstract (sommario):
Arc- and tail-like structures associated with disks around Herbig stars can be a consequence of infall events occurring after the initial collapse phase of a forming star, consistent with the observation of luminosity bursts. An encounter event of gas with an existing star can lead to the formation of a second-generation disk significantly after the initial protostellar collapse phase. Additionally, observations of shadows in disks can be well described by a configuration of a misaligned inner and outer disk, such that the inner disk casts a shadow on the outer disk. Carrying out altogether eleven 3D hydrodynamical models with the moving mesh code AREPO, we tested whether a late encounter of an existing star–disk system with a cloudlet of gas can lead to the formation of an outer disk that is misaligned with respect to the primordial inner disk. Our models demonstrate that a second-generation disk with a large misalignment with respect to an existing primordial disk can easily form if the infall angle is large. The second-generation outer disk is more eccentric, though the asymmetric infall also triggers eccentricity of the inner disk of e ≈ 0.05 to 0.1. Retrograde infall can lead to the formation of counter-rotating disks and enhanced accretion. As the angular momentum of the inner disk is reduced, the inner disk shrinks and a gap forms between the two disks. The resulting misaligned disk system can survive for ~100 kyr or longer without aligning with each other even for low primordial disk masses given an infall mass of ~10−4 M⊙. A synthetic image for one of our models reveals shadows in the outer disk similar to the ones observed in multiple transition disks that are caused by the misaligned inner disk. We conclude that late infall onto an existing star–disk system leads to the formation of a misaligned outer disk for infall that is inclined with respect to the orientation of the inner disk. Infall might therefore be responsible for observations of shadows in at least some transition disks.
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Tian, Hong, e Takeo Matsudaira. "Tribological Characteristics of Liquid Lubricant on Magnetic Disks Treated by Far-UV Radiation". Journal of Tribology 115, n. 3 (1 luglio 1993): 400–405. http://dx.doi.org/10.1115/1.2921650.
Testo completoAbstract (sommario):
In this paper, we have shown that a perfluoropolyether lubricant after far-UV irradiation treatment proposed by Saperstein and Lin (1990) was strongly bonded to disk surfaces without depletion from disks rotating at 6000 rpm in a 50°C temperature environment. Nonbonded lubricant (mobile lubricant) on disk surfaces or on the top of the UV-fixed lubricant was easily depleted from rotating disks. Depletion data of the mobile lubricant agreed well with predictions of an inter-slip model. It has been demonstrated experimentally that the mobile lubricant on disk surfaces was simply displaced from the contact or the sliding regions. After a long period of head contact, the lubricant was built up at the head/disk interface due to migration of the mobile lubricant around the contact regions. By contrast, no such build-up was observed for disks with the UV-fixed lubricant. Consequently, long-term stiction was observed for disks with only mobile lubricant, while no stiction was observed for disks with the UV-fixed lubricant. The UV-treated disks also adsorbed less water at high relative humidities compared with the nontreated disks.
30
Funes, S.J.,, José G., e Enrico Maria Corsini. "Galaxy Disks and Disk Galaxies1". Publications of the Astronomical Society of the Pacific 112, n. 777 (novembre 2000): 1510–11. http://dx.doi.org/10.1086/317703.
Testo completo
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Vorobyov, Eduard, e Christian Theis. "Structure Formation in Anisotropic Disks". Proceedings of the International Astronomical Union 2, S235 (agosto 2006): 143. http://dx.doi.org/10.1017/s1743921306005758.
Testo completoAbstract (sommario):
The majority of normal disk galaxies are characterized by non-axisymmetric structures like spirals or bars. These structural elements have been widely discussed in the literature as a result of gravitational instabilities which are connected to growing density waves or global instabilities of disks. A first insight into the properties of galactic discs was provided by linear stability analysis. However, a disadvantage of linear stability analysis remained its restriction to small perturbations, both in amplitude and wavelength. Thus, numerical simulations, especially hydrodynamical and stellar-hydrodynamical simulations became a primary tool for the analysis of galactic evolution.
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Leiendecker, Harrison, Hannah Jang-Condell, Neal J. Turner e Adam D. Myers. "Dust Rings and Cavities in the Protoplanetary Disks around HD 163296 and DoAr 44". Astrophysical Journal 941, n. 2 (1 dicembre 2022): 172. http://dx.doi.org/10.3847/1538-4357/aca32d.
Testo completoAbstract (sommario):
Abstract We model substructure in the protoplanetary disks around DoAr 44 and HD 163296 in order to better understand the conditions under which planets may form. We match archival millimeter-wavelength thermal emission against models of the disks’ structure that are in radiation balance with the starlight heating and in vertical hydrostatic equilibrium, and then we compare to archival polarized scattered near-infrared images of the disks. The millimeter emission arises in the interior, while the scattered near-infrared radiation probes the disks’ outer layers. Our best model of the HD 163296 disk has dust masses 81 ± 13 M ⊕ in the inner ring at 68 au and 82 − 16 + 26 M ⊕ in the outer ring at 102 au, both falling within the range of estimates from previous studies. Our DoAr 44 model has total dust mass 84 − 3.5 + 7.0 M ⊕. Unlike HD 163296, DoAr 44 as of yet has no detected planets. If the central cavity in the DoAr 44 disk is caused by a planet, the planet’s mass must be at least 0.5 M J and is unlikely to be greater than 1.6 M J. We demonstrate that the DoAr 44 disk's structure with a bright ring offset within a fainter skirt can be formed by dust particles drifting through a plausible distribution of gas.
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Robert, C. M. T., A. Crida, E. Lega, H. Méheut e A. Morbidelli. "Toward a new paradigm for Type II migration". Astronomy & Astrophysics 617 (settembre 2018): A98. http://dx.doi.org/10.1051/0004-6361/201833539.
Testo completoAbstract (sommario):
Context. Giant planets open gaps in their protoplanetary and subsequently suffer so-called type II migration. Schematically, planets are thought to be tightly locked within their surrounding disks, and forced to follow the viscous advection of gas onto the central star. This fundamental principle, however, has recently been questioned as migrating planets were shown to decouple from the gas’ radial drift. Aims. In this framework, we question whether the traditionally used linear scaling of migration rate of a giant planet with the disk’s viscosity still holds. Additionally, we assess the role of orbit-crossing material as part of the decoupling mechanism. Methods. We have performed 2D (r, θ) numerical simulations of point-mass planets embedded in locally isothermal α-disks in steady-state accretion, with various values of α. Arbitrary planetary accretion rates were used as a means to diminish or nullify orbit-crossing flows. Results. We confirm that the migration rate of a gap-opening planet is indeed proportional to the disk’s viscosity, but is not equal to the gas drift speed in the unperturbed disk. We show that the role of gap-crossing flows is in fact negligible. Conclusions. From these observations, we propose a new paradigm for type II migration: a giant planet feels a torque from the disk that promotes its migration, while the gap profile relative to the planet is restored on a viscous timescale, thus limiting the planet migration rate to be proportional to the disk’s viscosity. Hence, in disks with low viscosity in the planet region, type II migration should still be very slow.
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Visser, Ruud, Simon Bruderer, Paolo Cazzoletti, Stefano Facchini, Alan N. Heays e Ewine F. van Dishoeck. "Nitrogen isotope fractionation in protoplanetary disks". Astronomy & Astrophysics 615 (luglio 2018): A75. http://dx.doi.org/10.1051/0004-6361/201731898.
Testo completoAbstract (sommario):
Aims. The two stable isotopes of nitrogen, 14N and 15N, exhibit a range of abundance ratios both inside and outside the solar system. The elemental ratio in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC15N ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C15N ratio of 323 ± 30 in one T Tauri star. We aim to determine the dominant mechanism responsible for these enhancements of 15N: low-temperature exchange reactions or isotope-selective photodissociation of N2. Methods. Using the thermochemical code DALI, we model the nitrogen isotope chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical network is the first to include both fractionation mechanisms for nitrogen. The model produces abundance profiles and isotope ratios for several key N-bearing species. We study how these isotope ratios depend on various disk parameters. Results. The formation of CN and HCN is closely coupled to the vibrational excitation of H2 in the UV-irradiated surface layers of the disk. Isotope fractionation is completely dominated by isotope-selective photodissociation of N2. The column density ratio of HCN over HC15N in the disk’s inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. For larger grains, self-shielding of N2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. Between disk radii of ~50 and 200 au, the models predict HCN/HC15N and CN/C15N abundance ratios consistent with observations of disks and comets. The HCN/HC15N and CN/C15N column density ratios in the models are a factor of 2–3 higher than those inferred from the ALMA observations.
35
Agúndez, Marcelino, Evelyne Roueff, Franck Le Petit e Jacques Le Bourlot. "The chemistry of disks around T Tauri and Herbig Ae/Be stars". Astronomy & Astrophysics 616 (agosto 2018): A19. http://dx.doi.org/10.1051/0004-6361/201732518.
Testo completoAbstract (sommario):
Context. Infrared and (sub-)millimeter observations of disks around T Tauri and Herbig Ae/Be stars point to a chemical differentiation, with a lower detection rate of molecules in disks around hotter stars. Aims. We aim to investigate the underlying causes of the chemical differentiation indicated by observations and perform a comparative study of the chemistry of T Tauri and Herbig Ae/Be disks. This is one of the first studies to compare the chemistry in the outer regions of these two types of disk. Methods. We developed a model to compute the chemical composition of a generic protoplanetary disk, with particular attention to the photochemistry, and applied it to a T Tauri and a Herbig Ae/Be disk. We compiled cross sections and computed photodissociation and photoionization rates at each location in the disk by solving the far-ultraviolet (FUV) radiative transfer in a 1+1D approach using the Meudon PDR code and adopting observed stellar spectra. Results. The warmer disk temperatures and higher ultraviolet flux of Herbig stars compared to T Tauri stars induce some differences in the disk chemistry. In the hot inner regions, H2O, and simple organic molecules like C2H2, HCN, and CH4 are predicted to be very abundant in T Tauri disks and even more in Herbig Ae/Be disks, in contrast with infrared observations that find a much lower detection rate of water and simple organics toward disks around hotter stars. In the outer regions, the model indicates that the molecules typically observed in disks, like HCN, CN, C2H, H2CO, CS, SO, and HCO+, do not have drastic abundance differences between T Tauri and Herbig Ae disks. Some species produced under the action of photochemistry, like C2H and CN, are predicted to have slightly lower abundances around Herbig Ae stars due to a narrowing of the photochemically active layer. Observations indeed suggest that these radicals are somewhat less abundant in Herbig Ae disks, although in any case, the inferred abundance differences are small, of a factor of a few at most. A clear chemical differentiation between both types of disks concerns ices. Owing to the warmer temperatures of Herbig Ae disks, one expects snow lines lying farther away from the star and a lower mass of ices compared to T Tauri disks. Conclusions. The global chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main differences are driven by the warmer temperatures of the latter, which result in a larger reservoir or water and simple organics in the inner regions and a lower mass of ices in the outer disk.
36
Quiroz, Juan, Nicole L. Wallack, Bin Ren, Ruobing Dong, Jerry W. Xuan, Dimitri Mawet, Maxwell A. Millar-Blanchaer e Garreth Ruane. "Improving Planet Detection with Disk Modeling: Keck/NIRC2 Imaging of the HD 34282 Single-armed Protoplanetary Disk". Astrophysical Journal Letters 924, n. 1 (1 gennaio 2022): L4. http://dx.doi.org/10.3847/2041-8213/ac3e62.
Testo completoAbstract (sommario):
Abstract Formed in protoplanetary disks around young stars, giant planets can leave observational features such as spirals and gaps in their natal disks through planet–disk interactions. Although such features can indicate the existence of giant planets, protoplanetary disk signals can overwhelm the innate luminosity of planets. Therefore, in order to image planets that are embedded in disks, it is necessary to remove the contamination from the disks to reveal the planets possibly hiding within their natal environments. We observe and directly model the detected disk in the Keck/NIRC2 vortex coronagraph L′-band observations of the single-armed protoplanetary disk around HD 34282. Despite a nondetection of companions for HD 34282, this direct disk modeling improves planet detection sensitivity by up to a factor of 2 in flux ratio and ∼10 M Jupiter in mass. This suggests that performing disk modeling can improve directly imaged planet detection limits in systems with visible scattered light disks, and can help to better constrain the occurrence rates of self-luminous planets in these systems.
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Ulubay-Siddiki, Ayse, Ortwin Gerhard e Magda Arnaboldi. "Self-gravitating warped disks around nuclear black holes". Proceedings of the International Astronomical Union 2, S238 (agosto 2006): 467–68. http://dx.doi.org/10.1017/s1743921307005984.
Testo completoAbstract (sommario):
AbstractMany galactic nuclei contain disks of gas and possibly stars surrounding a supermassive black hole. These disks may play a key role in the evolution of galactic centers. Here we address the problem of finding stable warped equilibrium configurations for such disks, considering the attraction by the black hole and the disk self-gravity as the only acting forces. We model these disks as a collection of concentric, circular rings.We find the equilibria of such systems of rings, and determine how they scale with the ring parameters and the mass of the central black hole. We show that in some cases these disk equilibria may be highly warped. We then analyze the stability of these disks, using both direct time integration and linear stability analysis. This shows that the warped disks are stable for a range of disk-to-black hole mass ratios, when the rings extend over a limited range of radii.
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Chan, Chi-Ho, Tsvi Piran e Julian H. Krolik. "Nonlinear Evolution of the Magnetorotational Instability in Eccentric Disks". Astrophysical Journal 933, n. 1 (1 luglio 2022): 81. http://dx.doi.org/10.3847/1538-4357/ac68f3.
Testo completoAbstract (sommario):
Abstract The magnetorotational instability (MRI) has been extensively studied in circular magnetized disks, and its ability to drive accretion has been demonstrated in a multitude of scenarios. There are reasons to expect eccentric magnetized disks to also exist, but the behavior of the MRI in these disks remains largely uncharted territory. Here we present the first simulations that follow the nonlinear development of the MRI in eccentric disks. We find that the MRI in eccentric disks resembles circular disks in two ways, in the overall level of saturation and in the dependence of the detailed saturated state on magnetic topology. However, in contrast with circular disks, the Maxwell stress in eccentric disks can be negative in some disk sectors, even though the integrated stress is always positive. The angular momentum flux raises the eccentricity of the inner parts of the disk and diminishes the same of the outer parts. Because material accreting onto a black hole from an eccentric orbit possesses more energy than material tracing the innermost stable circular orbit, the radiative efficiency of eccentric disks may be significantly lower than circular disks. This may resolve the “inverse energy problem” seen in many tidal disruption events.
39
Sanchis, E., L. Testi, A. Natta, C. F. Manara, B. Ercolano, T. Preibisch, T. Henning et al. "Demographics of disks around young very low-mass stars and brown dwarfs in Lupus". Astronomy & Astrophysics 633 (gennaio 2020): A114. http://dx.doi.org/10.1051/0004-6361/201936913.
Testo completoAbstract (sommario):
We present new 890 μm continuum ALMA observations of five brown dwarfs (BDs) with infrared excess in Lupus I and III, which in combination with four previously observed BDs allowed us to study the millimeter properties of the full known BD disk population of one star-forming region. Emission is detected in five out of the nine BD disks. Dust disk mass, brightness profiles, and characteristic sizes of the BD population are inferred from continuum flux and modeling of the observations. Only one source is marginally resolved, allowing for the determination of its disk characteristic size. We conduct a demographic comparison between the properties of disks around BDs and stars in Lupus. Due to the small sample size, we cannot confirm or disprove a drop in the disk mass over stellar mass ratio for BDs, as suggested for Ophiuchus. Nevertheless, we find that all detected BD disks have an estimated dust mass between 0.2 and 3.2 M⊙; these results suggest that the measured solid masses in BD disks cannot explain the observed exoplanet population, analogous to earlier findings on disks around more massive stars. Combined with the low estimated accretion rates, and assuming that the mm-continuum emission is a reliable proxy for the total disk mass, we derive ratios of Ṁacc∕Mdisk that are significantly lower than in disks around more massive stars. If confirmed with more accurate measurements of disk gas masses, this result could imply a qualitatively different relationship between disk masses and inward gas transport in BD disks.
40
Bohn, A. J., M. Benisty, K. Perraut, N. van der Marel, L. Wölfer, E. F. van Dishoeck, S. Facchini et al. "Probing inner and outer disk misalignments in transition disks". Astronomy & Astrophysics 658 (febbraio 2022): A183. http://dx.doi.org/10.1051/0004-6361/202142070.
Testo completoAbstract (sommario):
Context. Transition disks are protoplanetary disks with dust-depleted cavities, possibly indicating substantial clearing of their dust content by a massive companion. For several known transition disks, dark regions interpreted as shadows have been observed in scattered light imaging and are hypothesized to originate from misalignments between distinct regions of the disk. Aims. We aim to investigate the presence of misalignments in transition disks. We study the inner disk (<1 au) geometries of a sample of 20 well-known transition disks with Very Large Telescope Interferometer (VLTI) GRAVITY observations and use complementary 12CO and 13CO molecular line archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) to derive the orientation of the outer disk regions (>10 au). Methods. We fit simple parametric models to the visibilities and closure phases of the GRAVITY data to derive the inclination and position angle of the inner disks. The outer disk geometries were derived from Keplerian fits to the ALMA velocity maps and compared to the inner disk constraints. We also predicted the locations of expected shadows for significantly misaligned systems. Results. Our analysis reveals six disks to exhibit significant misalignments between their inner and outer disk structures. The predicted shadow positions agree well with the scattered light images of HD 100453 and HD 142527, and we find supporting evidence for a shadow in the south of the disk around CQ Tau. In the other three targets for which we infer significantly misaligned disks, V1247 Ori, V1366 Ori, and RY Lup, we do not see any evident sign of shadows in the scattered light images. The scattered light shadows observed in DoAr 44, HD 135344 B, and HD 139614 are consistent with our observations, yet the underlying morphology is likely too complex to be described properly by our models and the accuracy achieved by our observations. Conclusions. The combination of near infrared and submillimeter interferometric observations allows us to assess the geometries of the innermost disk regions and those of the outer disk. Whereas we can derive precise constraints on the potential shadow positions for well-resolved inner disks around Herbig Ae/Be stars, the large statistical uncertainties for the marginally resolved inner disks around the T Tauri stars of our sample make it difficult to extract conclusive constraints for the presence of shadows in these systems.
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Öberg, Karin I. "The Chemistry of Nearby Disks". Proceedings of the International Astronomical Union 10, S314 (novembre 2015): 143–48. http://dx.doi.org/10.1017/s1743921315006237.
Testo completoAbstract (sommario):
AbstractThe gas and dust rich disks around young stars are the formation sites of planets. Observations of molecular trace species have great potential as probes of the disk structures and volatile compositions that together regulate planet formation. The disk around young star TW Hya has become a template for disk molecular studies due to a combination of proximity, a simple face-on geometry and richness in volatiles. It is unclear, however, how typical the chemistry of the TW disk is. In this proceeding, we review lessons learnt from exploring the TW Hya disk chemistry, focusing on the CO snowline, and on deuterium fractionation chemistry. We compare these results with new ALMA observations toward more distant, younger disks. We find that while all disks have some chemical structures in common, there are also substantial differences between the disks, which may be due to different initial conditions, structural or chemical evolutionary stages, or a combination of all three.
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Olson, Edward C. "Photometric Effects of Accretion Disks in Long-Period Eclipsing Binaries". International Astronomical Union Colloquium 107 (1989): 23–34. http://dx.doi.org/10.1017/s0252921100087649.
Testo completoAbstract (sommario):
Accretion disks are invoked in a variety of astrophysical settings, ranging from stellar-mass black holes to active galactic nuclei. There is now little doubt that true accretion disks can also occur in binaries containing non-degenerate stars (Peters 1980; Plavec et al. 1984; Polidan 1987). In this paper, I will discuss some of the properties of disks observed photometrically in the long-period systems KU Cyg and RZ Oph.The most intensively-studied disks are of course those in cataclysmic binaries. Accretion disks in such binaries are thought to have the classical ′alpha′ structure of Shakura and Sunyaev (1973), in which viscous dissipation arising from differential rotation converts gravitational potential energy to thermal energy in supplying the disk luminosity. This physical process requires that the viscous time scale be comparable to the radiative decay time of the disk (Pringle 1981). Most of the disk′s volume is optically thick, so disk luminosity is estimated by integrating black-body or stellar atmospheric fluxes over the disk (see Kriz and Hubeny 1986 for qualifications).
43
Yang, Yuan Hua, Xian Bin Xu, Shui Bing He e Yu Hua Wen. "A Statistics-Based Data Placement Strategy for Hybrid Storage". Applied Mechanics and Materials 644-650 (settembre 2014): 1620–24. http://dx.doi.org/10.4028/www.scientific.net/amm.644-650.1620.
Testo completoAbstract (sommario):
Although hard disk drives have been popular over several decades, there still exists the deficiency because of their slow speeds and high power consumptions. By contrast, flash-based solid state disks exhibit good performance and low power consumption. However, the limited lifetimes become a fatal flaw of solid state disks. In order to take full advantage of hard disk drives and solid state disks, we design a hybrid storage system to make them work in a complementary manner. Further, we propose a data placement scheme for this system to determine the data placement on the underlying solid state disks or hard disk drives based on the data access statistics. Experiment results show that the lifetime of solid state disks and the response time of the system can be significantly improved compared with the alone storage media.
44
Elbakyan, Vardan G., Anders Johansen, Michiel Lambrechts, Vitaly Akimkin e Eduard I. Vorobyov. "Gravitoviscous protoplanetary disks with a dust component". Astronomy & Astrophysics 637 (maggio 2020): A5. http://dx.doi.org/10.1051/0004-6361/201937198.
Testo completoAbstract (sommario):
Aims. We study the dynamics and growth of dust particles in circumstellar disks of different masses that are prone to gravitational instability during the critical first Myr of their evolution. Methods. We solved the hydrodynamics equations for a self-gravitating and viscous circumstellar disk in a thin-disk limit using the FEOSAD numerical hydrodynamics code. The dust component is made up of two different components: micron-sized dust and grown dust of evolving size. For the dust component, we considered the dust coagulation, fragmentation, momentum exchange with the gas, and dust self-gravity. Results. We found that the micron-sized dust particles grow rapidly in the circumstellar disk, reaching a few cm in size in the inner 100 au of the disk during less than 100 kyr after the disk formation, provided that fragmentation velocity is 30 ms−1. Due to the accretion of micron dust particles from the surrounding envelope, which serves as a micron dust reservoir, the approximately cm-sized dust particles continue to be present in the disk for more than 900 kyr after the disk formation and maintain a dust-to-gas ratio close to 0.01. We show that a strong correlation exists between the gas and pebble fluxes in the disk. We find that radial surface density distribution of pebbles in the disk shows power-law distribution with an index similar to that of the Minimum-mass solar nebula regardless the disk mass. We also show that the gas surface density in our models agrees well with measurements of dust in protoplanetary disks of AS 209, HD 163296, and DoAr 25 systems. Conclusions. Pebbles are formed during the very early stages of protoplanetary disk evolution. They play a crucial role in the planet formation process. Our disc simulations reveal the early onset (<105 yr) of an inwards-drifting flux of pebble-sized particles that makes up approximately between one hundredth and one tenth of the gas mass flux, which appears consistent with mm-observations of discs. Such a pebble flux would allow for the formation of planetesimals by streaming instability and the early growth of embryos by pebble accretion. We conclude that unlike the more common studies of isolated steady-state protoplanetary disks, more sophisticated global numerical simulations of circumstellar disk formation and evolution, including the pebble formation from the micron dust particles, are needed for performing realistic planet formation studies.
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Schlutt, M. G., e D. R. Dowling. "Volume Flow-Rate Measurements and Scaling Laws for a Transverse-Inlet Multiple-Disk Fan". Journal of Fluids Engineering 118, n. 4 (1 dicembre 1996): 857–60. http://dx.doi.org/10.1115/1.2835520.
Testo completoAbstract (sommario):
The air moving capacity of a transverse-inlet multiple-disk fan has been measured experimentally and the results successfully collapsed with a simple scaling law. The disk fan studied was an array of several hundred closely spaced plastic disks that rotated about a common axis inside an approximately semicylindrical housing. A splitter plate lying parallel to the axis of rotation and tangent to the disks separated the inflow and outflow streams. The volume flow rate of air per unit length along the axis of rotation was measured using standard LDV techniques. For disk spacings greater than 1 mm, the volume flow rate was found to be essentially proportional to ωhR3, where ω = radian rotation rate of the disks, h = disk spacing, and R = disk radius. This scaling law was derived from a simple force balance for a small fluid element moving between rotating disks. The Reynolds number of the experiments, ωh3R/ν, was varied from 2 × 102 to 103. In addition, the air-moving capability of the disk fan was found to be nearly independent of the geometrical placement of the rotating disks within the fan housing.
46
Semenov, Dmitry A., e Richard D. Teague. "Accretion disks around young stars: the cradles of planet formation". Europhysics News 51, n. 1 (gennaio 2020): 29–32. http://dx.doi.org/10.1051/epn/2020104.
Testo completoAbstract (sommario):
Protoplanetary disks around young stars are the birth sites of planetary systems like our own. Disks represent the gaseous dusty matter left after the formation of their central stars. The mass and luminosity of the star, initial disk mass and angular momentum, and gas viscosity govern disk evolution and accretion. Protoplanetary disks are the cosmic nurseries where microscopic dust grains grow into pebbles, planetesimals, and planets.
47
Morrow, Robert C., e Theodore W. Tibbitts. "Induction of Intumescence Injury on Leaf Disks". Journal of the American Society for Horticultural Science 112, n. 2 (marzo 1987): 304–6. http://dx.doi.org/10.21273/jashs.112.2.304.
Testo completoAbstract (sommario):
Abstract A procedure for inducing intumescence injury (oedema) on leaf disks was developed using a species of wild tomato, Lycopersicon hirsutum PI LA 1625. This procedure used Plexiglas vessels with which various combinations of irradiance direction and leaf disk orientation could be studied. Disks (1.4 cm in diameter) were cut from leaves and floated on distilled water in these vessels. Irradiation was provided by cool-white fluorescent lamps and was filtered through ultraviolet- (UV) absorbing Plexiglas. Disks were scored for injury after a period of 72 hr. Intumescences that developed on leaf disks using this procedure appeared similar in size, shape, and coloration to those on intact plants. Disks taken from leaves that were almost fully expanded developed injury most consistently, and the adaxial disk surface was much more sensitive to this injury than the abaxial surface. Intumescences were generally greater on the disk surface in contact with water. Besides L. hirsutum, injury was induced successfully on disks cut from eucalyptus (Eucalyptus globulus), sweet potato (Ipomoea batatas), tomato (Lycopersicon esculentum), ivy geranium (Pelargonium peltatum), European aspen (Populus tremula), and white potato (Solanum tuberosum).
48
Lubow, Stephen H., e Pawel Artymowicz. "Young Binary Star/Disk Interactions". International Astronomical Union Colloquium 163 (1997): 505–14. http://dx.doi.org/10.1017/s0252921100043116.
Testo completoAbstract (sommario):
AbstractYoung binary star systems have associated gaseous disks that affect their evolution. Strong tidal forces of a binary give rise to gaps that separate circumstellar (CS) disks from the circumbinary (CB) disk. We discuss two important processes that involve binary/disk interactions. The first is eccentricity evolution of a binary embedded in a disk. The second is accretion of gas across the gap that separates CS and CB disks. These results may also have implications for the recently discovered planets.
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Najita, Joan R., Scott J. Kenyon e Benjamin C. Bromley. "From Pebbles and Planetesimals to Planets and Dust: The Protoplanetary Disk–Debris Disk Connection". Astrophysical Journal 925, n. 1 (1 gennaio 2022): 45. http://dx.doi.org/10.3847/1538-4357/ac37b6.
Testo completoAbstract (sommario):
Abstract The similar orbital distances and detection rates of debris disks and the prominent rings observed in protoplanetary disks suggest a potential connection between these structures. We explore this connection with new calculations that follow the evolution of rings of pebbles and planetesimals as they grow into planets and generate dusty debris. Depending on the initial solid mass and planetesimal formation efficiency, the calculations predict diverse outcomes for the resulting planet masses and accompanying debris signature. When compared with debris disk incidence rates as a function of luminosity and time, the model results indicate that the known population of bright cold debris disks can be explained by rings of solids with the (high) initial masses inferred for protoplanetary disk rings and modest planetesimal formation efficiencies that are consistent with current theories of planetesimal formation. These results support the possibility that large protoplanetary disk rings evolve into the known cold debris disks. The inferred strong evolutionary connection between protoplanetary disks with large rings and mature stars with cold debris disks implies that the remaining majority population of low-mass stars with compact protoplanetary disks leaves behind only modest masses of residual solids at large radii and evolves primarily into mature stars without detectable debris beyond 30 au. The approach outlined here illustrates how combining observations with detailed evolutionary models of solids strongly constrains the global evolution of disk solids and underlying physical parameters such as the efficiency of planetesimal formation and the possible existence of invisible reservoirs of solids in protoplanetary disks.
50
Lee, Jung Hoon. "Weak reducing disks and disk surgery". Journal of Knot Theory and Its Ramifications 27, n. 12 (ottobre 2018): 1850069. http://dx.doi.org/10.1142/s0218216518500694.
Testo completoAbstract (sommario):
Let [Formula: see text] be an unknot in [Formula: see text]-bridge position in the [Formula: see text]-sphere. We give an example of a pair of weak reducing disks [Formula: see text] and [Formula: see text] for [Formula: see text] such that both disks obtained from [Formula: see text] ([Formula: see text]) by a surgery along any outermost disk in [Formula: see text], cut off by an outermost arc of [Formula: see text] in [Formula: see text], are not weak reducing disks, i.e. the property of weak reducibility of compressing disks is not preserved by a disk surgery.