Disk Model Research Articles

Wavelength-dependent far-infrared polarization of HL Tau observed with SOFIA/HAWC+

We present the first polarimetric observations of a circumstellar disk in the far-infrared wavelength range. We report flux and linear polarization measurements of the young stellar object HL Tau in the bands A ( um ), C ( um ), D ( um ), and E ( um ) with the High-resolution Airborne Wideband Camera-plus (HAWC+) on board of the Stratospheric Observatory for Infrared Astronomy (SOFIA). The orientation of the polarization vectors is strongly wavelength-dependent and can be attributed to different wavelength-dependent polarization mechanisms in the disk and its local environment. In bands A, C, and D ( um to um ), the orientation of the polarization is roughly consistent with a value of at the maximum emission. Hereby, the magnetic field direction is close to that of the spin axis of the disk. In contrast, in band E ( um ), the orientation is nearly parallel to the minor axis of the projection of the inclined disk. Based on a viscous accretion disk model combined with a surrounding envelope, we performed polarized three-dimensional Monte Carlo radiative transfer simulations. In particular, we considered polarization due to emission and absorption by aligned dust grains, and polarization due to scattering of the thermal reemission (self-scattering). At wavelengths of um um and um we were able to reproduce the observed orientation of the polarization vectors. Here, the origin of polarization is consistent with polarized emission by aligned non-spherical dust grains. In contrast, at a wavelength of um the polarization pattern could not be fully matched, however, applying self-scattering and assuming dust grain radii up to um we were able to reproduce the flip in the orientation of polarization. We conclude that the polarization is caused by dichroic emission of aligned dust grains in the envelope, while at longer wavelengths, the envelope becomes transparent and the polarization is dominated by self-scattering in the disk.

Thermal Profile of Accretion Disk Around Black Hole in 4D Einstein–Gauss–Bonnet Gravity

In this study, we investigate the properties of a thin accretion disk around a static spherically symmetric black hole in 4D Einstein–Gauss–Bonnet gravity, with an additional coupling constant, α, appearing in the spacetime metric. Using the Novikov–Thorne accretion disk model, we examine the thermal properties of the disk, finding that increasing α reduces the energy, angular momentum, and effective potential of a test particle orbiting the black hole. We demonstrate that α can mimic the spin of a Kerr black hole in general relativity up to a≃ 0.23 M for the maximum value of α. Our analysis of the thermal radiation flux shows that larger α values increase the flux and shift its maximum towards the central black hole, while far from the black hole, the solution recovers the Schwarzschild limit. The impact of α on the radiative efficiency of the disk is weak but can slightly alter it. Assuming black-body radiation, we observe that the disk’s temperature peaks near its inner edge and is higher for larger α values. Lastly, the electromagnetic spectra reveal that the disk’s luminosity is lower in Einstein–Gauss–Bonnet gravity compared to general relativity, with the peak luminosity shifting toward higher frequencies, corresponding to the soft X-ray band as α increases.

Unveiling the feeding response of Spodoptera litura to natural host and artificial diet

AbstractBehavioral studies play a crucial role in assessing food utilization in insects. However, there exists a gap in our understanding of the phenotypic plasticity in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) and how it impacts their growth, development, and potential for plant damage. This study aimed to investigate the phenotypic plasticity of S. litura, with a focus on its immature stages, and evaluate the consequences of plant damage and pest management. We conducted in vitro experiments using both a standard artificial diet and tobacco (Nicotiana tabacum L., Solanaceae) plants to assess the growth, development, survival, longevity, reproductive capacity, and egg‐laying capability of S. litura larvae and adults. A type II functional response was estimated for all larval instars, utilizing the Likelihood Holling's Disk model, which provided two key coefficients: the search rate (a) and handling time (Th). Our findings revealed that sixth instars exhibited exceptional agility and efficiency, requiring the shortest time to process each milligram of diet compared with other larval stages. Moreover, when larvae were fed on tobacco, they displayed the shortest developmental time and the highest fecundity. This trend extended to pupal weight, pupal duration, pupal emergence, fecundity, and longevity, which were all significantly higher in individuals fed on tobacco. Additionally, S. litura larvae that fed on tobacco exhibited greater efficiency in converting digested food and lower approximate digestibility compared to larvae fed the artificial diet. These results underscore the importance of focusing on the management of fourth, fifth, and sixth instars, which have the potential to cause severe plant damage. Understanding the functional and nutritional response of S. litura larvae enhances the efficiency of integrated pest management strategies (IPM) at early stages while minimizing ecological impact.

A Be star that turned bright and blue during a major outburst

Classical Be stars are rapidly rotating B-type stars exhibiting Balmer emission lines originating from circumstellar Keplerian gaseous disks, which likely form through episodic mass-ejection events. The currently favored model for Be star disks is the viscous decretion disk (VDD) model. However, the mechanism behind the mass ejection process during the formation of a VDD is a mystery. We present peculiar behavior of the Be star KIC 9715425, which exhibited several minor outbursts (MIBs) and one major outburst (MAB) observed as brightenings in broadband photometry, as well as transient H line emission. The variability may provide valuable keys to understanding the nature of Be outbursts. Based on Kepler the All-Sky Automated Survey for Supernovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS) time-series photometry, the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopy, multi-wavelength observations in the near-ultraviolet (NUV) from the Galaxy Evolution Explorer (GALEX) and optical from Gaia Xinglong 2.16m telescope and Isaac Newton Telescope (INT) of KIC,9715425 covering its outbursts, we determined fundamental parameters of the central star and the circumstellar disk. A frequency analysis and spectral modeling were carried out to characterize the events. During the major outburst, we find a 36% flux increase in the GALEX NUV band compared to 25% in the Kepler band, suggesting that whatever produced the flux increase should be hotter than the central B-type star. This is contradictory to the conventional scenario that the VDD should be cooler. In addition, such a high flux increase can only be accounted for by a luminous disk, which should produce a much stronger Hα emission than observed. The origin of this anomalously hot component remains unexplained. Except for the bluing, the available observations of KIC 9715425 are perfectly compatible with being a normal Be star. If the bluing was physically related to the MAB, the most economical effort to identify the nature of the underlying process could be adaptive NUV and far-ultraviolet (FUV) monitoring of Be stars with cyclically repeating MABs.

At Work with Sustainable Well-Being and Sustainable Performance: Testing the DISC Model Among Office Workers

There is increasing interest in sustainable employment throughout employees’ careers, which makes sustainable work environments more and more important. This study investigates key components of sustainable work systems (i.e., job demands and job resources) and their association with employee sustainable well-being and sustainable performance. Specifically, using two prominent theoretical frameworks, the interaction between job demands and job resources was studied on the one hand and sustainable well-being and performance on the other. A cross-sectional survey study using online questionnaires was performed among 154 office workers of a business operations department. Moderated regression analyses revealed that emotional demands were negatively associated with sustainable performance in the case of low emotional resources (−1 SD, b = −0.14, p = 0.025), and this relation was buffered (and even reversed) in the case of high emotional resources (+1 SD, b = 0.11, p = 0.042). Regarding sustainable well-being, results revealed that higher cognitive job resources were associated with higher sustainable well-being (b = 0.13, p = 0.041). It can be concluded that enhancing job resources as key drivers of sustainable well-being and sustainable performance is important. The discussion addresses theoretical and practical implications, adding to the expanding knowledge of sustainable work systems.

Exploring the dark matter disk model in dwarf galaxies: Insights from the LITTLE THINGS sample

We conducted an analysis of the velocity field of dwarf galaxies in the sample collected by the Local Irregulars That Trace Luminosity Extremes and The H I Nearby Galaxy Survey (LITTLE THINGS) and focused on deriving 2D velocity maps that encompass the transverse and radial velocity fields. Within the range of radial distances in which velocity anisotropies are sufficiently small for the disk to be considered supported by rotation and where the warped geometry of the disk can be neglected, we reconstructed the rotation curve while taking the effect of the asymmetric drift into account. To fit the rotation curves, we employed the standard halo model and the dark matter disk (DMD) model, which assumes that dark matter is primarily confined to the galactic disks and can be traced by the distribution of H I. Interestingly, our analysis revealed that the fits from the DMD model are statistically comparable to those obtained using the standard halo model, but the inferred masses of the galaxies in the DMD model are approximately 10 to 100 times lower than the masses inferred in the standard halo model. In the DMD model, the inner slope of the rotation curve is directly related to a linear combination of the surface density profiles of the stellar and gas components, which generally exhibit a flat core. Consequently, the observation of a linear relation between the rotation curve and the radius in the central disk regions is consistent with the framework of the DMD model.

Warped Accretion Disks and Quasars with Episodic Periodicity of Long-term Variations

It has been found from long-term monitoring campaigns that some quasars are undergoing quasiperiodic variations (most of them with damped amplitudes) in optical bands, but how to explain the origin of such light-curve variations still remains an open question. In this paper, we use the warped accretion disks model to explain the quasiperiodical variations. This model employs a free-bending wave traveling in an accretion disk, which causes the orientation of the central part of the disk to oscillate from the line of sight, resulting in a quasiperiodical variation. We numerically solve the governing equation of warp propagation and calculate the simulated R-band light curves, finding that the periodical light curves generated by this model have damped amplitudes. To compare with observations, we select SDSSJ134820.42+194831.5 as a preliminary example from a sample of periodic quasar candidates by combining CRTS with other public survey data and fitting its light curve with different observational angles. Our result gives a reduced χ 2 ≃ 2.4, implying that the model might give insights into the future application of the warped disk model.

Peculiarities of radio pulsars with long periods

The analysis of parameters of radio pulsars with periods P 5 sec has been carried out. It was found that there is not a clear dependence dP / dt on P on the diagram {dP / dt, P}. Such lack of any dependence can be explained in the frame of the disk model. It is shown that the pulse width decreases with increasing period for the sample considered. It is the opposite of the dependence in the generally accepted pulsar model. Such a behavior can be explained by the dependence of the level of radiation on the period, The alternative explanation is a non-dipole structure of magnetic field in the region of the generation of observed radiation. We consider the possibility of the description of extremely long intervals between successive pulses in pulsars J0901–4046 and J0250+5854 using the model of drift waves at the periphery of the magnetosphere. In the drift model rotation periods of these pulsars are several times shorter than the intervals between observed pulses.

The actuator farm model for large eddy simulation (LES) of wind-farm-induced atmospheric gravity waves and farm–farm interaction

Abstract. This study introduces the actuator farm model (AFM), a novel parameterization for simulating wind turbines within large eddy simulations (LESs) of wind farms. Unlike conventional models like the actuator disk (AD) or actuator line (AL), the AFM utilizes a single actuator point at the rotor center and only requires two to three mesh cells across the rotor diameter. Turbine force is distributed to the surrounding cells using a new projection function characterized by an axisymmetric spatial support in the rotor plane and Gaussian decay in the streamwise direction. The spatial support's size is controlled by three parameters: the half-decay radius r1/2, smoothness s, and streamwise standard deviation σ. Numerical experiments on an isolated National Renewable Energy Laboratory (NREL) 5MW wind turbine demonstrate that selecting r1/2=R (where R is the turbine radius), s between 6 and 10, and σ≈Δx/1.6 (where Δx is the grid size in the streamwise direction) yields wake deficit profiles, turbine thrust, and power predictions similar to those obtained using the actuator disk model (ADM), irrespective of horizontal grid spacing down to the order of the rotor radius. Using these parameters, LESs of a small cluster of 25 turbines in both staggered and aligned layouts are conducted at different horizontal grid resolutions using the AFM. Results are compared against ADM simulations employing a spatial resolution that places at least 10 grid points across the rotor diameter. The wind farm is placed in a neutral atmospheric boundary layer (ABL) with turbulent inflow conditions interpolated from a previous simulation without turbines. At horizontal resolutions finer than or equal to R/2, the AFM yields similar velocity, shear stress, turbine thrust, and power as the ADM. Coarser resolutions reveal the AFM's ability to accurately capture power at the non-waked wind farm rows, although it underestimates the power of waked turbines. However, the far wake of the cluster can be predicted well even when the cell size is of the order of the turbine radius. Finally, combining the AFM with a domain nesting method allows us to conduct simulations of two aligned wind farms in a fully neutral ABL and of wind-farm-induced atmospheric gravity waves under a conventionally neutral ABL, obtaining excellent agreement with ADM simulations but with much lower computational cost. The simulations highlight the AFM's ability to investigate the mutual interactions between large turbine arrays and the thermally stratified atmosphere.

Magnetic seed generation by plasma heat flux in accretion disks

Magnetic batteries are potential sources that may drive the generation of a seed magnetic field, even if this field is initially zero. These batteries can be the result of nonaligned thermodynamic gradients in plasmas, as well as of special and general-relativistic effects. So far, magnetic batteries have only been studied in ideal magnetized fluids. We studied the nonideal fluid effects introduced by the energy flux in the vortical dynamics of a magnetized plasma in curved space-time. We propose a novel mechanism for generating a heat-flux-driven magnetic seed within a simple accretion disk model around a Schwarzschild black hole. We used the 3+1 formalism for the splitting of the space-time metric into space-like and time-like components. We studied the vortical dynamics of a magnetized fluid with a heat flux in the Schwarzschild geometry in which thermodynamic and hydrodynamic quantities are only dependent on the radial coordinate. Assuming that the magnetic field is initially zero, we estimated the linear time evolution of the magnetic field due to the inclusion of nonideal fluid effects. When the thermodynamic and hydrodynamic quantities vary only radially, the effect of the coupling between the heat flux, the space-time curvature, and the fluid velocity acts as the primary driver for an initial linearly time-growing magnetic field. The plasma heat flux completely dominates the magnetic field generation at a specific distance from the black hole, where the fluid vorticity vanishes. This distance depends on the thermodynamical properties of the Keplerian plasma accretion disk. These properties control the strength of the nonideal effects in the generation of seed magnetic fields. We find that heat flux is the main driver of a seed magnetic field in black hole accretion disks if the geometry, plasma dynamics, and thermodynamics share the same axial symmetry. This suggests that nonideal fluid effects may play a major role in the magnetization of astrophysical plasmas.

Numerical modeling of rupture disk as a method of controlling pressure in oil well annulus

The work objective is to present a methodology for computational modeling for pressure control in confined annulus of oil wells using rupture disks. Throughout the oil wellbore cycle life, different operations cause variations in its temperature, causing pressure increases in its annular spaces, a phenomenon called APB (Annular Pressure Build-Up). In some cases, the difference between internal and external pressures on the casing or production columns can result in the loss of their respective integrity caused by burst failure or collapse. When using a rupture disk, once the pressure difference over the column is greater than the disk collapse pressure, there is a hydraulic communication between the annulus, balancing the volume and pressure between them. This pressure balance significantly increases the safety factors of the coatings, whether for burst failure or collapse. The methodology adopted to achieve the objective of this work consists of five macro steps: i) bibliographical review on the use of rupture discs in the context of APB, identifying their functioning and advantages; ii) definition and numerical implementation of pressure balance models following those available in the literature; iii) simulation of a reference scenario to verify pressure balance models; iv) simulations of variations in the rupture disc positioning in the reference scenario and calculation of the casing safety factors (burst and collapse); and v) final evaluation of the disk's ability to control annular pressures and protect the casings. The results indicate that the proposed methodology presents good results compared to the state-of-the-art. Additionally, possible modeling adjustments can allow a closer approximation to reality in the field. The main work contribution is to advance studies on pressure control methods in annular using a methodology proposed in the literature. In addition, to developing a computational tool that reproduces this methodology.

Physics-Informed Neural Network Surrogate Model for Small-Sample Mistuned Bladed Disk

Mistuning in bladed disk structures disrupts their cyclic symmetry, potentially resulting in high cycle fatigue. The stochastic nature of mistuning greatly increases the computational cost of related studies. To improve computational efficiency, previous research has primarily focused on reduced-order models based on finite element methods, whereas the increasing demand for rapid computation and precise predictions has led to the emergence of data-driven approaches. These methods directly use vibration data from bladed disks, eliminating the need for building complex physical models. The accuracy of these methods is dependent on the size of the data, which, however, are often difficult to obtain. To overcome the challenge of insufficient data, this study introduces a Physics-Informed Neural Network Surrogate Model for Small-Sample Mistuned Bladed Disk (PINN-SMBD). This new method combines neural networks with the knowledge of bladed disk dynamics mainly reflected in the following three features: the integration of the governing equations of a mistuned bladed disk in the network architecture, the selection of the input and output according to the frequency independence of the response, and the data augmentation using the cyclic characteristic of bladed disks. Moreover, a dataset construction strategy is proposed that the use of datasets with high variance of mistuning can speed up model convergence and improve accuracy. Finally, a PINN-SMBD model for a dummy bladed disk is built by using only 30 samples which demonstrates remarkable accuracy.

In vitro and ex vivo screening of microRNA combinations with enhanced cell penetrating peptides to stimulate intervertebral disc regeneration.

Low back pain (LBP) is predominantly caused by degeneration of the intervertebral disc (IVD) and central nucleus pulposus (NP) region. Conservative treatments fail to restore disc function, motivating the exploration of nucleic acid therapies, such as the use of microRNAs (miRNAs). miRNAs have the potential to modulate expression of discogenic factors, while silencing the catabolic cascade associated with degeneration. To deliver these miRNAs, nonviral cell penetrating peptides (CPPs) are gaining favor given their low immunogenicity and strong targeting ability. Single miRNA therapies have been investigated for IVD repair, however dual miRNA delivery strategies have not been commonly examined and may augment regeneration. Transfection of four pro-discogenic miRNAs (miRNA mimics:140-5p; 149-5p and inhibitors: 141-3p; 221-3p) and dual delivery of six miRNA pairings was performed using two CPPs, RALA and GET peptide (FLR), in primary rat NP monolayer culture, and in an ex vivo organ culture model of rat caudal discs. Protein expression of discogenic (aggrecan, collagen type II, and SOX9) and catabolic markers (ADAMTS5 and MMP13) were assessed. Monolayer investigations signified enhanced discogenic marker expression following dual miRNA delivery, signifying a synergistic effect when compared to single miRNA transfection. Utilization of an appropriate model was emphasized in our ex vivo organ culture experiment, revealing the establishment of a regenerative microenvironment characterized by reduced catabolic enzyme activity and enhanced matrix deposition, particularly following concurrent delivery of FLR-miRNA-149-5p mimic and miRNA-221-3p inhibitor. Bioinformatics analysis of miRNA-149-5p mimic and miRNA-221-3p inhibitor identified distinct targets, pathways, and interactions, suggesting a mode of action for this amplified response. Our findings suggest the potential of FLR-miRNA-149-5p + miRNA-221-3p inhibitor to create an anti-catabolic niche within the disc to foster regeneration in moderate cases of disc degeneration, which could be utilized in further studies with the overarching aim of developing treatments for LBP.

Kinematical signatures: Distinguishing between warps and radial flows

Context. Increasing evidence shows that warped disks are common, challenging the methods used to model their velocity fields. Molecular line emission of these disks is characterized by a twisted pattern, similar to the signal from radial flows, complicating the study of warped disk kinematics. Previous attempts to model these features have encountered difficulties in distinguishing between the underlying kinematics of different disks. Aims. This study aims to advance gas kinematics modeling capabilities by extending the Extracting Disk Dynamics (eddy) package to include warped geometries and radial flows. We assess the performance of eddy in recovering input parameters for scenarios involving warps, radial flows, and combinations of the two. Additionally, we provide a basis to break the visual degeneracy between warped disks and radial flow, establishing a criterion to distinguish them. Methods. We extended the eddy package to handle warped geometries by including a parametric prescription of a warped disk and a ray-casting algorithm to account for the surface self-obscuration arising from the 3D to 2D projection. The effectiveness of the tool was tested using the radiative transfer code RADMC3D, generating synthetic models for disks with radial flows, warped disks, and warped disks with radial flows. Results. We demonstrate the efficacy of our tool in accurately recovering the geometrical parameters of systems, particularly in data with sufficient angular resolution. Importantly, we observe minimal impact from thermal noise levels typical in Atacama Large Millimeter/submillimeter Array (ALMA) observations. Furthermore, our findings reveal that fitting an incorrect model type produces characteristic residual signatures, which serve as kinematic criteria for disk classification. Conclusions. Characterizing gas kinematics requires careful consideration of twisted motions. While our model provides insights into disk geometries, caution is needed when interpreting parameters in regions with complex kinematics or low-resolution data. Future ALMA baseline observations should help clarify warped disk kinematics.

PMN J1310–5552: A Gamma-Ray-emitting Blazar Candidate Harboring a Cosmic Monster

Relativistic jets manifest some of the most intriguing activities in the nuclear regions of active galaxies. Identifying the most powerful relativistic jets permits us to probe the most luminous accretion systems and, in turn, the most massive black holes. This paper reports the identification of one such object, PMN J1310−5552 (z = 1.56), a blazar candidate of uncertain type detected with the Fermi Large Area Telescope (LAT) and Swift Burst Alert Telescope. The detection of broad emission lines in its optical spectra taken with the X-Shooter and Goodman spectrographs classifies it to be a flat-spectrum radio quasar. The analysis of the Goodman optical spectrum has revealed PMN J1310−5552 harbors a massive black hole (log scale M BH = 9.90 ± 0.07, in M ⊙) and luminous accretion disk (log scale L disk = 46.86 ± 0.03, in erg s−1). The fitting of the observed big blue bump with the standard accretion disk model resulted in the log scale M BH =9.81−0.20+0.19 (in M ⊙) and L disk =46.86−0.09+0.09 (in erg s−1), respectively. These parameters suggest PMN J1310−5552 hosts one of the most massive black holes and the most luminous accretion disks among the blazar population. The physical properties of this enigmatic blazar were studied by modeling the broadband spectral energy distribution considering the data from NuSTAR, Swift, Fermi-LAT, and archival observations. Overall, PMN J1310−5552 is a powerful “MeV” blazar with physical parameters similar to other members of this unique class of blazars. These results provide glimpses of monsters lurking among the unknown high-energy emitters and demonstrate the importance of ongoing wide-field sky surveys to discover them.

Non-cadaveric spine surgery simulator training in neurosurgical residency

BackgroundSpine surgical training faces increasing challenges due to restricted working hours and greater subspecialization. Modern simulators offer a promising approach to teaching both simple and complex spinal procedures. This study evaluated the acceptance and efficacy of spine simulator training using a lumbar herniated disc model tested by 16 neurosurgical residents (PGY-1-6), and compared 3D and 2D teaching methods. MethodsSixteen residents utilized the Realists RealSpine L4/L5 disc simulator with both microscope and exoscope. A mixed-methods analysis assessed the efficacy and acceptance of the training. Six PGY-1 residents participated in a learning curve study, divided into exoscopic and microscopic cohorts. Each group watched a tutorial in either 3D or 2D, followed by three surgical sessions. Endpoints included surgical progress within 30 minutes and complication rates. Microsurgical skills and mental concepts were evaluated on a numeric Likert Scale. ResultsParticipants rated the simulator training favorably, with a median score of 8/10 across six categories. The learning curve study showed a 30% improvement in microsurgical performance. The completion rate of herniated disc removal increased from 50% at T2 to 100% at T3 and T4. Significant improvement in mental concept was observed (p=0.035), with slightly better consolidation in the exoscope group. Self-assessments revealed significantly improved skills across all participants. ConclusionSpine simulator training was well-received and resulted in improvements in both mental concept and microsurgical performance, with enhanced outcomes in the 3D teaching/exoscope group. This study supports the integration of spine simulators into spine surgical residency, particularly for early-stage training, to improve both cognitive and practical surgical skills.

Revisiting the Chemical Composition of WD 1145+017: Impact of Circumstellar Disk Contamination on Photospheric Abundances

We performed a chemical analysis of the asteroid-bearing white dwarf WD 1145+017 using optical and ultraviolet spectroscopic data from 25 epochs between 2015 and 2023. We present an updated gas disk model with improved opacity calculations and temperature profiles to properly account for all circumstellar absorption features. Incorporating these changes into our models, we identified at least 11 elements in the disk, including a detection of circumstellar Na. We detected 16 elements in the photosphere, including new detections of P, Co, and Cu. At 16 elements, WD 1145+017 ties GD 362 as one of the most polluted white dwarfs in terms of the number of elements detected. We find that both the disk and photosphere compositions align, to first order, with CI Chondrite. Our study underscores the importance of accounting for circumstellar absorption, as neglecting them leads to significant abundance errors. Additionally, the analysis of the disk’s opacity highlighted an ultraviolet flux reduction due to a pseudo-continuum due to an optically thick component. This result may affect previous analyses of other polluted white dwarfs, suggesting a need for revisiting some studies.

The influence of disks deformation on the stability analysis of an aircraft braking system

Friction-induced vibration emanating from aircraft braking system is a key issue in the design phase, due to the significant damage it can cause to the brake structure. Although the problem of unstable vibrations in aircraft braking systems has been studied by a number of researchers, the suitability of the mechanical modeling strategy for predicting instabilities remains an open problem. The need for relevant numerical models is therefore essential in order to be as predictive as possible during the design phase. Preliminary studies must therefore be carried out to validate or invalidate the modeling hypotheses traditionally used. Indeed the stability analysis of an aircraft braking system is performed in order to study a low-frequency instability. An industrial model is used, hence reducing the number of degrees of freedom (DoF) is of utmost importance in order to have reasonable computation times. When studying low-frequency phenomena, this can be achieved by neglecting the deformations of the disks. However, no current study has shown that this hypothesis is realistic. So the aim of this paper is to assess the effect of the rigidity hypothesis on the results predicted by the stability analysis. In order to do so, the stability analysis results of a model with rigid disks and one with non-rigid disks are compared, with a particular attention on the main instability phenomenon. It is found that considering rigid disks has a very limited influence on the frequency of the low-frequency eigenmodes, but it over-predicts the real part of the unstable eigenmode. Besides, a component mode synthesis (CMS) technique is shown to reduce significantly the size of the non-rigid disks model while ensuring a satisfying precision regarding eigenmodes prediction.

Examining the brightness variability, accretion disk, and evolutionary stage of the binary OGLE-LMC-ECL-14413

Several intermediate-mass close binary systems exhibit photometric cycles longer than their orbital periods, potentially due to changes in their accretion disks. Past studies indicate that analyzing historical light curves can provide valuable insights into disk evolution and track variations in mass transfer rates within these systems. Our study aims to elucidate both short-term and long-term variations in the light curve of the eclipsing system with a particular focus on the unusual reversals in eclipse depth. We aim to clarify the role of the accretion disk in these fluctuations, especially in long-cycle changes spanning hundreds of days. Additionally, we seek to determine the evolutionary stage of the system and gain insights into the internal structure of its stellar components. We analyzed photometric time series from the Optical Gravitational Lensing Experiment (OGLE) project in the $I$ and $V$ bands, and from the MAssive Compact Halo Objects (MACHO) project in the $B_ M $ and $R_ M $ bands, covering a period of 30.85 years. Using light curve data from 27 epochs, we constructed models of the accretion disk. An optimized simplex algorithm was employed to solve the inverse problem, deriving the best-fit parameters for the stars, orbit, and disk. We also utilized the Modules for Experiments in Stellar Astrophysics ( MESA ) software to assess the evolutionary stage of the binary system, investigating the progenitors and potential future developments. We found an orbital period of 38.15917 pm 0.00054 days and a long-term cycle of approximately 780 days. Temperature, mass, radius, and surface gravity values were determined for both stars. The photometric orbital cycle and the long-term cycle are consistent with a disk containing variable physical properties, including two shock regions. The disk encircles the more massive star and the system brightness variations align with the long-term cycle at orbital phase 0.25. Our mass transfer rate calculations correspond to these brightness changes. MESA simulations indicate weak magnetic fields in the donor star's subsurface, which are insufficient to influence mass transfer rates significantly.

Lack of emission lines in the optical spectra of SAX J1808.4–3658 during reflaring of the 2019 outburst

Aims. We present spectroscopy of the accreting X-ray binary and millisecond pulsar SAX J1808.4–3658. These observations are the first to be obtained during a reflaring phase. We collected spectroscopic data during the beginning of the reflaring of the 2019 outburst and compared them to previous datasets taken at different epochs, both of the same outburst and across the years. To this end, we also present spectra of the source taken during quiescence in 2007, one year before the next outburst. Methods. We made use of data taken by the Very Large Telescope (VLT) X-shooter spectrograph on August 31, 2019, three weeks after the outburst peak. For flux calibration, we used photometric data taken during the same night by the 1m telescopes from the Las Cumbres Observatory network that are located in Chile. We compare our spectra to the quiescent data taken by the VLT-FORS1 spectrograph in September, 2007. We inspected the spectral energy distribution by fitting our data with a multicolored accretion-disk model and sampled the posterior probability density function for the model parameters with a Markov chain Monte Carlo (MCMC) algorithm. Results. We find the optical spectra of the 2019 outburst to be unusually featureless, with no emission lines present despite the high resolution of the instrument. Fitting the UV-optical spectral energy distribution with a disk plus irradiated star model results in a very large value for the inner disk radius of ∼5130 ± 240 km, which could suggest that the disk was emptied of material during the outburst, possibly accounting for the emission-less spectra. Alternatively, the absence of emission lines could be due to a significant contribution of the jet emission at optical wavelengths.