scholarly journals Tomography of cool giant and supergiant star atmospheres

2019 ◽  
Vol 632 ◽  
pp. A28 ◽  
Author(s):  
K. Kravchenko ◽  
A. Chiavassa ◽  
S. Van Eck ◽  
A. Jorissen ◽  
T. Merle ◽  
...  
Keyword(s):  
Velocity Field ◽  
Phase Shift ◽  
Hysteresis Loop ◽  
Acoustic Waves ◽  
Hysteresis Loops ◽  
Stellar Disk ◽  
Photometric Variability ◽  
Tomographic Method ◽  
Red Supergiants ◽  
Supergiant Star

Context. Red supergiants are cool massive stars and are the largest and the most luminous stars in the Universe. They are characterized by irregular or semi-regular photometric variations, the physics of which is not clearly understood. Aims. The paper aims to derive the velocity field in the red supergiant star μ Cep and to relate it to the photometric variability with the help of the tomographic method. Methods. The tomographic method allows one to recover the line-of-sight velocity distribution over the stellar disk and within different optical-depth slices. This method was applied to a series of high-resolution spectra of μ Cep, and these results are compared to those obtained from 3D radiative-hydrodynamics CO5BOLD simulations of red supergiants. Fluctuations in the velocity field are compared with photometric and spectroscopic variations, the latter were derived from the TiO band strength and serve, at least partly, as a proxy of the variations in effective temperature. Results. The tomographic method reveals a phase shift between the velocity and spectroscopic and photometric variations. This phase shift results in a hysteresis loop in the temperature – velocity plane with a timescale of a few hundred days, which is similar to the photometric one. The similarity between the hysteresis loop timescale measured in μ Cep and the timescale of acoustic waves disturbing the convective pattern suggests that such waves play an important role in triggering the hysteresis loops.

Tomography of the red supergiant star μ Cep

2018 ◽  
Vol 14 (S343) ◽  
pp. 441-442
Author(s):  
K. Kravchenko ◽  
A. Chiavassa ◽  
S. Van Eck ◽  
A. Jorissen ◽  
T. Merle ◽  
...  
Keyword(s):  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Photometric Variability ◽  
Tomographic Method ◽  
Supergiant Star ◽  
Atmospheric Motions

AbstractA tomographic method, aiming at probing velocity fields at depth in stellar atmospheres, is applied to the red supergiant star μ Cep and to snapshots of 3D radiative-hydrodynamics simulation in order to constrain atmospheric motions and relate them to photometric variability.

A polycrystal hysteresis model for ferroelectric ceramics

2006 ◽  
Vol 462 (2069) ◽  
pp. 1573-1592 ◽  
Author(s):  
Y Su ◽  
G.J Weng
Keyword(s):  
Electric Field ◽  
Hysteresis Loop ◽  
Ferroelectric Properties ◽  
Electric Displacement ◽  
Hysteresis Loops ◽  
Ferroelectric Ceramics ◽  
Domain Growth ◽  
Thermodynamic Approach ◽  
Hysteresis Model ◽  
Self Consistent

Most key elements of ferroelectric properties are defined through the hysteresis loops. For a ferroelectric ceramic, its loop is contributed collectively by its constituent grains, each having its own hysteresis loop when the ceramic polycrystal is under a cyclic electric field. In this paper, we propose a polycrystal hysteresis model so that the hysteresis loop of a ceramic can be calculated from the loops of its constituent grains. In this model a micromechanics-based thermodynamic approach is developed to determine the hysteresis behaviour of the constituent grains, and a self-consistent scheme is introduced to translate these behaviours to the polycrystal level. This theory differs from the classical phenomenological ones in that it is a micromechanics-based thermodynamic approach and it can provide the evolution of new domain concentration among the constituent grains. It also differs from some recent micromechanics studies in its secant form of self-consistent formulation and in its application of irreversible thermodynamics to derive the kinetic equation of domain growth. To put this two-level micromechanics theory in perspective, it is applied to a ceramic PLZT 8/65/35, to calculate its hysteresis loop between the electric displacement and the electric field ( D versus E ), and the butterfly-shaped longitudinal strain versus the electric field relation ( ϵ versus E ). The calculated results are found to be in good quantitative agreement with the test data. The corresponding evolution of new domain concentration c 1 and the individual hysteresis loops of several selected grains—along with those of the overall polycrystal—are also illustrated.

scholarly journals CMOS Realization of All-Positive Pinched Hysteresis Loops

Complexity ◽  
2017 ◽  
Vol 2017 ◽  
pp. 1-15 ◽  
Author(s):  
B. J. Maundy ◽  
A. S. Elwakil ◽  
C. Psychalinos
Keyword(s):  
Hysteresis Loop ◽  
Detailed Analysis ◽  
Hysteresis Loops ◽  
Nonlinear Circuits ◽  
Experimental Results ◽  
First Order ◽  
Pinched Hysteresis Loop ◽  
A Charge ◽  
Pinched Hysteresis ◽  
Nmos Transistors

Two novel nonlinear circuits that exhibit an all-positive pinched hysteresis loop are proposed. These circuits employ two NMOS transistors, one of which operates in its triode region, in addition to two first-order filter sections. We show the equivalency to a charge-controlled resistance (memristance) in a decremental state via detailed analysis. Simulation and experimental results verify the proposed theory.

Exchange Bias Properties in Bulk Ni45Co5Mn38Sn12 Heusler Alloy

2014 ◽  
Vol 875-877 ◽  
pp. 272-276 ◽  
Author(s):  
Chao Jing ◽  
Ye Jun Yang ◽  
Dong Hua Yu ◽  
Zhe Li ◽  
Xiao Long Wang ◽  
...  
Keyword(s):  
Hysteresis Loop ◽  
Exchange Bias ◽  
Heusler Alloy ◽  
Magnetic Hysteresis ◽  
Hysteresis Loops ◽  
Bias Field ◽  
Magnetic Hysteresis Loop ◽  
Zero Field ◽  
Co Substitution ◽  
Exchange Bias Field

We report the exchange bias properties in the bulk Ni45Co5Mn38Sn12quaternary Heusler alloy. The ferromagnetic (FM) –antiferromagnetic (AFM) interactions get reinforced after the Co substitution for Ni in the Ni-Mn-Sn alloy, which increase the exchange bias field (HE). A maximum shift in hysteresis loops of 306 Oe was observed in the 10 kOe field cooled sample. The origin of this large exchange bias field has been discussed. Magnetic hysteresis loop obtained in the zero field cooled (ZFC) mode shows double-shifted loop, and the reason of this phenomenon has been explained in detail.

Shape of piezoelectric hysteresis loop for non-ferroelastic switching

2003 ◽  
Vol 784 ◽  
Author(s):  
A. K. Tagantsev ◽  
P. Muralt ◽  
J. Fousek
Keyword(s):  
Thin Films ◽  
Electric Field ◽  
Hysteresis Loop ◽  
Single Crystals ◽  
Applied Electric Field ◽  
Piezoelectric Coefficient ◽  
Hysteresis Loops ◽  
Simple Theory ◽  
The Difference

ABSTRACTA simple theory for the shape of the piezoelectric hysteresis loops (piezoelectric coefficient d vs. applied electric field E) is developed for the case of non-ferroelelastic 180° switching in ferroelectrics. The theory provides explanations for specific features of piezoelectric hysteresis loops, which have been observed in single crystals, thin films and in ceramics in particular. The piezoelectric coefficient may show a “hump”, i.e. when E decreases from the tip of the loop down to zero, d passes through a maximum, and a “nose”, i.e. a self-crossing of the loop close to its tips. The theory also explains the difference in the coercive fields seen in the polarization and piezoelectric loops.

scholarly journals Activity time series of old stars from late F to early K

2019 ◽  
Vol 629 ◽  
pp. A42 ◽  
Author(s):  
N. Meunier ◽  
A.-M. Lagrange
Keyword(s):  
Time Series ◽  
Main Sequence ◽  
Stellar Disk ◽  
Physical Parameters ◽  
Activity Levels ◽  
Main Sequence Stars ◽  
Low Mass Stars ◽  
Photometric Variability ◽  
Chromospheric Emission ◽  
Brightness Variability

Context. A number of high-precision time series have recently become available for many stars as a result of data from CoRoT, Kepler, and TESS. These data have been widely used to study stellar activity. Photometry provides information that is integrated over the stellar disk. Therefore, there are many degeneracies between spots and plages or sizes and contrasts. In addition, it is important to relate activity indicators, derived from photometric light curves, to other indicators (Log R′HK and radial velocities). Aims. Our aim is to understand how to relate photometric variability to physical parameters in order to help the interpretation of these observations. Methods. We used a large number of synthetic time series of brightness variations for old main sequence stars within the F6-K4 range. Simultaneously, we computed using consistent modeling for radial velocity, astrometry, and chromospheric emission. We analyzed these time series to study the effect of the star spectral type on brightness variability, the relationship between brightness variability and chromospheric emission, and the interpretation of brightness variability as a function of spot and plage properties. We then studied spot-dominated or plage-dominated regimes. Results. We find that within our range of activity levels, the brightness variability increases toward low-mass stars, as suggested by Kepler results. However, many elements can create an interpretation bias. Brightness variability roughly correlates to Log R′HK level. There is, however, a large dispersion in this relationship, mostly caused by spot contrast and inclination. It is also directly related to the number of structures, and we show that it can not be interpreted solely in terms of spot sizes. Finally, a detailed analysis of its relation with Log R′HK shows that in the activity range of old main-sequence stars, we can obtain both spot or plage dominated regimes, as was shown by observations in previous works. The same star can also be observed in both regimes depending on inclination. Furthermore, only strong correlations between chromospheric emission and brightness variability are significant. Conclusions. Our realistic time series proves to be extremely useful when interpreting observations and understanding their limitations, most notably in terms of activity interpretation. Inclination is crucial and affects many properties, such as amplitudes and the respective role of spots and plages.

scholarly journals Convective cells in Betelgeuse: imaging through spectropolarimetry

2018 ◽  
Vol 620 ◽  
pp. A199 ◽  
Author(s):  
A. López Ariste ◽  
P. Mathias ◽  
B. Tessore ◽  
A. Lèbre ◽  
M. Aurière ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Linear Polarization ◽  
Rayleigh Scattering ◽  
Characteristic Size ◽  
Spectral Lines ◽  
Stellar Disk ◽  
Convective Structures ◽  
Weak Magnetic Fields ◽  
Red Supergiants ◽  
Convective Cells

Aims. We assess the ability to image the photosphere of red supergiants and, in particular Betelgeuse, through the modelling of the observed linear polarization in atomic spectral lines. We also aim to analyse the resulting images over time, to measure the size and dynamics of the convective structures in these stars. Methods. Rayleigh scattering polarizes the continuum and spectral lines depolarize it. This depolarization is seen as a linear polarization signal parallel to the radial direction on the stellar disk. Integrated over the disk, it would result in a null signal, except if brightness asymmetries/inhomogeneities are present. This is the basic concept behind our imaging technique. Through several tests and comparisons, we have tried to assess and extend its validity, and to determine what can be learnt unambiguously through it. Results. The several tests and comparisons performed prove that our technique reliably retrieves the salient brightness structures in the photosphere of Betelgeuse, and should be relevant to other red supergiants. For Betelgeuse, we demonstrate that these structures we infer are convective cells, with a characteristic size of more than 60% of the stellar radius. We also derive the characteristic upflow and downflow speeds, 22 and 10 km s−1, respectively. We find weak magnetic fields concentrated in the downflow lanes in between granules, similar to the quiet sun magnetism. We follow those convective structures in time. Changes happen on timescales of 1 week, but individual structures can be tracked over 4 yr of observations. Conclusions. The measured characteristics of the convection in Betelgeuse confirm the predictions of numerical simulations in both the strong, supersonic upflows and the size of the convective cells. They also concur in the presence of weak magnetic fields that are completely dominated by the convective flows and constrained to the dark lanes of down-flowing plasma.

Frequency and Amplitude Dependent Dynamic Model of Rotor Elastomeric Damper with Refined Nonlinear Stiffness

2013 ◽  
Vol 275-277 ◽  
pp. 888-893
Author(s):  
Rui Rui Li ◽  
Wei Dong Yang ◽  
Zhi Hao Yu
Keyword(s):  
Hysteresis Loop ◽  
Dynamic Model ◽  
Dynamic Stiffness ◽  
Excitation Frequency ◽  
Hysteresis Loops ◽  
Helicopter Rotor ◽  
Internal Variables ◽  
Load Prediction ◽  
Nonlinear Dynamic Model ◽  
Elastomeric Dampers

Elastomeric damper is a very important component for helicopter rotor system; its dynamic property has strong nonlinear behavior characterized by complex hysteresis loops, and dependence on excitation frequency, amplitude and temperature. Based on internal variables theory, combined with the nonlinear spring model, a time domain nonlinear dynamic model of elastomeric damper used for helicopter rotor load prediction is presented. The model was characterized by using the genetic algorithm, the hysteresis loop of elastomeric dampers made of different elastomeric materials under several excitation frequencies and strain amplitudes was calculated with this model and compared with experimental data. It is shown that the presented model can predict the hysteresis loop of the elastomeric dampers with little relative errors, and the model is able to catch the variation of dynamic stiffness. Therefore, the presented method can be used for helicopter rotor load prediction and aeroelastic analysis.

Resonance dynamics in compressible cavity flows using time-resolved velocity and surface pressure fields

2017 ◽  
Vol 830 ◽  
pp. 494-527 ◽  
Author(s):  
Justin L. Wagner ◽  
Steven J. Beresh ◽  
Katya M. Casper ◽  
Edward P. DeMauro ◽  
Srinivasan Arunajatesan
Keyword(s):  
Velocity Field ◽  
Shear Layer ◽  
Surface Pressure ◽  
Coherent Structures ◽  
Acoustic Waves ◽  
Mode Switching ◽  
Recirculation Region ◽  
Mean Flow ◽  
Time Resolved ◽  
Phase Velocities

The resonance modes in Mach 0.94 turbulent flow over a cavity having a length-to-depth ratio of five were explored using time-resolved particle image velocimetry (TR-PIV) and time-resolved pressure sensitive paint (TR-PSP). Mode switching was quantified in the velocity field simultaneous with the pressure field. As the mode number increased from one through three, the resonance activity moved from a region downstream within the recirculation region to areas further upstream in the shear layer, an observation consistent with linear stability analysis. The second and third modes contained organized structures associated with shear layer vortices. Coherent structures occurring in the velocity field during modes two and three exhibited a clear modulation in size with streamwise distance. The streamwise periodicity was attributable to the interference of downstream-propagating vortical disturbances with upstream-travelling acoustic waves. The coherent structure oscillations were approximately $180^{\circ }$ out of phase with the modal surface pressure fluctuations, analogous to a standing wave. Modal propagation (or phase) velocities, based on cross-correlations of bandpass-filtered velocity fields were found for each mode. The phase velocities also showed streamwise periodicity and were greatest at regions of maximum constructive interference where coherent structures were the largest. Overall, the phase velocities increased with modal frequency, which coincided with the modal activity residing at higher portions of the cavity where the local mean flow velocity was elevated. Together, the TR-PIV and TR-PSP provide unique details not only on the distribution of modal activity throughout the cavity, but also new understanding of the resonance mechanism as observed in the velocity field.

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