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匹配条件: “S. Mathis” ,找到相关结果约324704条。
Melting the core of giant planets: impact on tidal dissipation
S. Mathis
Physics , 2015,
Abstract: Giant planets are believed to host central dense rocky/icy cores that are key actors in the core-accretion scenario for their formation. In the same time, some of their components are unstable in the temperature and pressure regimes of central regions of giant planets and only ab-initio EOS computations can address the question of the state of matter. In this framework, several works demonstrated that erosion and redistribution of core materials in the envelope must be taken into account. These complex mechanisms thus may deeply modify giant planet interiors for which signatures of strong tidal dissipation have been obtained for Jupiter and Saturn. The best candidates to explain this dissipation are the viscoelastic dissipation in the central dense core and turbulent friction acting on tidal inertial waves in their fluid convective envelope. In this work, we study the consequences of the possible melting of central regions for the efficiency of each of these mechanisms.
The variation of the tidal quality factor of convective envelopes of rotating low-mass stars along their evolution
S. Mathis
Physics , 2015,
Abstract: More than 1500 exoplanets have been discovered around a large diversity of host stars (from M- to A-type stars). Tidal dissipation in their convective envelope is a key actor that shapes the orbital architecture of short-period systems and that still remains unknown. Using a simplified two-layer assumption and grids of stellar models, we compute analytically an equivalent modified tidal quality factor, which is proportional to the inverse of the frequency-averaged dissipation due to the viscous friction applied by turbulent convection on tidal waves. It leads the conversion of their kinetic energy into heat and tidal evolution of orbits and spin. During their Pre-Main-Sequence, all low-mass stars have a decrease of the equivalent modified tidal quality factor for a fixed angular velocity of their convective envelope. Next, it evolves on the Main Sequence to an asymptotic value that is minimum for $0.6M_{\odot}$ K-type stars and that increases by several orders of magnitude with increasing stellar mass. Finally, the rotational evolution of low-mass stars strengthens tidal dissipation during the Pre-Main-Sequence.
The variation of tidal dissipation in the convective envelope of low-mass stars along their evolution
S. Mathis
Physics , 2015, DOI: 10.1051/0004-6361/201526472
Abstract: Since 1995, more than 1500 exoplanets have been discovered around a large diversity of host stars (from M- to A-type stars). Tidal dissipation in stellar convective envelopes is a key actor that shapes the orbital architecture of short-period systems. Our objective is to understand and evaluate how tidal dissipation in the convective envelope of low-mass stars (from M to F types) depends on their mass, evolutionary stage and rotation. Using a simplified two-layer assumption, we compute analytically the frequency-averaged tidal dissipation in their convective envelope. This dissipation is due to the conversion into heat of the kinetic energy of tidal non wave-like/equilibrium flow and inertial waves because of the viscous friction applied by turbulent convection. Using grids of stellar models allows us to study the variation of the dissipation as a function of stellar mass and age on the Pre-Main-Sequence and on the Main-Sequence for stars with masses spanning from $0.4$ to $1.4M_{\odot}$. As shown by observations, tidal dissipation in stars varies over several orders of magnitude as a function of stellar mass, age and rotation. During their Pre-Main-Sequence, all low-mass stars have an increase of the frequency-averaged tidal dissipation for a fixed angular velocity in their convective envelope until they reach a critical aspect and mass ratios. Next, the dissipation evolves on the Main Sequence to an asymptotic value that becomes maximum for $0.6M_{\odot}$ K-type stars and that decreases by several orders of magnitude with increasing stellar mass. Finally, the rotational evolution of low-mass stars strengthens the importance of tidal dissipation during the Pre-Main-Sequence for star-planet and multiple star systems.
Transformation of passive electrical networks with distributed PWL-sources using a Model Order Reduction
S. Ludwig,W. Mathis
Advances in Radio Science : Kleinheubacher Berichte , 2009,
Abstract: In modeling of distributed systems with distributed sources large networks with RLC-elements and independent sources arise. This high complexity leads to a high effort in simulations. Therefore model reduction can be used to reduce these networks, preserving the behavior at the observed nodes in the networks. For the reduction of networks with a large number of independent sources only a weak reduction is enabled with standard model reduction techniques. In this paper an efficient reduction of networks with a large number of sources with piece-wise-linear waveforms is presented, using the decomposition of piece-wise-linear functions. With the proposed method a higher reduction of the network and/or a higher accuracy can be achieved with model reduction. The validity and efficiency of the proposed method is shown by reducing a RCI-Grid model.
Large-Scale Magnetic Fields in Stellar Interiors I. Equilibrium Configurations to Model Fossil Fields
V. Duez,S. Mathis
Physics , 2009,
Abstract: Context. The understanding of fossil fields origin, topology and stability is one of the corner stones of the stellar magnetism theory. On one hand, since they survive over secular time-scales, they may modify the structure and the evolution of their host stars. On the other hand, they must have a complex stable structure since it has been demonstrated by Tayler and collaborators that simplest purely poloidal or toroidal fields are unstable on dynamical time-scales. In this context, the only stable configuration which has been found today is the one resulting of a numerical simulation by Braithwaite and collaborators who have studied the evolution of an initial stochastic magnetic field, which is found to relax on a mixed stable configuration (poloidal and toroidal) that seems to be in equilibrium and then diffuses. Aims. In this work, we thus go on the track of such type of equilibrium field in a semi-analytical way. Methods. In this first article, we study the barotropic magnetohydrostatic equilibrium states; the problem reduces to a Grad-Shafranov-like equation with arbitrary functions. Those latters are constrained by deriving the lowest-energy equilibrium states for given invariants of the considered axisymmetric problem and in particular for a given helicity which is known to be one of the main actor of such problems. Then, we obtain the generalization of the force-free Taylor's relaxation states obtained in laboratory experiments (in spheromaks) that become non force-free in the self-gravitating stellar case. The case of general baroclinic equilibrium states will be studied in Paper II. Results. Those theoretical results are applied to realistic stellar cases, namely to the solar radiative core and to the envelope of an Ap star, and discussed. In both cases we assume that the field is initially confined in the stellar radiation zone.
The Warm Ionized Medium in the Milky Way and Other Galaxies
John S. Mathis
Physics , 2000, DOI: 10.1086/317210
Abstract: Observations of the "Warm Ionized Medium" (or, equivalently, the "Diffuse Ionized Gas") of the local ISM, the Perseus arm in the Milky Way, and also in several other galaxies show strong [NII]6563 (~H-alpha in some cases) and [SII]6717/[NII]6583 = 0.6 - 0.7 in all locations and objects. Other line ratios (e.g., [O III]5007/H-beta) vary considerably. Simple photoionization models reproduce the observed spectra, providing extra heating beyond that supplied by photoionization is assumed (Reynolds, Haffner, & Tufte 1999). With observed gas-phase abundances (not solar), the line ratios in the local arm at b = 0 deg are fitted with no extra heating and (S/H) = 13 ppm (solar is 20 ppm). Local gas observed at b = -35 deg requires extra heating of about gamma = 0.75, where gamma is the extra heating in units of 10^{-25} erg H^{-1} s^{-1}. In the Perseus arm, there are similar results, with a domposition consistent with the Galactic abundance gradient. The requirements for NGC 891 are similar to the Perseus arm: little or no extra heating at |z| = 1 kpc and gamma 3 at 2 kpc. In NGC 891 there is also an increase of 5007/H-alpha with |z| that can only come about if most of the ionizing radiation is supplied by stars with T~50000 K. Either their radiation must propagate from the plane to high |z| through very little intervening matter, or else the stars are located at high |z|. The total power requirement of the extra heating is <15% of the photoionization power. [O~II]3727/H-beta can serve as a useful diagnostic of extra heating, but [S~III] 9065,9531/H-alpha is not useful in this regard.
Making a Be star: the role of rotation and pulsations
C. Neiner,S. Mathis
Physics , 2013, DOI: 10.1017/S174392131301507X
Abstract: The Be phenomenon, i.e. the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT satellite brought clear evidence that Be outbursts are directly correlated to pulsations and rapid rotation. In particular the stochastic excitation of gravito-inertial modes, such as those detected by CoRoT in the hot Be star HD 51452, is enhanced thanks to rapid rotation. These waves increase the transport of angular momentum and help to bring the already rapid stellar rotation to its critical value at the surface, allowing the star to eject material. Below we summarize the recent observational and theoretical findings and describe the new picture of the Be phenomenon which arose from these results.
Modellierung von Quanteneffekten in einem ladungsbasierten MOS-Transistor-Modell zur Simulation von nanoskalierten CMOS-Analogschaltungen
S. Stegemann,J. Xiong,W. Mathis
Advances in Radio Science : Kleinheubacher Berichte , 2009,
Abstract: Aufgrund der fortschreitenden Miniaturisierung der Bauelemente in CMOS-Schaltungen und den dadurch erreichten Strukturgr en nehmen quantenmechanische Effekte zunehmenden Einfluss auf die Funktion von Transistoren und damit auf die gesamte Schaltung. Unter Einbeziehung der Energiequantisierung an der Si/SiO2-Grenzfl che wird untersucht, wie sich durch eine Modifikation der Beschreibung des Oberfl chenpotenzials die Inversionsladung quantenmechanisch formulieren l sst. Im Hinblick auf den Entwurf und die Simulation von CMOS-Analogschaltungen wird dazu ein ladungsbasiertes MOS-Transistor-Modell zugrunde gelegt. Die sich daraus ergebenden Ver nderungen für die Kapazit ten und die Inversionsladung werden dabei für die Modellierung des quasiballistischen Drain-Source-Stromes verwendet. Dazu wird innerhalb dieses Modells ein Streufaktor berechnet, mit dem nanoskalierte MOS-Transistoren mit einer Kanall nge von unter 20 nm simuliert werden k nnen. Ausgehend von Parametern eines CMOS-Prozesses werden mit MATLAB die Einflüsse der quantenmechanischen Effekte bei der Skalierung des Transistors analysiert.
Laserangioplastie der Koronargef e bei chronischen Totalverschlüssen und kalzifizierten Stenosen - Ein systemischer Review
Johansson T,Mathis S,Mittermayr T
Journal für Kardiologie , 2010,
Abstract: Die Laserangioplastie bei hochgradig kalzifizierten Stenosen und chronischen Totalverschlüssen wurde entwickelt, um das Lumen für eine Ballondilatation/Stenting zu erweitern. In dieser übersichtarbeit wurden ein RCT und 2 Beobachtungsstudien mit einer Gesamtpopulation von 444 Patienten eingeschlossen. Es wurde keine Studie identifiziert, die eine Aussage zur Wirksamkeit der Laserangioplastie (Laserablationskatheter) bei mit herk mmlichen Ballonkathetern nicht passierbaren Stenosen oder chronischen Totalverschlüssen zulassen. In der einzigen Wirksamkeitsstudie wurde ein laserbasierter Führungsdraht zur Passage chronischer Totalverschlüsse untersucht mit dem Ergebnis, dass der Laserdraht keinen Vorteil gegenüber konventionellen mechanischen Dr hten aufweist. Aufgrund des gegebenen Studiendesigns ein RCT mit einer per Randomisierung und Protokollanalyse ("allocation-switch") und 2 Fallserien ohne Kontrollgruppen ist die St rke der Evidenz zur Wirksamkeit und Sicherheit niedrig. Eine Aufnahme in den Leistungskatalog der sterreichischen Spit ler wird derzeit nicht empfohlen.
The equilibrium tide in stars and giant planets: I - the coplanar case
F. Remus,S. Mathis,J. -P. Zahn
Physics , 2012, DOI: 10.1051/0004-6361/201118160
Abstract: Since 1995, more than 500 extrasolar planets have been discovered orbiting very close to their parent star, where they experience strong tidal interactions. Their orbital evolution depends on the physical mechanisms that cause tidal dissipation, and these are still not well understood. We refine the theory of the equilibrium tide in fluid bodies that are partly or entirely convective, to predict the dynamical evolution of the systems. In particular, we examine the validity of modeling the tidal dissipation by the quality factor Q, as is commonly done. We consider here the simplest case where the considered star or planet rotates uniformly, all spins are aligned, and the companion is reduced to a point-mass. The first manifestation of the tide is to distort the shape of the star or planet adiabatically along the line of centers. This generates the divergence-free velocity field of the adiabatic equilibrium tide which is decoupled from the dynamical tide. The tidal kinetic energy is dissipated into heat through turbulent friction, which is modeled here as an eddy-viscosity acting on the adiabatic tidal flow. This dissipation induces a second velocity field, the dissipative equilibrium tide, which is in quadrature with the exciting potential; it is responsible for the imaginary part of the disturbing function, which is implemented in the dynamical evolution equations, from which one derives characteristic evolution times. The rate at which the system evolves depends on the physical properties of tidal dissipation, and specifically on how the eddy viscosity varies with tidal frequency and on the thickness of the convective envelope for the fluid equilibrium tide. At low frequency, this tide retards by a constant time delay, whereas it lags by a constant angle when the tidal frequency exceeds the convective turnover rate.

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