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1.
We investigate the photometric variability of magnetized stars, particularly neutron stars, accreting through a magnetic Rayleigh–Taylor-type instability at the disc–magnetosphere interface, and compare it with the variability during stable accretion, with the goal of looking for possible quasi-periodic oscillations (QPOs). The light curves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, light curves during unstable accretion through tongues penetrating the magnetosphere are more chaotic due to the stochastic behaviour of the tongues, and produce noisier power spectra. However, the power spectra do show some signs of quasi-periodic variability. Most importantly, the rotation frequency of the tongues and the resulting hotspots are close to the inner-disc orbital frequency, except in the most strongly unstable cases. There is therefore a high probability of observing QPOs at that frequency in longer simulations. In addition, the light curves in the unstable regime show periodicity at the star's rotation frequency in many of the cases investigated here, again except in the most strongly unstable cases which lack funnel flows and the resulting antipodal hotspots. The noisier power spectra result in the fractional rms amplitudes of the Fourier peaks being smaller.
We also study in detail the effect of the misalignment angle between the rotation and magnetic axes of the star on the variability, and find that at misalignment angles  ≳25°  the star's period always appears in the light curves.  相似文献   

2.
Jack Wisdom 《Icarus》2008,193(2):637-640
Expressions for tidal dissipation in a body in synchronous rotation at arbitrary orbital eccentricity and obliquity are derived. The rate of tidal dissipation for a synchronously rotating body is compared to that in a body in asymptotic nonsynchronous rotation.  相似文献   

3.
The aim of the present paper has been to investigate the effects, on radial velocities of the components of close binary systems, of atmospheric gas motions caused by mutual irradiation of the two stars. Such motions can produce systematic differences between the observed radial velocity and that of the centre of mass of the respective star, varying with the phase, and give rise to spurious, deformations of the star's radial-velocity curve which have nothing to do with its axial rotation or orbital motion.Quantitative determination of this effect will be given for the simplified case in which the symmetry of gas motion can be described in terms of zonal harmonics of arbitrary degree, and a brief comparison made with the observed radial velocities of the B9-component of the eclipsing system U Cephei, which is known to move in a circular orbit, but exhibits a radial-velocity curve of marked skew-symmetry.  相似文献   

4.
The ice crust of Europa probably floats over a deep liquid-water ocean, and has been continually resurfaced by tectonic and thermal processes driven by tides. Tidal working causes rotational torque, surface stress, internal heating, and orbital evolution. The stress patterns expected on such a crust due to reorientation of the tidal bulge by non-synchronous rotation and due to orbital eccentricity, which introduces periodic ('diurnal') variations in the tide, are shown as global maps. By taking into account the finite rate of crack propagation, global maps are generated of cycloidal features and other distinctive patterns, including the crack shapes characteristic of the wedges region and its antipode on the sub-Jovian hemisphere. Theoretical maps of tidal stress and cracking can be compared with observed tectonics, with the possibility of reconstructing the rotational history of the satellite.  相似文献   

5.
An asynchronous magnetic white dwarf affects the rate of orbital evolution in AM Herculis binaries. An over-synchronous star leads to a positive orbital magnetic torque which reduces the rate of shrinkage of the secondary star's Roche lobe, and hence reduces the mass transfer rate. An opposing effect occurs as a result of the orbital angular momentum loss via secondary mass transfer in the absence of an accretion disc. The modification of the magnetic braking-driven synchronous mass transfer rate is calculated for a range of degrees of asynchronism, and its effect is compared at different orbital periods.  相似文献   

6.
The precession of the orbital plane in a close binary system can provide an important observational tool for investigating dynamical properties of the components. Tidal evolution will always tend to align the rotation axes perpendicular to the orbital plane, thereby eliminating precession. It is pointed out, however, that if observations indicate the existence of a circular orbit and synchronous rotation of the components-which is the outcome of tidal evolution-then precession may still be present, provided the interior of one of the components is, or recently has been, radiative, and is not strongly coupled to the surface layers (where tidal dissipation is greatest). The equations governing precession and nutation are derived in a concise form, and applied to the numerical study of two binary systems. The observational effects are also discussed. Finally, it is pointed out that precession may be present in a subclass of the X-ray binary systems, and its observational significance is briefly discussed.  相似文献   

7.
This report is a review of Darwin’s classical theory of bodily tides in which we present the analytical expressions for the orbital and rotational evolution of the bodies and for the energy dissipation rates due to their tidal interaction. General formulas are given which do not depend on any assumption linking the tidal lags to the frequencies of the corresponding tidal waves (except that equal frequency harmonics are assumed to span equal lags). Emphasis is given to the cases of companions having reached one of the two possible final states: (1) the super-synchronous stationary rotation resulting from the vanishing of the average tidal torque; (2) capture into the 1:1 spin-orbit resonance (true synchronization). In these cases, the energy dissipation is controlled by the tidal harmonic with period equal to the orbital period (instead of the semi-diurnal tide) and the singularity due to the vanishing of the geometric phase lag does not exist. It is also shown that the true synchronization with non-zero eccentricity is only possible if an extra torque exists opposite to the tidal torque. The theory is developed assuming that this additional torque is produced by an equatorial permanent asymmetry in the companion. The results are model-dependent and the theory is developed only to the second degree in eccentricity and inclination (obliquity). It can easily be extended to higher orders, but formal accuracy will not be a real improvement as long as the physics of the processes leading to tidal lags is not better known.  相似文献   

8.
the hypothesis that lineaments on Europa are fractures produced by tidal distortion and planetary volume change is examined by comparing the orientations of dark bands, triple bands, and cuspate ridges to fracture patterns predicted for tidal distortion due to orbital recession and orbital eccentricity. If short, reticulate dark band nnear the anti-Jove point are tension cracks which formed in response to tidal distortion, they could only have been produced by orbital eccentricity. Long, arcuate dark band and triple bands peripheral to the anti-Jove point orientations which suggest that they are strike-slip faults which formed in response to orbital recession. If cuspate ridges are compressional features, their orientations and distribution suggest that they formed in response to combined orbital recession and a decrease in planetary volume. Stresses due to orbital eccentricit could have produced tension cracks near the anti-Jove point only if tensile failure occurred either prior to the accumulation of orbital recession stresses or after they had relaxed. Surface fracturing, if a consequence of tidal deformation, places important constraints on the orbital evolution of Europa.  相似文献   

9.
Since 20 years, a large population of close-in planets orbiting various classes of low-mass stars (from M-type to A-type stars) has been discovered. In such systems, the dissipation of the kinetic energy of tidal flows in the host star may modify its rotational evolution and shape the orbital architecture of the surrounding planetary system. In this context, recent observational and theoretical works demonstrated that the amplitude of this dissipation can vary over several orders of magnitude as a function of stellar mass, age and rotation. In addition, stellar spin-up occurring during the Pre-Main-Sequence (PMS) phase because of the contraction of stars and their spin-down because of the torque applied by magnetized stellar winds strongly impact angular momentum exchanges within star–planet systems. Therefore, it is now necessary to take into account the structural and rotational evolution of stars when studying the orbital evolution of close-in planets. At the same time, the presence of planets may modify the rotational dynamics of the host stars and as a consequence their evolution, magnetic activity and mixing. In this work, we present the first study of the dynamics of close-in planets of various masses orbiting low-mass stars (from \(0.6~M_\odot \) to \(1.2~M_\odot \)) where we compute the simultaneous evolution of the star’s structure, rotation and tidal dissipation in its external convective envelope. We demonstrate that tidal friction due to the stellar dynamical tide, i.e. tidal inertial waves excited in the convection zone, can be larger by several orders of magnitude than the one of the equilibrium tide currently used in Celestial Mechanics, especially during the PMS phase. Moreover, because of this stronger tidal friction in the star, the orbital migration of the planet is now more pronounced and depends more on the stellar mass, rotation and age. This would very weakly affect the planets in the habitable zone because they are located at orbital distances such that stellar tide-induced migration happens on very long timescales. We also demonstrate that the rotational evolution of host stars is only weakly affected by the presence of planets except for massive companions.  相似文献   

10.
The orbital motion of the Sun has been linked with solar variability, but the underlying physics remains unknown. A coupling of the solar axial rotation and the barycentric orbital revolution might account for the relationships found. Some recent published studies addressing the physics of this problem have made use of equations from rotational physics in order to model particle motions. However, our standard equations for rotational velocity do not accurately describe particle motions due to orbital revolution. The Sun's orbital motion is a state of free fall; in consequence, aside from very small tidal motions, the associated particle velocities do not vary as a function of position on or within the body of the Sun. In this note, I describe and illustrate the fundamental difference between particle motions in rotation and revolution, in order to dispel some part of the confusion that has arisen in the past and that which may yet arise in the future. This discussion highlights the principal physical difficulty that must be addressed and overcome by future dynamical spin–orbit coupling hypotheses.  相似文献   

11.
Most transiting planets orbit very close to their parent star, causing strong tidal forces between the two bodies. Tidal interaction can modify the dynamics of the system through orbital alignment, circularization, synchronization and orbital decay by exchange of angular moment. Evidence for tidal circularization in close-in giant planet is well known. Here, we review the evidence for excess rotation of the parent stars due to the pull of tidal forces towards spin-orbit synchronization. We find suggestive empirical evidence for such a process in the present sample of transiting planetary systems. The corresponding angular momentum exchange would imply that some planets have spiralled towards their star by substantial amounts since the dissipation of the protoplanetary disc. We suggest that this could quantitatively account for the observed mass–period relation of close-in gas giants. We discuss how this scenario can be further tested and point out some consequences for theoretical studies of tidal interactions and for the detection and confirmation of transiting planets from radial velocity and photometric surveys.  相似文献   

12.
海潮模型的比较及海潮对地球自转变化的影响   总被引：2，自引：0，他引：2  
近年来,由于卫星测高工作的开展,提供了丰富准确的观测资料,产生出许多新的海潮模型。这些海潮模型的相互比较为研究海洋的精细结构、海潮的动力学、地球动力学提供了依据。另一方面,由现代空间技术和新方法来监测地球自转中的高频变化研究领域也有长足的进展。用这些技术可检测出地球自转中的周日和半日变化,从而激发地球自转的变化。一般来说,海潮影响地球自转的高频变化有两种不同的激发机制。地球的惯性张量的变化即质量项  相似文献   

13.
张瑞玉  李焱 《天文学报》2012,53(4):274-290
通过计算双星演化中的角动量转移,研究了潮汐作用下双星系统自转与公转周期的变化以及潮汐作用对双星演化的影响.结果表明,密近双星系统在主序演化时,潮汐摩擦会在较短的时间内使自转与公转达到比较接近的状态,此后经过一个较长时间的调整才能使自转与公转达到同步转动.物质交换阶段开始后,半相接双星系统更容易出现非同步转动,而相接双星系统物质交换很难破坏系统的同步状态.同时比较了非同步双星模型与同步双星模型演化曲线在赫罗图上的不同,结果表明非同步模型在物质交换阶段主星演化曲线向赫罗图光度和有效温度高的方向移动.最后通过对统计的观测数据进行分析后发现,采用该模型可以解释观测上双星超过潮汐锁定时标后仍然存在非同步转动的现象.  相似文献   

14.
In this paper, we study the behavior of a pair of co-orbital planets, both orbiting a central star on the same plane and undergoing tidal interactions. Our goal is to investigate final orbital configurations of the planets, initially involved in the 1/1 mean-motion resonance (MMR), after long-lasting tidal evolution. The study is done in the form of purely numerical simulations of the exact equations of motions accounting for gravitational and tidal forces. The results obtained show that, at least for equal mass planets, the combined effects of the resonant and tidal interactions provoke the orbital instability of the system, often resulting in collision between the planets. We first discuss the case of two hot-super-Earth planets, whose orbital dynamics can be easily understood in the frame of our semi-analytical model of the 1/1 MMR. Systems consisting of two hot-Saturn planets are also briefly discussed.  相似文献   

15.
Protostars in a group exert gravitational tidal torques on an aspherical nebula located in the group. The net torque transfers angular momentum from the orbital motions of the stars to rotation of the nebula. A relation can be derived between the parameters describing the protostars and the final angular momentum of the nebula. While the parameters concerned are uncertain, a conservative choice results in a value for the angular momentum equal to about 1/3 of that of the present solar system. This suggests that if the Sun formed in a group, tidal interactions with other protostars may account for a significant part of the angular momentum of the solar system.  相似文献   

16.
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17.
T.A. Hurford  R. Greenberg 《Icarus》2007,186(1):218-233
Cycloidal crack patterns on Europa are influenced by tides induced by orbital eccentricity, which in turn is driven by the Laplace orbital resonance. Their shapes potentially record the location of their formation (relative to the direction of Jupiter), as well as the parameters of crack formation. Hoppa et al. [Hoppa, G., Tufts, B.R., Greenberg, R., Geissler, P., 1999a. Icarus 141, 287-298] modeled several cycloid chains using a fixed set of material parameters, but some details did not fit. We now allow material parameters to vary for each arc of an observed cycloid. In general, with minimal variation of model parameters between the arcs, fits are greatly improved. Furthermore, accounting for tidal stress accumulated during non-synchronous rotation, in addition to diurnal stress, allows even better fits. Even with the added freedom in the model our fits allow us to better constrain the location where each cycloid may have formed. Our results support Hoppa et al.'s finding that only a few cracks form ridges per cycle of non-synchronous rotation in the region examined, probably because cracking relieves built up stress until further substantial rotation occurs.  相似文献   

18.
We numerically investigate the dynamics of rotation of several close-in terrestrial exoplanet candidates. In our model, the rotation of the planet is disturbed by the torque of the central star due to the asymmetric equilibrium figure of the planet. We model the shape of the planet by a Jeans spheroid. We use surfaces of section and spectral analysis to explore numerically the rotation phase space of the systems adopting different sets of parameters and initial conditions close to the main spin–orbit resonant states. One of the parameters, the orbital eccentricity, is critically discussed here within the domain of validity of orbital circularization timescales given by tidal models. We show that, depending on some parameters of the system like the radius and mass of the planet, eccentricity etc., the rotation can be strongly perturbed and a chaotic layer around the synchronous state may occupy a significant region of the phase space. 55 Cnc e is an example.  相似文献   

19.
Tidal torque drives the rotational and orbital evolution of planet–satellite and star–exoplanet systems. This paper presents one analytical tidal theory for a viscoelastic multi-layered body with an arbitrary number of homogeneous layers. Starting with the static equilibrium figure, modified to include tide and differential rotation, and using the Newtonian creep approach, we find the dynamical equilibrium figure of the deformed body, which allows us to calculate the tidal potential and the forces acting on the tide generating body, as well as the rotation and orbital elements variations. In the particular case of the two-layer model, we study the tidal synchronization when the gravitational coupling and the friction in the interface between the layers is added. For high relaxation factors (low viscosity), the stationary solution of each layer is synchronous with the orbital mean motion (n) when the orbit is circular, but the rotational frequencies increase if the orbital eccentricity increases. This behavior is characteristic in the classical Darwinian theories and in the homogeneous case of the creep tide theory. For low relaxation factors (high viscosity), as in planetary satellites, if friction remains low, each layer can be trapped in different spin-orbit resonances with frequencies \(n/2,n,3n/2,2n,\ldots \). When the friction increases, attractors with differential rotations are destroyed, surviving only commensurabilities in which core and shell have the same velocity of rotation. We apply the theory to Titan. The main results are: (i) the rotational constraint does not allow us to confirm or reject the existence of a subsurface ocean in Titan; and (ii) the crust-atmosphere exchange of angular momentum can be neglected. Using the rotation estimate based on Cassini’s observation (Meriggiola et al. in Icarus 275:183–192, 2016), we limit the possible value of the shell relaxation factor, when a deep subsurface ocean is assumed, to \(\gamma _s\lesssim 10^{-9}\,\hbox {s}^{-1}\), which corresponds to a shell’s viscosity \(\eta _s\gtrsim 10^{18}\,\hbox {Pa}\,\hbox {s}\), depending on the ocean’s thickness and viscosity values. In the case in which a subsurface ocean does not exist, the maximum shell relaxation factor is one order of magnitude smaller and the corresponding minimum shell’s viscosity is one order higher.  相似文献   

20.
We present the results of a long-term photometric monitoring of the young single main-sequence star HD134319. It shows short-term variability of the optical-band continuum flux with a period of 4.448 days. The variability is attributed to dark spots or spots groups unevenly distributed in longitude on the star's photosphere, whose visibility is modulated by the star's rotation. Maps of the photospheric spot pattern have been obtained with light curve inversion techniques based on the Maximum Entropy and the Tikhonov regularization criteria. The overall spot pattern shows evidence for two long-lasting active longitudes located about 180° apart, with a total area of at least 16% of the stellar surface (assuming an inclination of the stellar rotation axis of 90° on the line of sight). The longitude distribution of the spot pattern and its total area do not show any clear evidence for a long-term variation along the five years of observations. A comparison with recent mean field dynamo models is also addressed, suggesting a possible interpretation of such a behaviour. Singularity, spectral type, youth and a high level of photospheric and chromospheric activity make HD134319 a suitable proxy for studying the magnetic activity of the young Sun not far after its arrival on the zero age main sequence.  相似文献   

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