共查询到20条相似文献，搜索用时 968 毫秒
1.
We investigate the secular dynamics of three-body circumbinary systems under the effect of tides. We use the octupolar non-restricted approximation for the orbital interactions, general relativity corrections, the quadrupolar approximation for the spins, and the viscous linear model for tides. We derive the averaged equations of motion in a simplified vectorial formalism, which is suitable to model the long-term evolution of a wide variety of circumbinary systems in very eccentric and inclined orbits. In particular, this vectorial approach can be used to derive constraints for tidal migration, capture in Cassini states, and stellar spin–orbit misalignment. We show that circumbinary planets with initial arbitrary orbital inclination can become coplanar through a secular resonance between the precession of the orbit and the precession of the spin of one of the stars. We also show that circumbinary systems for which the pericenter of the inner orbit is initially in libration present chaotic motion for the spins and for the eccentricity of the outer orbit. Because our model is valid for the non-restricted problem, it can also be applied to any three-body hierarchical system such as star–planet–satellite systems and triple stellar systems. 相似文献
2.
Bengt Gustafsson Jorge Meléndez Martin Asplund David Yong 《Astrophysics and Space Science》2010,328(1-2):185-191
The question whether the solar chemical composition is typical for solar-type stars is analysed by comparing the Sun with different stellar samples, including a sample of stars with very similar parameters, solar twins. Although typical in terms of overall metallicity for stars of solar age and galactic orbit, the solar atmosphere is found to have abundances, as compared with solar twins, that indicate that its gas has once been affected by dust formation and dust separation. It is concluded that this may be related to the formation of the solar planetary system and its special properties. 相似文献
3.
Su-Shu Huang 《Icarus》1973,18(3):339-376
The article deals with the occurrence of planetary systems in the Universe. In Section I, the terms “planet” and “planet-like objects” are defined. Two definitions proposed for the term “planetary system” are examined from the point of view (1) of the relation between planetary systems and binary and multiple star systems and (2) of planetary systems as abodes of intelligent beings. In Section II, the observational search for extrasolar planetary systems is described, as performable by earthbound optical telescopes, by space probes, by long baseline radio interferometry, and finally by inference from the reception of signals sent by intelligent beings in other worlds.In Section III we show that any planetary system must be preceded by a rotating disk of gas and dust around a central mass. Both observational evidence and theoretical reasons indicate the ease of formation of such disk structures in the cosmos. The time scale of collapse of a gaseous medium into a disk and that of the latter's dissipation are examined. This provides us with a new empirical approach and leads us to consider the problem of the frequency of occurrence of planetary systems to be ripe for scientific study. In Section IV, a brief review of theories of the formation of the solar system is given along with a proposed scheme for classification of these theories. In Section V, the evidence for magnetic activity in the early stages of stellar evolution is presented, as developed from six independent clues: the nuclear abundance of light elements, the behavior of flare stars, the intensities of H and K emission in stars, the nonthermal radiation of premain sequence stars, the properties of meteorites, and finally the existence of contact binaries. The magnetic braking theories of solar and stellar rotation are discussed in Section VI, thereby introducing the idea of formation of a rotating disk of gas and dust around stars in Section VII. From this disk a planetary system emerges.Section VIII gives an estimate for the frequency of occurrence of planetary systems in the Universe. It is based on the rotational behavior of main-sequence stars, and concludes that planetary systems have a far greater chance to appear around single main-sequence stars of spectral types later than F5 than around any other kind of star. The combined probability distribution of sizes and masses could be obtained. From physical considerations, it appears that sizes of planetary systems around stars of any given spectral type may not vary greatly from one to another. 相似文献
4.
《天文和天体物理学研究(英文版)》2015,(6)
We present different mass ratio distributions of massive black hole(MBH)binaries due to different mechanisms involved in binary evolution. A binary system of MBHs forms after the merger of two galaxies, which has three stages: the dynamical friction stage, the stellar scattering or circumbinary disk stage, and the gravitational radiation stage. The second stage was once believed to be the "final parsec problem"(FPP) as the binary stalled at this stage because of the depletion of stars. Now, the FPP has been shown to no longer be a problem. Here we get two different mass ratio distributions of MBH binaries under two mechanisms, stellar scattering and the circumbinary disk interaction. For the circumbinary disk mechanism, we assume that the binary shrinks by interaction with a circumbinary disk and the two black holes(BHs)have different accretion rates in the simulation. We apply this simple assumption to the hierarchical coevolution model of MBHs and dark matter halos, and we find that there will be more equal-mass MBH binaries in the final coalescence for the case where the circumbinary mechanism operates. This is mainly because the secondary BH in the circumbinary disk system accretes at a higher rate than the primary one. 相似文献
5.
Using the GADGET-2 code modified by us, we have computed hydrodynamic models of a protoplanetary disk perturbed by a low-mass companion. We have considered the cases of circular and eccentric orbits coplanar with the disk and inclined relative to its midplane. During our simulations we computed the column density of test particles on the line of sight between the central star and observer. On this basis we computed the column density of circumstellar dust by assuming the dust and gas to be well mixed with a mass ratio of 1: 100. To study the influence of the disk orientation relative to the observer on the interstellar extinction, we performed our computations for four inclinations of the line of sight to the disk plane and eight azimuthal directions. The column densities in the circumstellar disk of the central star and the circumbinary disk were computed separately. Our computations have shown that periodic column density oscillations can arise in both inner and circumbinary disks. The amplitude and shape of these oscillations depend on the system’s parameters (the orbital eccentricity and inclination, the component mass ratio) and its orientation in space. The results of our simulations can be used to explain the cyclic brightness variations of young UX Ori stars. 相似文献
6.
P. M. Williams 《Astrophysics and Space Science》1995,224(1-2):267-270
The presence of heated circumstellar dust around WC type Wolf-Rayet stars requires the episodic or persistent condensation of carbon grains in their stellar winds. In order to survive in the stars' strong ultraviolet radiation fields, the grains must be located at least 100AU from the stellar surfaces. The densities in isotropic winds at such large distances are too low to allow grain growth and anisotropies such as clumps, disks or wind-collision wakes in colliding-wind binary systems are required to provide grain nurseries. Observational evidence for such features in grain-forming W-R stars is examined. 相似文献
7.
As low- and intermediate-mass stars reach the asymptotic giant branch (AGB), they have developed into intriguing and complex objects that are major players in the cosmic gas/dust cycle. At this stage, their appearance and evolution are strongly affected by a range of dynamical processes. Large-scale convective flows bring newly-formed chemical elements to the stellar surface and, together with pulsations, they trigger shock waves in the extended stellar atmosphere. There, massive outflows of gas and dust have their origin, which enrich the interstellar medium and, eventually, lead to a transformation of the cool luminous giants into white dwarfs. Dust grains forming in the upper atmospheric layers play a critical role in the wind acceleration process, by scattering and absorbing stellar photons and transferring their outward-directed momentum to the surrounding gas through collisions. Recent progress in high-angular-resolution instrumentation, from the visual to the radio regime, is leading to valuable new insights into the complex dynamical atmospheres of AGB stars and their wind-forming regions. Observations are revealing asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months, as well as more long-lived large-scale structures in the circumstellar envelopes. High-angular-resolution observations indicate at what distances from the stars dust condensation occurs, and they give information on the chemical composition and sizes of dust grains in the close vicinity of cool giants. These are essential constraints for building realistic models of wind acceleration and developing a predictive theory of mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical evolution models. At present, it is still not fully possible to model all these phenomena from first principles, and to predict the mass-loss rate based on fundamental stellar parameters only. However, much progress has been made in recent years, which is described in this review. We complement this by discussing how observations of emission from circumstellar molecules and dust can be used to estimate the characteristics of the mass loss along the AGB, and in different environments. We also briefly touch upon the issue of binarity. 相似文献
8.
A. Kučinskas 《Astrophysics and Space Science》1997,251(1-2):231-234
We have calculated terminal gas outflow velocities from the circumstellar shells of the AGB stars. Our results confirm that,
in case of the stationary dust driven winds, the dynamics of the outflow is governed essentially by the dust properties and
stellar luminosities, while only marginally by other stellar parameters. Although being far from comprehensive, our calculations
also indicate that the dust driven winds may only occur in cool, luminous, low mass objects.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
9.
Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Their X-ray spectra have been important in
constraining physical processes that heat plasma in stellar environments to temperatures exceeding one million degrees. Low-mass
stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population
in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona.
The Sun itself as a typical example of a main-sequence cool star has been a pivotal testbed for physical models to be applied
to cool stars. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal
plasma, although plasma parameters such as temperature, density, and element abundances vary widely. Coronal structure, its
thermal stratification and geometric extent can also be interpreted based on various spectral diagnostics. New features have
been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence
on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate
the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy
and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in
these stars, and put constraints on their most peculiar feature: the stellar wind. Medium and high- resolution spectroscopy
have shed new light on these objects as well. Here, we review recent advances in our understanding of cool and hot stars through
the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion
streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars. 相似文献
10.
The atmospheres and circumstellar envelopes of AGB stars are characterized by complex physical phenomena like shock waves
caused by stellar pulsation or formation of molecules and dust which often lead to a heavy mass loss and have a strong influence
on IR properties as observed by ISO. To allow a physical interpretation of various observations we have constructed improved
dynamical model atmospheres of long-period variables. In this contribution we mainly investigate the dependence of the atmospheric
structure and its variability on stellar pulsation, molecular opacities and time-dependent dust formation. IR spectra resulting
from our models are discussed in detail by Loidl et al. (1997b) and compared to ISO-SWS spectra obtained by Hron et al. (1997).
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
11.
We consider the problem of dust grain survival in the disk winds from T Tauri and Herbig Ae stars. For our analysis, we have chosen a disk wind model in which the gas component of the wind is heated through ambipolar diffusion to a temperature of ～104 K. We show that the heating of dust grains through their collisions with gas atoms is inefficient compared to their heating by stellar radiation and, hence, the grains survive even in the hot wind component. As a result, the disk wind can be opaque to the ultraviolet and optical stellar radiation and is capable of absorbing an appreciable fraction of it. Calculations show that the fraction of the wind-absorbed radiation for T Tauri stars can be from 20 to 40% of the total stellar luminosity at an accretion rate ? a = 10?8-10?6 M ⊙yr?1. This means that the disk winds from T Tauri stars can play the same role as the puffed-up inner rim in current accretion disk models. In Herbig Ae stars, the inner layers of the disk wind (r ≤ 0.5 AU) are dust-free, since the dust in this region sublimates under the effect of stellar radiation. Therefore, the fraction of the radiation absorbed by the disk wind in this case is considerably smaller and can be comparable to the effect from the puffed-up inner rim only at an accretion rate of the order of or higher than 10?6 M ⊙yr?1. Since the disk wind is structurally inhomogeneous, its optical depth toward the observer can be variable, which should be reflected in the photometric activity of young stars. For the same reason, moving shadows from gas and dust streams with a spiral-like shape can be observed in high-angular-resolution circumstellar disk images. 相似文献
12.
《中国天文和天体物理学报》2014,(11)
A high angular resolution near-infrared image that shows the intensity of polarization for the GG Tau A binary system was obtained with the Subaru Telescope.The image shows a circumbinary disk scattering the light from the central binary. The azimuthal profile of the intensity of polarization for the circumbinary disk is roughly reproduced by a simple disk model with the Henyey-Greenstein phase function and the Rayleigh function, indicating there are small dust grains at the surface of the disk.Combined with a previous observation of the circumbinary disk, our image indicates that the gap structure in the circumbinary disk orbits counterclockwise, but material in the disk orbits clockwise. We propose that there is a shadow caused by material located between the central binary and the circumbinary disk. The separations and position angles of the stellar components of the binary in the past 20 yr are consistent with the binary orbit with a = 33.4 AU and e = 0.34. 相似文献
13.
Possible configurations of the planetary systems of the binary stars α Cen A–BandEZAqr A–C are analyzed. The P-type orbits—circumbinary ones, i.e., the orbits around both stars of the binary, are studied. The choice of these systems is dictated by the fact that α Cen is closest to us in the Galaxy, while EZ Aqr is the closest system whose circumbinary planets, as it turns out, may reside in the “habitability zone.” The analysis has been performed within the framework of the planar restricted three-body problem. The stability diagrams of circumbinary motion have been constructed: on representative sets of initial data (in the pericentric distance–eccentricity plane), we have computed the Lyapunov spectra of planetary motion and identified the domains of regular and chaotic motion through their statistical analysis. Based on present views of the dynamics and architecture of circumbinary planetary systems, we have determined the most probable planetary orbits to be at the centers of the main resonance cells, at the boundary of the dynamical chaos domain around the parent binary star, which allows the semimajor axes of the orbits to be predicted. In the case of EZ Aqr, the orbit of the circumbinary planet is near the habitability zone and, given that the boundary of this zone is uncertain, may belong to it. 相似文献
14.
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16.
Shashanka R. Gurumath K. M. Hiremath V. Ramasubramanian 《Journal of Astrophysics and Astronomy》2017,38(2):19
By considering the physical and orbital characteristics of G type stars and their exoplanets, we examine the association between stellar mass and its metallicity that follows a power law. Similar relationship is also obtained in case of single and multiplanetary stellar systems suggesting that, \(\hbox {Sun}^{\prime }\)s present mass is about 1% higher than the estimated value for its metallicity. Further, for all the stellar systems with exoplanets, association between the planetary mass and the stellar metallicity is investigated, that suggests planetary mass is independent of stellar metallicity. Interestingly, in case of multiplanetary systems, planetary mass is linearly dependent on the stellar absolute metallicity, that suggests, metal rich stars produce massive (\(\ge \)1 Jupiter mass) planets compared to metal poor stars. This study also suggests that there is a solar system planetary missing mass of \({\sim }\)0.8 Jupiter mass. It is argued that probably 80% of missing mass is accreted onto the Sun and about 20% of missing mass might have been blown off to the outer solar system (beyond the present Kuiper belt) during early history of solar system formation. We find that, in case of single planetary systems, planetary mass is independent of stellar metallicity with an implication of their non-origin in the host star’s protoplanetary disk and probably are captured from the space. Final investigation of dependency of the orbital distances of planets on the host stars metallicity reveals that inward migration of planets is dominant in case of single planetary systems supporting the result that most of the planets in single planetary systems are captured from the space. 相似文献
17.
We present archival and recent IUE high dispersion spectra of late B stars which reveal the presence of accreting gas with velocities as high as 350 km s–1, collisional ionization of the accreting gas to temperatures above the stellar Teff, and column densities intermediate between those observed toward classical Herbig Ae/Be stars and the nearby proto-planetary system Pictoris. One of the stars, HD 176386, while lacking obvious optical signatures of youth, is a member of the R CrA star formation region, and with an inferred age of 2.8 Myr has not yet arrived on the zero-age main sequence (ZAMS). The other object, an isolated, field B star with pronounced IR excess due to warm, circumstellar dust, 51 Oph, exhibits only modest h emission. The combination of high velocity, accreting gas in systems with IR excesses due to circumstellar dust suggests that not only are these objects candidate proto-planetary systems, but that they may represent an extension to higher stellar masses of the weak-emission pre-main sequence (PMS) stars.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A. 相似文献
18.
V. G. Karetnikov 《Astrophysics and Space Science》1996,246(2):309-314
It is shown that during contact eclipsing binaries evolution under the influence of stellar wind, magnetic stellar wind and with matter transfer by gas flow, in binary stellar systems there may take place a process of star merger (low mass stars) within 105–107 yr and a fast increase of distance between stars of massive binaries. W UMa-type stars are a finite evolutionary stage of very close and low mass binary pairs. As for contact systems of early spectral types (CE-systems), they are more varied in evolution. 相似文献
19.
D. E. Backman S. B. Fajardo-Acosta R. E. Stencel J. R. Stauffer 《Astrophysics and Space Science》1997,255(1-2):91-101
Many nearby main sequence stars are surrounded by cool dust radiating strongly in the far-IR. The characteristics of some
of the prototype systems will be discussed. The dust is understood to represent debris related to planet formation, and the
known disk systems may be dense analogs of the Kuiper Belt structure in our outer solar system. ISO observing programs plan
to address questions regarding how common planetary material is around normal stars and how its amount and location depend
on system age. These questions are central to an understanding of the place of the "Vega/β Pic" disks in stellar evolution.
Preliminary results from the first year of ISO operation will be reported.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
20.
From the concentration of stars, cold gas, and dust in the direction l = 14.?7; b = ?0.?9, we have proved the existence of the SerOB1A association at a distance of about 2.1 kpc from the Sun and described its structure. For 29 stars belonging to the association and its vicinity, we present data on radial velocities and profiles of stellar and interstellar lines derived from original high-resolution spectra and obtained from the literature. New spectroscopic data on the unique couple HD168607 and HD168625 are given including the evidence on belonging of both stars to luminous blue variables (LBVs), proving their mutual proximity and membership in SerOB1 A, and also indicating their participation in evolution of the association. 相似文献