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1.
In this work a model has been proposed to explain how the nucleus of a Galaxy can accumulate mass and becomes supermassive — ultimately giving way to gravitational instability leading to an explosion in the nucleus. The process may be repeated many times during the life-span of a Galaxy. The mass shed by the evolved stars populating the central region of the Galaxy can be attracted toward the nuclear core by gravitational pull. Since the incident gas possesses rotational velocity, the centrifugal repulsion of the gas may balance the gravitational pull of the nucleus; thus infall of mass into the nucleus will ordinarily be inhibited. But dissipative agents — such as the prevailing magnetic field and the viscosity of gas — may be sufficient to destroy the rotational velocity of the incident gas and keep the accretion process efficient. The correlation between rotational velocity of gas and its distance from the centre of the Galaxy has been deduced. The radial equation of motion has been solved and the time-scale during which the nucleus accumulates mass sufficient for explosion, has been estimated. 相似文献
2.
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
3.
J. G. Hills 《Astrophysics and Space Science》1976,45(1):243-248
Physical arguments suggest the spiral arms may be manifestations of the galaxy not being in dynamical equilibrium — in the sense that the kinetic energy of tis stars and gas is less relative to its binding energy than that dictated by the virial theorem. Without constant cooling of the galactic disk (i.e., a progressive increase in the binding energy of the galaxy) such a departure from dynamical equilibrium would be corrected and the spiral arms destroyed in about 109 yr due to an increase in the velocity dispersion of the stars in the disk resulting from their interacting with the spiral arms. The rate of cooling required to maintain the spiral arms, about 6×104
L
, may be provided by mass loss from stars in the disk population. The cooling arises from the average scale-heights and velocities of these stars being larger than that of the gas in the disk, so that there is a net loss of kinetic energy and an increase in the binding energy of the galaxy due to the ejected gas settling down to a lower terminal velocity and scale-height in the galactic disk. 相似文献
4.
The model of the solar wind interaction with interstellar medium suggested by Baranovet al. (1970) is developed. In this model (TSM) the presence of two shock waves is assumed, through which the solar wind and interstellar gas pass, the latter moving relative to the Sun at supersonic speed (20 km s–1).The distance between shocks was considered earlier (Baranovet al., 1970; Baranov and Krasnobaev, 1971) to be small compared with their distance from the Sun, due to the hypersonic character of the flow. The structure of the subsonic flow portion may not be taken into account.In the present paper the distribution of the gas parameters in the region between shocks is calculated which, in particular, allows us to estimate the possibility of its experimental detection, observing radio-scintillation on interstellar irregularities (Baranovet al., 1975).The possible influence on the model of galactic hydrogen neutral atoms penetrating into interplanetary medium is estimated. 相似文献
5.
6.
A theory of secondary vorticity is suggested on grounds of an isotropic Hot Big Bang. Vortical motions are generated in front of the shock waves, which appear as a result of supersonic hydrodynamic processes induced by gravitational instability at a later epoch of cosmological expansion. Inside large layers of gas compressed by shocks subsonic turbulence with both vortical and acoustic modes develops. Every such layer may be treated as a protocluster; and eddies, if strong enough, would give rise to protogalaxies. An effective mechanism of vorticity generation — scattering of density inhomogeneities on shock fronts — is examined in detail. Quantitative estimates show that the vortices that are due to this mechanism can be at least the order of magnitude to account for the angular mometa of spiral galaxies. The spectrum of initial irrotational perturbations remains open to discussion, but characteristic amplitudes required do not contradict any observational or theoretical restrictions. 相似文献
7.
A system for obtaining high-precision radial velocities of solar-type stars by spectral cross-correlation has been established at the Mt John University Observatory. The use of a fibre-feed between the telescope and échelle spectrograph has enabled a stability such that we can achieve a precision of better than 50 m s–1.A programme of radial-velocity observations of 29 southern solar-type stars—of which two are IAU radial velocity standard stars—is under way with the prime objective being a search for low-mass companions to the stars. Ten further IAU radial-velocity standard stars are also being monitored. 相似文献
8.
A. Rodríguez-González J. Cantó A. Esquivel A. C. Raga P. F. Velázquez 《Monthly notices of the Royal Astronomical Society》2007,380(3):1198-1206
We present the analytic and numerical models of the 'cluster wind' resulting from the multiple interactions of the winds ejected by the stars of a dense cluster of massive stars. We consider the case in which the distribution of stars (i.e. the number of stars per unit volume) within the cluster is spherically symmetric, has a power-law radial dependence, and drops discontinuously to zero at the outer radius of the cluster. We carry out comparisons between an analytic model (in which the stars are considered in terms of a spatially continuous injection of mass and energy) and 3D gasdynamic simulations (in which we include 100 stars with identical winds, located in 3D space by statistically sampling the stellar distribution function). From the analytic model, we find that for stellar distributions with steep enough radial dependencies, the cluster wind flow develops a very high central density and a non-zero central velocity, and for steeper dependencies, it becomes fully supersonic throughout the volume of the cluster (these properties are partially reproduced by the 3D numerical simulations). Therefore, the wind solutions obtained for stratified clusters can differ dramatically from the case of a homogeneous stellar distribution (which produces a cluster wind with zero central velocity, and a fully subsonic flow within the cluster radius). Finally, from our numerical simulations, we compute predictions of X-ray emission maps and luminosities, which can be directly compared with observations of cluster wind flows. 相似文献
9.
Zdeněk Kopal 《Astrophysics and Space Science》1988,144(1-2):557-586
The aim of the present paper will be to investigate the effects, on the observed radial velocities of the components of close binary systems, of atmospheric motions caused by mutual irradiation of the two stars. Such motions can (and, in general, will) produce systematic differences between the observed radial velocity and that of the centre of mass of the respective star — differences varying with with the phase and thus giving rise to spurious deformations of the star's radial-velocity curves due to orbital motion. A failure to separate the two could (and in general, will) vitiate the physical elements deduced from these curves —such as the masses or absolute dimensions of the components and of the shape of their orbit; but in order to do so, an investigation of atmospheric motions invoked by irradiation becomes a necessary prerequisite.In the Introduction following this abstract, the problem at issue will be described in general terms, and phenomena outlined which should arise in this connection (together with the observations indicating their presence). In Section 2, general expressions for the radial velocity at any point of stellar surface arising from atmospheric motions will be formulated while Section 3 will isolate such velocities for components of close binary systems as are produced by mutual irradiation of their mates, in terms of hydrodynamical equations of radiative transfer describing the problem. In Sections 4 and 5, the effects of non-rotational motions on the observed radial velocities will be specified, and hydrodynamical equations formulated which specify atmospheric convection caused by irradiation of each component of a close binary by its mate. Linearized versions of such equations will be constructed in Section 6; while Section 7 contains an evaluation of the effects which such gas streams exert on the observed radial velocity of the stars.In the concluding Section 8 applications to practical cases are carried out. It will be shown that no reliable spectroscopic elements of close binary systems (including the masses and absolute dimensions of their components) can be obtained until the effects of atmospheric convection caused by mutual irradiation have been accounted for to permit us to convert the observed radial velocities (influenced as they are by the motion of as in which they originate) to those of the centre of mass of the respective stars.Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
10.
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
11.
With the deepening of our knowledge on the stellar Universe, the problem is set forth concerning a further development of the spectral classification of stars —the actual means for investigation of the realm of stars.A new word on the spectral classification of stars — the idea of the MK Process — has been coined by Morgan; it marks a new stage in the study of our Universe. 相似文献
12.
A. Niedzielski 《Astrophysics and Space Science》2003,286(1-2):237-244
The Wolf-Rayet stars represent an advanced stage of evolution of the most massive stars. Their next immediate stadium will
be supernova explosion. The most striking property of this very rare but exceptionally hot and bright objects is their extreme
mass loss, of the order of 10-5 solar mass per year. In turn of evolution before and during the Wolf-Rayet phase such stars eject a lot of matter (∼ 10 M⊙) with velocity up to 3000 km/s that surrounds the min the form of gas and dust. In the case of binary Wolf-Rayet star such
expanding envelope may interact with a companion (usually hot OB star) wind forming a tail extended for ∼ 100 AU. This spectacular
phenomenon as well as some other connected with Wolf-Rayet stars that can be studied with high spatial resolution instruments
(both astrometric and imaging) are reviewed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
R. E. Gershberg 《Astrophysics》1980,16(2):223-227
Conclusions The observations of flares of UV Ceti stars give direct evidence for the existence in these eruptive processes of fast electrons.The photometric, colorimetric, and x-ray characteristics of the flares of the red dwarf stars which are regarded by Gurzadyan as observational confirmations of the fast-electron hypothesis are in reality not such confirmations.Independent data — colorimetric, infrared, and x-ray properties of the flares of UV Ceti stars — demonstrate the incorrectness of Gurzadyan's idea, according to which all manifestations of the activity of flare stars are due ultimately to only Compton radiation and nonthermal bremsstrahlung of fast electrons.Crimean Astrophysical Observatory. Translated from Astrofizika, Vol. 16, No. 2, pp. 375–381, April–June, 1980. 相似文献
14.
J. P. Vallée 《Astrophysics and Space Science》1994,220(2):243-252
Early theories of stellar winds from an association of OB stars predicted the formation of a common superbubble enclosed by athin, dense supershell, through the combined effort of the winds from the stars at the center. These early theories were adequate for explaining the primary observational features (defined as: shell age, outer radius, shell speed, shell mass, shell energy), but they were not adequate to explain the secondary features (defined as: shell thickness, shell magnetic field, shell gas density). A recently published theory for a stellar-wind-bubble and supershell, predicting a range ofthick supershells, can now be compared with the secondary observational features.Using the observed parameters from all large (> 100 pc) interstellar magnetized supershells near the sun (< 1 kpc away), I assemble for comparison with stellar-wind theories: (a) the primary observational features of these shells — I find: large shell age, large outer radius, low shell speed, large shell mass, large shell energy; and (b) some of their secondary observational features — I find: thick shell, low shell magnetic field strengh, low shell gas density. 相似文献
15.
Taking advantage of the fact that Be stars produce a characteristic infra-red excess, the DENIS data — in combination with UBV and IRAS photometric data — can provide us with a quite complete sample of Be stars within about 4 kpc around the sun. This will give us the possibility to carry out further studies to see if there presence is correlated with particular physical characteristics of the interstellar medium surrounding them. 相似文献
16.
During the previous years spacecraft observations of so-called Energetic Neutral Atoms (ENAs) have become an important remote-sensing
technique in planetary science for analyzing the solar wind plasma flow around the upper atmospheric environments of Solar
System bodies. ENAs are produced whenever solar- or stellar wind protons interact via charge exchange with a neutral particle
from a planetary atmosphere so that their signals constrain both, ion distributions and neutral gas densities. The observation
of ENAs which have been generated due to charge exchange with stellar wind plasma have been used for the indirect mass loss
and stellar wind property estimation of Sun-like stars by observing the interaction regions carved out by the collisions between
stellar winds and the interstellar medium. In this work we review ENA-observations and data interpretations at Solar System
planets and recent hydrogen-cloud observations in UV Lyman-α absorption around hydrogen-rich extra-solar gas giants. We discuss the production of stellar wind related hydrogen ENA-clouds
around close-in exoplanets and show how a detailed analysis of attenuation spectra obtained for transiting hydrogen-rich close-in
gas giants can be used for the study of the upper atmosphere structure, the planet’s magnetosphere and to obtain information
on stellar wind properties. Finally, we discuss how future hydrogen cloud observations around exoplanets by space observatories
like the Russia-led World Space Observatory-UV (WSO-UV) together with ESAs planned PLATO mission can be used for the reconstruction
of the solar wind history or the test of magnetosphere evolution hypotheses. 相似文献
17.
R. Ignace † L. M. Oskinova C. Foullon 《Monthly notices of the Royal Astronomical Society》2000,318(1):214-226
We review existing ROSAT detections of single Galactic Wolf–Rayet (WR) stars and develop wind models to interpret the X-ray emission. The ROSAT data, consisting of bandpass detections from the ROSAT All-Sky Survey (RASS) and some pointed observations, exhibit no correlations of the WR X-ray luminosity ( L X ) with any star or wind parameters of interest (e.g. bolometric luminosity, mass-loss rate or wind kinetic energy), although the dispersion in the measurements is quite large. The lack of correlation between X-ray luminosity and wind parameters among the WR stars is unlike that of their progenitors, the O stars, which show trends with such parameters. In this paper we seek to (i) test by how much the X-ray properties of the WR stars differ from the O stars and (ii) place limits on the temperature T X and filling factor f X of the X-ray-emitting gas in the WR winds. Adopting empirically derived relationships for T X and f X from O-star winds, the predicted X-ray emission from WR stars is much smaller than observed with ROSAT . Abandoning the T X relation from O stars, we maximize the cooling from a single-temperature hot gas to derive lower limits for the filling factors in WR winds. Although these filling factors are consistently found to be an order of magnitude greater than those for O stars, we find that the data are consistent (albeit the data are noisy) with a trend of in WR stars, as is also the case for O stars. 相似文献
18.
The time dependent one dimensional hydrodynamic equations describe the evolution of the thermal plasma flow along closed magnetic field lines outside of the plasmasphere. The convection of the supersonic polar wind onto a closed field line results in the assumed formation of collisionless plasma shocks. These shocks move earthward as the field line with its ‘frozen-in’ plasma remains fixed or contracts with time to smaller L coordinates. The high equatorial plasma temperature (of the order of electron volts) produced by the shock process decreases with time if the flow is isothermal but it will increase if the contraction is under adiabatic conditions. Assuming adiabaticity a peak in the temperature forms at the equator in conjunction with a depression in the ion density. After an initial contraction, if the flux tube drifts to higher L coordinates the direction of the shock motion can be reversed so that the supersonic region will expand along the field line towards the state characterizing the supersonic polar wind. A rapid expansion will lower the equatorial density while the temperature decreases with time under adiabatic but not isothermal conditions. 相似文献
19.
Joanna P. Anosova 《Astrophysics and Space Science》1991,177(1-2):351-358
The general problem of investigating multiple stellar systems is formulated. It is shown that the complete solution of this problem requires 1) a complex of astrometric and astrophysical observations of multiple stars, and the 2) maximum attainable precision of such observations. The conditions under which this precision can be achieved are discussed.The most important characteristics of the dynamical states of multiple systems are the total energy E and the relative energies Eij of the bodies in these systems. For eight triple systems (ADS 1630, 2926, 6175, 6650, 6811, 7114, 9626, 9909), statistical tests — a method of calculating the uncertainties E and Eij from the errors of the observational data — are used to find the probabilities for dynamical states and the values Etr±Etr and Ebin±Ebin.Only three triple stars appear certain to be physical systems with a dynamical connection between the components — ADS 6175 and 9909 with the probabilities P>0.80 are dynamical non-hierarchical unstable triple systems with a complicated motions of the components; the final state of these systems is an escape. In the triple system ADS 1630 a qualitative course of the component motions has not been determined because of the larger errors in the observed data. The dynamical evolution of the triple system ADS 9909 is under study. 相似文献
20.
Ju. V. Glagolevsky 《Astrophysics》2005,48(2):194-207
Some difficulties in explaining the slow rotation of CP stars are discussed. The most likely hypotheses are (1) a loss of angular momentum involving a magnetic field during “pre-main sequence” evolution and (2) the slow rotation existed from the very start of the creation of these stars. The braking hypothesis is supported by only one property of CP stars— the lower the mass of the star is, the greater the difference between its average rotation velocity vsini and that of normal stars. On the other hand, there is another property— the lower the rotation speeds of CP stars are, the greater their fraction among normal stars. The latter property supports the hypothesis that the lower the initial rotation speed of a star is when it is created, the greater the probability will become chemically peculiar. This property is inherent in chemically peculiar stars both with and without a magnetic field. It is proposed that the cause of the slow rotation of CP stars must be sought in the very earliest phases of their formation, as should the cause of the separation into chemically peculiar magnetic, chemically peculiar nonmagnetic, and normal stars.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 229–245 (May 2005). 相似文献