共查询到20条相似文献，搜索用时 62 毫秒
1.
Places of formation have been derived for a sample of 23 Cepheids with well-determined atmospheric abundances in an attempt
to study the chemical inhomogeneities in the local interstellar medium. The abundance data available for the sample Cepheids
is compiled and critically reviewed to adopt the most reliable esimates. We find that the most conspicuous irregularity in
the metallicity is exhibited by stars that are born in the local arm or in the interarm region. We propose a scenario to explain
these local variations in terms of supernova-induced star formation in interstellar gas enriched by massive stars formed in
the density wave. 相似文献
2.
V. P. Dokuchaev 《Astronomy Letters》2003,29(3):170-174
Radially pulsating stars are shown to radiate fast and slow magnetoacoustic waves into the interstellar gas. No Alfvén waves are excited, because the oscillations are radially symmetric. Calculations were performed for the following two limiting cases: hot, weakly magnetized interstellar plasma and cold plasma with a strong magnetic field. In these limiting cases, pulsating stars excite mostly fast magnetoacoustic waves, while the excitation of slow magnetoacoustic waves is weak. Magnetogasdynamic fields of density, velocity, and magnetic-field perturbations in the interstellar medium were found. Relations were derived to calculate the radiated power and its estimates are given for various conditions in the medium. It is shown that radially stratified wave structures with wavelengths from 1 AU to several tenths of a parsec must exist in the vicinity of pulsating stars. 相似文献
3.
M. A. Barstow D. D. Boyce J. K. Barstow A. E. Forbes B. Y. Welsh R. Lallement 《Astrophysics and Space Science》2009,320(1-3):91-95
We report preliminary results of a search for O VI absorption in the spectra of ～100 hot DA white dwarfs observed by the FUSE satellite. We have carried out a detailed analysis of the radial velocities of interstellar and (where present) stellar absorption lines for the entire sample of stars. In many cases, the velocity differences between the interstellar and photospheric components are below the resolution of the FUSE spectrographs. However, in a significant number of cases the interstellar and photospheric contributions can be separated. In the majority of stars where we find O VI absorption lines, the material is clearly associated with the stellar photosphere and not the interstellar medium. There are a small number of lines-of-sight where the gas is interstellar in nature but the stars are located beyond the boundaries of the local cavity. 相似文献
4.
When the shock wave from a supernova expands, it sweeps up not only interstellar matter but also magnetic field. The field is greatly amplified by compression and will provide the dominant pressure during the cool radiative phase of an expanding supernova shell. We examine a hydromagnetic instability in this system (a form of the Parker instability) and find that it will concentrate gas at intervals of the order of parsecs. The length and time scales make the instability promising as an explanation of the stellar clustering that is seen in Canis Major R1. 相似文献
5.
One of the most important topics in astrophysics concern to the study of how the stars are born. Observational facts show that nearly all stars were born in groups within large massive complexes of gas and dust. An idea almost universally accepted, comes from the fact that gas associated with star formation is very inhomogeneous over a large range of size scales. The stars need to be formed by the fragmentation of large gas complexes. Studies of the interstellar medium show the existence of a hierarchical structure covering at least four orders of magnitude in density and length scale.Recently various studies have been made in an attempt to understand the process, or processes, responsible for the decay of large scales to small ones. Several processes have been suggested, however, some of them seem to be responsible only for generating the hierarchical structure on small scales.A study of the velocity distribution of molecular clouds in the solar vicinity, shows the existence of large velocity gradients perpendicular to the galactic plane. As can be shown, these gradients may be one of the most important sources of the energy responsible for the production of the interstellar turbulence on the largest size scales.On leave from Observatorio Astronomico da Serra da Piedade, Depto. de Fisica, ICEx, UFMG, C.P. 702, 30.161, Belo Horizonte-MG, Brasil. 相似文献
6.
G. A. Pellegatti Franco 《Astrophysics and Space Science》1983,96(1):195-203
Analysis of observational data of OB stars show an, excellent agreement of the density distributions in space ?(x, y, z) as well as in velocity space \(\rho (\dot x,\dot y,\dot z)\) with the predictions of the density wave theory, the values for the density and velocity fluctuations are explained only by the non-linear theory. These theoretical calculations predict perturbations greater than ±10 km s?1, consistent with the observations for the velocity field. Thus one should disregard analytical treatments of the linearized equations since they predict maximum perturbations of ±5km s?1. Another consequence of this is the fact that the Gould's Belt is not a local anomaly, but a local feature of the density waves. The analysis of observational data show that the wave pattern is similar to that of the gas and dust. 相似文献
7.
《Chinese Astronomy and Astrophysics》1981,5(2):132-138
This paper discusses the role played by the interstellar gas in spiral galaxies,using a two-disk model, one for the stars, one for the gas. The following conclusions are drawn. 1. When stars and gas have different velocity dispersions,the stellar and gaseous arm must separate. 2. Such a separation makes the spiral mode of density waves unstable. 3. The ratio η between the densities of gas and stars must be less than a certain value for density waves to be maintained.4.The smaller η is,the more tightly wound will be the arms. 相似文献
8.
Yu. A. Shchekinov 《Astrophysics and Space Science》1989,152(1):43-56
One of interactions of young active stars with interstellar gas is excitation of shock waves, that compress the gas and favour the formation of new generation of stars. Thus, a positive feedback between stellar and gaseous constituents is realized. When spread from point to point this interaction gives rise to a stationary wave of star formation. The properties of such a wave are analyzed both in homogeneous and clumped media.The stationary wave of star formation is a natural mechanism that can provide a coherent behaviour (such as global star bursts) of large star-gas systems. Particularly, the origin of extreme and intermediate halo populations in our Galaxy are possibly produced by large-scale star burst, that was initiated by stationary wave of formation of Population III stars. 相似文献
9.
Cornelis de Jager 《Astronomy and Astrophysics Review》1998,8(3):145-180
We list the main stellar data of known hypergiants and similarly luminous stars, and then concentrate on a review of the
yellow hypergiants. These stars are post-red supergiants evolving along blueward loops in the Hertzsprung-Russell diagram.
Their properties, their location in the Hertzsprung-Russell diagram and their occasional mass ejections are related to a region
of atmospheric instability in the H-R diagram, the Yellow Void. The ‘bouncing against the border of the Void’ of three objects:
Cas, HR8752 and IRC+10420, is described. The apparent atmospheric instability of yellow hypergiants is related to the atmospheric
pulsations. There are indications that the approach to the Void is associated with an increased amplitude of the pulsation
and with enhanced mass loss. The observed small-scale motion field is only apparently strongly supersonic; the observed large
stochastic velocities are the quasi-stochastically varying thermal motions in the many hot sheets that occur in the wakes
of many small shocks, while the real hydrodynamic velocity component is small and subsonic. This shock-wave field is also
responsible for the observed rate of mass loss and for emission in the wings of H. Most yellow hypergiants have envelopes containing gas and dust, but a thick extended envelope, presumably dissipating and
showing bipolar outflow, is only known around IRC+10420. At the interface of the bipolar wind and the interstellar medium
one or more stationary shocks may develop as is observed in the case of IRC+10420 and suspected with Cas.
Received 1 July 1997 相似文献
10.
The critical accretion flow of gas onto compact stars with mass of 0.6M
is investigated by numerical integrations of the time-dependent hydrodynamic equations in the sphericallysymmetric and optically thick case. For the compact stars surrounded by such a dense cloud of gas, the radiation pressure force decelerates the infall gas significantly and free fall regime of the gas is not at all attained. This results in incident low velocities at the standing shock front close to the stellar surface, low temperatures of the gas around the compact stars, and no X-ray in white dwarfs but soft X-rays in neutron stars, respectively. Some applications of the results to the X-ray sources are discussed. 相似文献
11.
Sachin Kaothekar 《Journal of Astrophysics and Astronomy》2016,37(3):23
The effects of finite ion Larmor radius (FLR) corrections, Hall current and radiative heat-loss function on the thermal instability of an infinite homogeneous, viscous plasma incorporating the effects of finite electrical resistivity, thermal conductivity and permeability for star formation in interstellar medium have been investigated. A general dispersion relation is derived using the normal mode analysis method with the help of relevant linearized perturbation equations of the problem. The wave propagation is discussed for longitudinal and transverse directions to the external magnetic field and the conditions of modified thermal instabilities and stabilities are discussed in different cases. We find that the thermal instability criterion gets modified into radiative instability criterion. The finite electrical resistivity removes the effect of magnetic field and the viscosity of the medium removes the effect of FLR from the condition of radiative instability. The Hall parameter affects only the longitudinal mode of propagation and it has no effect on the transverse mode of propagation. Numerical calculation shows stabilizing effect of viscosity, heat-loss function and FLR corrections, and destabilizing effect of finite resistivity and permeability on the thermal instability. The outcome of the problem discussed the formation of star in the interstellar medium. 相似文献
12.
K. O. Thielheim 《Astrophysics and Space Science》1980,73(2):499-502
N-body simulations performed by us suggest a mechanism for the generation of spiral waves in galaxies in which a mutual quasi-ellipsoidal rotating equilibrium configuration increasing slowly by accretion from the surrounding disk influences the density distribution of stars in the disk such as to give rise to a trailing spiral density wave. Interaction of the spiral wave with the viscous interstellar gas and mutual gravitation between the stars in the disk are believed to influence the form of the spiral. Nevertheless the basic assumption of conventional density wave theory according to which the mutual interaction of stars in the disk is essential for the formation of spirals may not be true. 相似文献
13.
Shin'ichirou Yoshida † Shigeyuki Karino Shijun Yoshida Yoshiharu Eriguchi 《Monthly notices of the Royal Astronomical Society》2000,316(1):L1-L4
The first results of numerical analysis of classical r-modes of rapidly rotating compressible stellar models are reported. The full set of linear perturbation equations of rotating stars in Newtonian gravity is solved numerically without the slow rotation approximation. A critical curve of gravitational wave emission induced instability, which restricts the rotational frequencies of hot young neutron stars, is obtained. Taking the standard cooling mechanisms of neutron stars into account, we also show the 'evolutionary curves' along which neutron stars are supposed to evolve as cooling and spinning down proceed. Rotational frequencies of 1.4-M⊙ stars suffering from this instability decrease to around 100 Hz when the standard cooling mechanism of neutron stars is employed. This result confirms the results of other authors, who adopted the slow rotation approximation. 相似文献
14.
Numerical simulations of two-component (stars + gas) self-gravitating galactic disks show that the interstellar gas can significantly affect the dynamical evolution of the disk even if its mass fraction (relative to the total galaxy mass) is as low as several percent. Aided by efficient energy dissipation, the gas becomes gravitationally unstable onlocal scale and forms massive clumps. Gravitational scattering of stars by these clumps leads to suppression of bar instability usually seen in heavy stellar disks. In this case, gas inflow towards the galactic center is driven by dynamical friction which gas clumps suffer instead of bar forcing. 相似文献
15.
Gravitational instability of an infinitely conducting hydromagnetic composite rotating plasma is considered to include simultaneously the finite Larmor radius effects and the frictional effects with neutrals. It is found that Jeans' criterion of instability holds good in the presence of rotation, finite Larmor radius and collisions with neutrals. The particular cases of the above effects on the waves propagated along and perpendicular to the magnetic field have been discussed. The effect of rotation is to decrease the Larmor radius by an amount depending upon the wave number of perturbation. 相似文献
16.
《New Astronomy》2014
Most rapidly and differentially rotating disk galaxies, in which the sound speed (thermal velocity dispersion) is smaller than the orbital velocity, display graceful spiral patterns. Yet, over almost 240 yr after their discovery in M51 by Charles Messier, we still do not fully understand how they originate. In this first paper of a series, the dynamical behavior of a rotating galactic disk is examined numerically by a high-order Godunov hydrodynamic code. The code is implemented to simulate a two-dimensional flow driven by an internal Jeans gravitational instability in a nonresonant wave–“fluid” interaction in an infinitesimally thin disk composed of stars or gas clouds. A goal of this work is to explore the local and linear regimes of density wave formation, employed by Lin, Shu, Yuan and many others in connection with the problem of spiral pattern of rotationally supported galaxies, by means of computer-generated models and to compare those numerical results with the generalized fluid-dynamical wave theory. The focus is on a statistical analysis of time-evolution of density wave structures seen in the simulations. The leading role of collective processes in the formation of both the circular and spiral density waves (“heavy sound”) is emphasized. The main new result is that the disk evolution in the initial, quasilinear stage of the instability in our global simulations is fairly well described using the local approximation of the generalized wave theory. Certain applications of the simulation to actual gas-rich spiral galaxies are also explored. 相似文献
17.
Based on our H α interferometry and 21-cm and CO observations, we analyze the structure and kinematics of the interstellar medium around the stars WR 134 and WR 135. We conclude that the HI bubble found here previously is associated with WR 135, not with WR 134. High-velocity motions of ionized gas that can be interpreted as expansion of the gas swept up by the stellar wind with a velocity up to 50–80 km s?1 are observed around both stars. The line-of-sight velocity field of the ionized hydrogen in the Cygnus arm is shown to agree with the large-scale line-of-sight velocity distribution of the CO emission. 相似文献
18.
19.
Kuniaki Masai 《Astrophysics and Space Science》1984,98(2):367-395
Effects of non-equilibrium ionization on emission spectra are studied. Ionization rate equations are solved with the eigenvalue-method to avoid a numerical instability near the equilibrium and also to save time of computations. Our scheme is valid for linear problems, and is applicable to solve closely coupled equations of time-dependent problems. As an example, we calculate emission spectra (1–160 Å) in the transient phase of ionization under interstellar conditions applicable to a shock-heated plasma. Our results are useful for an analysis of the spectra from tenuous ionizing plasmas-such as supernova remnants. 相似文献
20.
In the last few years, the spatial distribution of old, isolated neutron stars has become of great interest (see, for example, Treves and Colpi (1991)). Several sources of this size have been observed by ROSAT. We present here a computer model of the distribution of the luminosity produced by old, isolated neutron stars accreting from the interstellar medium.We use direct calculations of trajectories in the Galaxy potential, taken in the form given by Paczynski (1990). The system of differential equations was solved numerically. We made calculations on a grid with cell size 100 pc in the R direction and 10 pc in the Z direction (centered at R=50 pc, Z=5 pc and so on). Stars were born in the Galactic plane with a specified velocity distribution corresponding to non-symmetrical supernova explosions.In our model, we assumed that the birth rate of neutron stars is proportional to the square of the local density. The local density was calculated using the data and formulas of Bochkarev (1993) and Zane et al. (1995). We then calculated the luminosity using the Bondi formula (in the inner kiloparsec our results are only a rough estimate).We show that for various mean velocities for the old isolated neutron stars, the distribution of the luminosity has a torus-like structure, with the maximum at 5kpc. Since we made very general assumptions, we argue that this type of distribution is not unique to our Galaxy, and all spiral galaxies should have such a distribution of the luminosity density, associated with accreting old, isolated neutron stars. 相似文献