共查询到20条相似文献，搜索用时 46 毫秒
1.
Javier Ballesteros-Paredes 《Astrophysics and Space Science》2004,292(1-4):193-205
We review the main results from recent numerical simulations of turbulent fragmentation and star formation. Specifically, we discuss the observed scaling relationships, the “quiescent” (subsonic) nature of many star-forming cores, their energy balance, their synthesized polarized dust emission, the ages of stars associated with the molecular gas from which they have formed, the mass spectra of clumps, and the density and column density probability distribution function of the gas. We then give a critical discussion on recent attempts to explain and/or predict the star formation efficiency and the stellar initial mass function from the statistical nature of turbulent fields. Finally, it appears that turbulent fragmentation alone cannot account for the final stages of fragmentation: although the turbulent velocity field is able to produce filaments, the spatial distribution of cores in such filaments is better explained in terms of gravitational fragmentation. 相似文献
2.
G. P. Horedt 《Astrophysics and Space Science》1972,15(1):129-136
We describe two possibilities for the formation of a star by transition of molecular hydrogen from gaseous to the liquid phase in the gravitational field of a spherical mass.Model A.: Contraction through condensation of a polytropic respectively isothermal gas sphere.Model B: Condensation in the isothermal atmosphere which surrounds a core.Model B is sooner possible, because model A introduces certain restrictions of mass and radius of the protostar. 相似文献
3.
Jan Pflamm-Altenburg Pavel Kroupa 《Monthly notices of the Royal Astronomical Society》2009,397(1):488-494
We explore the gravitational influence of pressure-supported stellar systems on the internal density distribution of a gaseous environment. We conclude that compact massive star clusters with masses ≳106 M⊙ act as cloud condensation nuclei and are able to accrete gas recurrently from a warm interstellar medium which may cause further star formation events and account for multiple stellar populations in the most massive globular and nuclear star clusters. The same analytical arguments can be used to decide whether an arbitrary spherical stellar system is able to keep warm or hot interstellar material or not. These mass thresholds coincide with transition masses between pressure supported galaxies of different morphological types. 相似文献
4.
《New Astronomy》2021
Various compact dark matter objects (CDOs) were discussed in the literature. Typically parameters of CDOs, such as the mass and the distance, were evaluated by using the gravitational microlensing effect. However, this method has limitations. We propose an alternative method for detecting and measuring parameters of CDOs. It is based on the scenario where there is a star having one planet, such that the orbital plane of the planet does not contain the star. This indicates the presence of a gravitating object located far away at the axis directed from the star to the planetary orbital plane. If in this direction there is no visible star, this could mean that the distant gravitating object is a CDO. We derived analytical expressions for determining the unknown mass of the CDO and its unknown distance from the star based on the parameters of the planetary orbit. We believe that this method could help obtaining additional observational data on the CDOs in particular and therefore on dark matter in general. 相似文献
5.
E. I. Staritsin 《Astronomy Letters》2010,36(11):796-807
An upper limit for the mass loss rate of rapidly rotating main-sequence O9-B4 stars has been determined. Themaximum mass loss
rate of a rotating star is determined by the ability of radiation pressure in lines to remove matter from the gravitational
potential well of the star. The maximum mass loss rate in the case of extremely rapid stellar rotation is a factor of 3–7
higher than that in the case of a nonrotating star. A simple formula for determining the ratio of the maximum mass loss rate
of a rotating star to the maximum mass loss rate of a nonrotating star with the same mass, luminosity, and volume is suggested. 相似文献
6.
Shin'ichirou Yoshida Yoshiharu Eriguchi 《Monthly notices of the Royal Astronomical Society》2000,316(4):917-922
We have investigated the influence of the r-mode instability on hypercritically accreting neutron stars in close binary systems during their common envelope phases, based on the scenario proposed by Brown et al. On the one hand, neutron stars are heated by the accreted matter at the stellar surface, but on the other hand they are also cooled down by the neutrino radiation. At the same time, the accreted matter transports its angular momentum and mass to the star. We have studied the evolution of the stellar mass, temperature and rotational frequency.
The gravitational-wave-driven instability of the r-mode oscillation strongly suppresses spinning up of the star, the final rotational frequency of which is well below the mass-shedding limit, in fact typically as low as 10 per cent of that of the mass-shedding state. On a very short time-scale the rotational frequency tends to approach a certain constant value and saturates there, as long as the amount of accreted mass does not exceed a certain limit to collapse to a black hole. This implies that a similar mechanism of gravitational radiation to that in the so-called 'Wagoner star' may work in this process. The star is spun up by accretion until the angular momentum loss by gravitational radiation balances the accretion torque. The time-integrated dimensionless strain of the radiated gravitational wave may be large enough to be detectable by gravitational wave detectors such as LIGO II. 相似文献
The gravitational-wave-driven instability of the r-mode oscillation strongly suppresses spinning up of the star, the final rotational frequency of which is well below the mass-shedding limit, in fact typically as low as 10 per cent of that of the mass-shedding state. On a very short time-scale the rotational frequency tends to approach a certain constant value and saturates there, as long as the amount of accreted mass does not exceed a certain limit to collapse to a black hole. This implies that a similar mechanism of gravitational radiation to that in the so-called 'Wagoner star' may work in this process. The star is spun up by accretion until the angular momentum loss by gravitational radiation balances the accretion torque. The time-integrated dimensionless strain of the radiated gravitational wave may be large enough to be detectable by gravitational wave detectors such as LIGO II. 相似文献
7.
Using numerical simulations, we studied several coupled translational and rotational solutions of the two-finite-body problem with one spherical and one triaxial body. The aim was to investigate which types of orbits and planetary bodies could produce spin-induced orbital perturbations relevant enough to add to models dealing with other perturbations. To fully assess the strengths and consequences of this perturbation, we did not include any other perturbation even when a more realistic scenario would have required it. Interesting results concern planet–star mass ratios like a hot Jupiter or a super-Jupiter around a star like the Sun or the red dwarf Proxima Centauri. The short-period chaotic effect of the gravitational spin–orbit perturbation on highly eccentric orbits in the vicinity of the Roche limit can be a prominent feature. It should be taken into account when studying the tidal evolution of such a planet or its interactions with any companion in the neighborhood of the star. 相似文献
8.
M. K. Mak T. Harko Department of Physics The Hong Kong University of Science Technology Clear Water Bay Hong Kong Department of Physics The University of Hong Kong Pokfulam Hong Kong 《中国天文和天体物理学报》2002,2(3):248-259
We present an exact analytical solution of the gravitational field equations describing a static spherically symmetric anisotropic quark matter distribution. The radial pressure inside the star is assumed to obey a linear equation of state, while the tangential pressure is a complicated function of the radial coordinate. In order to obtain the general solution of the field equations a particular density profile inside the star is also assumed. The anisotropic pressure distribution leads to an increase in the maximum radius and mass of the quark star, which in the present model is around three solar masses. 相似文献
9.
10.
The gravitational collapse of molecular clouds or cloud cores is expected to lead to the formation of stars that begin their lives in a state of rapid rotation. It is known that, in at least some specific cases, rapidly rotating, slf-gravitating bodies are subject to instabilities that cause them to assume ellipsoidal shapes. In this paper we investigate the consequences of such instabilities on the angular momentum evolution of a star in the process of formation from a collapsing cloud, and surrounded by a protostellar disk, with a view toward applications to the formation of the Solar System. We use a specific model of star formation to demonstrate the possibility that such a star would become unstable, that the resulting distortion of the star would generate spiral density waves in the circumstellar disk, and that the torque associated with these waves would regulate the angular momentum of the star as it feeds angular momentum to the disk. We conclude that the angular momentum so transported to the disk would not spread the disk to, say, Solar System dimensions, by the action of the spiral density waves alone. However, a viscous disk could effectively extract stellar angular momentum and attain Solar System size. Our results also indicate that viscous disks could feed mass and angular momentum to a growing protostar in such a manner that distortions of the star would occur before gravitational torques could balance the influx of angular momentum. In other situations (in which the viscosity was small), a gap could be cleared between the disk and star. 相似文献
11.
Using a consistent perturbation theory for collisionless disk-like and spherical star clusters, we construct a theory of slow
modes for systems having an extended central region with a nearly harmonic potential due to the presence of a fairly homogeneous
(on the scales of the stellar system) heavy, dynamically passive halo. In such systems, the stellar orbits are slowly precessing,
centrally symmetric ellipses (2: 1 orbits). We consider star clusters with monoenergetic distribution functions that monotonically
increase with angular momentum in the entire range of angular momenta (from purely radial orbits to circular ones) or have
a growing region only at low angular momenta. In these cases, there are orbits with a retrograde precession, i.e., in a direction
opposite to the orbital rotation of the star. The presence of a gravitational loss-cone instability, which is also observed
in systems of 1: 1 orbits in near-Keplerian potentials, is associated with such orbits. In contrast to 1: 1 systems, the loss-cone
instability takes place even for distribution functions monotonically increasing with angular momentum, including those for
systems with circular orbits. The regions of phase space with retrograde orbits do not disappear when the distribution function
is smeared in energy. We investigate the influence of a weak inhomogeneity of a heavy halo with a density that decreases with
distance from the center. 相似文献
12.
Nils Andersson & Kostas D. Kokkotas 《Monthly notices of the Royal Astronomical Society》1998,299(4):1059-1068
We present new results for pulsating neutron stars. We have calculated the eigenfrequencies of the modes that one would expect to be the most important gravitational wave sources: the fundamental fluid f mode, the first pressure p mode and the first gravitational wave w mode, for twelve realistic equations of state. From these numerical data we have inferred a set of 'empirical relations' between the mode frequencies and the parameters of the star (the radius R and the mass M ). Some of these relations prove to be surprisingly robust, and we show how they can be used to extract the details of the star from observed modes. The results indicate that, should the various pulsation modes be detected by the new generation of gravitational wave detectors that come online in a few years, the mass and the radius of neutron stars can be deduced with errors no larger than a few per cent. 相似文献
13.
The evolution of the family of binaries with a low-mass star and a compact neutron star companion (low-mass X-ray binaries (LMXBs) with neutron stars) ismodeled by the method of population synthesis. Continuous Roche-lobe filling by the optical star in LMXBs is assumed to be maintained by the removal of orbital angular momentum from the binary by a magnetic stellar wind from the optical star and the radiation of gravitational waves by the binary. The developed model of LMXB evolution has the following significant distinctions: (1) allowance for the effect of the rotational evolution of a magnetized compact remnant on themass transfer scenario in the binary, (2) amore accurate allowance for the response of the donor star to mass loss at the Roche-lobe filling stage. The results of theoretical calculations are shown to be in good agreement with the observed orbital period-X-ray luminosity diagrams for persistent Galactic LMXBs and their X-ray luminosity function. This suggests that the main elements of binary evolution, on the whole, are correctly reflected in the developed code. It is shown that most of the Galactic bulge LMXBs at luminosities L x > 1037 erg s?1 should have a post-main-sequence Roche-lobe-filling secondary component (low-mass giants). Almost all of the models considered predict a deficit of LMXBs at X-ray luminosities near ～1036.5 erg s?1 due to the transition of the binary from the regime of angular momentum removal by a magnetic stellar wind to the regime of gravitational waves (analogous to the widely known period gap in cataclysmic variables, accreting white dwarfs). At low luminosities, the shape of the model luminosity function for LMXBs is affected significantly by their transient behavior-the accretion rate onto the compact companion is not always equal to the mass transfer rate due to instabilities in the accretion disk around the compact object. The best agreement with observed binaries is achieved in the models suggesting that heavy neutron stars with masses 1.4–1.9M ⊙ can be born. 相似文献
14.
Fast rotation of compact stars (at sub-millisecond period) and, in particular, their stability, are sensitive to the equation of state (EOS) of dense matter. Recent observations of XTE J1739-285 suggest that it contains a neutron star rotating at 1122 Hz. At such rotational frequency the effects of rotation on star’s structure are significant. We study the interplay of fast rotation, EOS, and gravitational mass of a sub-millisecond pulsar. We discuss the EOS dependence of spin-up to a sub-millisecond period, via mass accretion from a disk in a low-mass X-ray binary. 相似文献
15.
GP. Horedt 《Astrophysics and Space Science》1976,45(2):353-367
In dense interstellar clouds or in the surroundings of just-formed stars, the larger grains form proto-cores by segregating from the gas under the influence of the gravitational field of the cloud during intervals of the order 108 yr.If the mass of the proto-cores is smaller than a certain limit, the object possesses a negligible atmosphere and remains composed mainly of solid grain material. If the mass of the proto-core exceeds the limiting mass, the proto-core can bind around it an atmosphere. When the temperature of the opaque part of the atmosphere is only fractions of a degree above the temperature of the surrounding tenuous gas, gravitational contraction of the opaque part of the atmosphere sets in, forming gaseous objects up to stellar mass range.Binary and multiple systems originate from neighbouring proto-cores, through gravitational contraction of both their separate and common outer atmospheres.Pycnonuclear reactions are not able to prevent a star with mass 0.08M
from cooling to the black dwarf stadium. 相似文献
16.
T. Naylor A. Allan K. S. Long 《Monthly notices of the Royal Astronomical Society》2005,361(3):1091-1101
We present a phase-resolved spectroscopic study of the secondary star in the cataclysmic variable (CV) U Gem. We use our data to measure the radial velocity semi-amplitude, systemic velocity and rotational velocity of the secondary star. Combining this with literature data allows us to determine masses and radii for both the secondary star and white dwarf, which are independent of any assumptions about their structure. We use these to compare their properties to those of field stars and find that both components follow field mass–radius relationships. The secondary star has the mass, radius, luminosity and photometric temperature of an M2 star, but a spectroscopic temperature of M4. The latter may well be due to a high metallicity. There is a troubling inconsistency between the radius of the white dwarf inferred from its gravitational redshift and inclination and that inferred from its temperature, flux and astrometric distance.
We find that there are two fundamental limits to the accuracy of the parameters we can derive. First, the radial velocity curve of the secondary star deviates from a sinusoid, in part because of its asphericity (which can be modelled) and in part because the line flux is not evenly distributed over its surface. Secondly, we cannot be certain which spectral type is the best match for the lines of the secondary star, and the derived rotational velocity is a function of the spectral type of the template star used. 相似文献
We find that there are two fundamental limits to the accuracy of the parameters we can derive. First, the radial velocity curve of the secondary star deviates from a sinusoid, in part because of its asphericity (which can be modelled) and in part because the line flux is not evenly distributed over its surface. Secondly, we cannot be certain which spectral type is the best match for the lines of the secondary star, and the derived rotational velocity is a function of the spectral type of the template star used. 相似文献
17.
We argue that gravitational microlensing is a feasible technique for measuring the mass function of brown dwarf stars in distant galaxies. Microlensing surveys of the bulge of M31, and of M87 in the Virgo cluster, may provide enough events to differentiate the behaviour of the mass function of lenses below the hydrogen-burning limit (although we find that M87 is a more favourable target). Such objects may provide a significant supply of baryonic dark matter, an interesting possibility for the study of galactic dynamics. Furthermore, these systems have different metallicities from the solar neighbourhood, which may affect the mass function. These considerations are relevant in the context of star formation studies. 相似文献
18.
V. S. Imshennik 《Astronomy Letters》2008,34(6):375-388
We consider the evolution of a neutron star binary system under the effect of two factors: gravitational radiation and mass transfer between the components. Gravitational radiation is specified under the justified assumption of a circular orbit and point masses and in the approximation of a weak gravitational field at nonrelativistic velocities of the binary components. During the first evolutionary phase determined only by gravitational radiation, the neutron stars approach each other according to a simple analytical solution. The second evolutionary phase begins at the time of Roche-lobe filling by the low-mass component, when the second factor, mass transfer as a result of mass loss by the latter, also begins to affect the evolution. Under the simplest assumptions of conservative mass transfer and exact equality between the Roche-lobe radius and the radius of the low-mass neutron star, it is still possible to extend the analytical solution of the problem of evolution to its second phase. We present this complete solution at both phases and, in particular, give theoretical light curves of gravitational radiation that depend only on two dimensionless parameters (m t and δ 0). Based on the solution found, we analyze the theoretical gravitational signals from SN 1987A; this analysis includes the hypothesis about the rotational explosion mechanism for collapsing supernovae. 相似文献
19.
Manjari Bagchi Rachid Ouyed Jan Staff Subharthi Ray Mira Dey † Jishnu Dey ‡ 《Monthly notices of the Royal Astronomical Society》2008,387(1):115-120
We study the effects of temperature on strange stars. It is found that the maximum mass of the star decreases with the increase of temperature, as at high temperatures the equations of state become softer. Moreover, if the temperature of a strange star increases, keeping its baryon number fixed, its gravitational mass increases and its radius decreases. This leads to a limiting temperature, where it turns into a black hole. These features are the result of a combined effect of the change of gluon mass and the quark distribution with temperature. We report on a new type of radial oscillation of strange stars, driven by what we call 'chromothermal' instability. We also discuss the relevance of our findings in the astrophysics of core collapse supernovae and gamma-ray bursts. 相似文献
20.
A gravitationless black hole model is proposed in accord with a five-dimensional fully covariant Kaluza-Klein (K-K) theory
with a scalar field, which unifies the four-dimensional Einsteinian general theory of relativity and Maxwellian electromagnetic
theory. It is shown that a dense compact core of a star, when it collapses to a critical density, suddenly turns off or shields
its gravitational field. The core, if its mass exceeds an upper limit, directly collapses into a black hole. Otherwise, the
extremely large pressure, as the gravity is turned off, immediately stops the collapse and drives the mantle material of supernova
moving outward, which leads to an impulsive explosion and forms a neutron star as a remnant. A neutron star can further evolve
into a black hole when it accretes enough matter from a companion star such that the total mass exceeds a lower limit. The
black hole in the K-K theory is gravitationless at the surface because the scalar field is infinitely strong, which varies
the equivalent gravitational constant to zero. In general, a star, at the end of its evolution, is relatively harder to collapse
into a gravitationless K-K black hole than a strong gravitational Schwarzschild black hole. This is consistent with the observation
of some very massive stars to form neutron stars rather than expected black holes. In addition, the gravitationless K-K black
hole should be easier to generate jets than a Schwarzschild black hole. 相似文献