首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 843 毫秒
1.
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   

2.
We consider gravitational waves emitted by various populations of compact binaries at cosmological distances. We use population synthesis models to characterize the properties of double neutron stars, double black holes and double white dwarf binaries, and white dwarf–neutron star, white dwarf–black hole and black hole–neutron star systems.
We use the observationally determined cosmic star formation history to reconstruct the redshift distribution of these sources and their merging rate evolution.
The gravitational signals emitted by each source during its early spiralling in phase add randomly to produce a stochastic background in the low-frequency band with spectral strain amplitude between ~10−18 and ~5×10−17 Hz−1/2 at frequencies in the interval ~5×10−6–5×10−5 Hz.
The overall signal, which at frequencies above 10−4 Hz is largely dominated by double white dwarf systems, might be detectable with LISA in the frequency range 1–10 mHz and acts like a confusion-limited noise component, which might limit the LISA sensitivity at frequencies above 1 mHz.  相似文献   

3.
We calculate the disc and boundary layer luminosities for accreting rapidly rotating neutron stars with low magnetic fields in a fully general relativistic manner. Rotation increases the disc luminosity and decreases the boundary layer luminosity. A rapid rotation of the neutron star substantially modifies these quantities as compared with the static limit. For a neutron star rotating close to the centrifugal mass shed limit, the total luminosity has contribution only from the extended disc. For such maximal rotation rates, we find that well before the maximum stable gravitational mass configuration is reached, there exists a limiting central density, for which particles in the innermost stable orbit will be more tightly bound than those at the surface of the neutron star. We also calculate the angular velocity profiles of particles in Keplerian orbits around the rapidly rotating neutron star. The results are illustrated for a representative set of equation of state models of neutron star matter.  相似文献   

4.
Using time evolutions of the relevant linearized equations, we study non-axisymmetric oscillations of rapidly rotating and superfluid neutron stars. We consider perturbations of Newtonian axisymmetric background configurations and account for the presence of superfluid components via the standard two-fluid model. Within the Cowling approximation, we are able to carry out evolutions for uniformly rotating stars up to the mass-shedding limit. This leads to the first detailed analysis of superfluid neutron star oscillations in the fast rotation regime, where the star is significantly deformed by the centrifugal force. For simplicity, we focus on background models where the two fluids (superfluid neutrons and protons) corotate, are in β-equilibrium and co-exist throughout the volume of the star. We construct sequences of rotating stars for two analytical model equations of state. These models represent relatively simple generalizations of single fluid, polytropic stars. We study the effects of entrainment, rotation and symmetry energy on non-radial oscillations of these models. Our results show that entrainment and symmetry energy can have a significant effect on the rotational splitting of non-axisymmetric modes. In particular, the symmetry energy modifies the inertial mode frequencies considerably in the regime of fast rotation.  相似文献   

5.
The gravitational radiation of n = 1 polytropes undergoing quasiradial pulsations is examined. The intensity of the gravitational radiation and the gravitational wave amplitudes are calculated for polytropic models of white dwarfs and neutron stars when the energy of rotation of the object serves as the source of the radiated energy. Calculations of h0 show that objects with a polytropic equation of state can describe the expected gravitational radiation from white dwarfs and neutron stars. The gravitational radiation of polytropic models of galactic nuclei and quasars is also examined. These objects can create a high enough background of gravitational radiation at frequencies of 10-8–10-11 Hz for gravitational wave detectors operating in this frequency range. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 603–612 (November 2005).  相似文献   

6.
The purpose of this paper is to present new full evolutionary calculations for DA white dwarf stars with the major aim of providing a physically sound reference frame for exploring the pulsation properties of the resulting models in future communications. Here, white dwarf evolution is followed in a self-consistent way with the predictions of time-dependent element diffusion and nuclear burning. In addition, full account is taken of the evolutionary stages prior to white dwarf formation. In particular, we follow the evolution of a 3-M⊙ model from the zero-age main sequence (the adopted metallicity is   Z =0.02)  , all the way from the stages of hydrogen and helium burning in the core up to the thermally pulsing phase. After experiencing 11 thermal pulses, the model is forced to evolve towards its white dwarf configuration by invoking strong mass loss episodes. Further evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch.
Emphasis is placed on the evolution of the chemical abundance distribution caused by diffusion processes and the role played by hydrogen burning during the white dwarf evolution. We find that discontinuities in the abundance distribution at the start of the cooling branch are considerably smoothed out by diffusion processes by the time the ZZ Ceti domain is reached. Nuclear burning during the white dwarf stage does not represent a major source of energy, as expected for a progenitor star of initially high metallicity. We also find that thermal diffusion lessens even further the importance of nuclear burning.
Furthermore, the implications of our evolutionary models for the main quantities relevant for adiabatic pulsation analysis are discussed. Interestingly, the shape of the Ledoux term is markedly smoother compared with previous detailed studies of white dwarfs. This is translated into a different behaviour of the Brunt–Väisälä frequency.  相似文献   

7.
The purpose of this work is to explore the evolution of helium-core white dwarf stars in a self-consistent way with the predictions of detailed non-grey model atmospheres and element diffusion. To this end, we consider helium-core white dwarf models with stellar masses of 0.406, 0.360, 0.327, 0.292, 0.242, 0.196 and 0.169 M⊙ and follow their evolution from the end of mass-loss episodes, during their pre-white dwarf evolution, down to very low surface luminosities.
We find that when the effective temperature decreases below 4000 K, the emergent spectrum of these stars becomes bluer within time-scales of astrophysical interest. In particular, we analyse the evolution of our models in the colour–colour and in the colour–magnitude diagrams and find that helium-core white dwarfs with masses ranging from ∼0.18 to 0.3 M⊙ can reach the turn-off in their colours and become blue again within cooling times much less than 15 Gyr and then remain brighter than M V ≈16.5 . In view of these results, many low-mass helium white dwarfs could have had enough time to evolve to the domain of collision-induced absorption from molecular hydrogen, showing blue colours.  相似文献   

8.
A white dwarf rotating at a maximal angular velocity can take a form of a triaxial ellipsoid due to the rotation and to the presence of mountains on its surface. Such an object emits gravitational waves at a frequency of 2, where is the angular velocity of rotation, and the source of the radiated energy is the rotational kinetic energy. It is shown that the gravitational waves from rapidly rotating white dwarfs at an average distance of 50 pc from an terrestrial observer have an amplitude on the order of 10–24, so they can be detected by the new generation of detectors. Gravitational radiation from a pulsating white dwarf with a rough surface is also examined. It is shown that quasiradial pulsations of a white dwarf are long-lived; that is, once perturbed, a white dwarf will emit gravitational waves during all lifetime.Translated from Astrofizika, Vol. 48, No. 1, pp. 69–78 (February 2005).  相似文献   

9.
We present computed spectra, as seen by a distant observer, from the accretion disc around a rapidly rotating neutron star. Our calculations are carried out in a fully general relativistic framework, with an exact treatment of rotation. We take into account the Doppler shift, gravitational redshift and light-bending effects in order to compute the observed spectrum. We find that light bending significantly modifies the high-energy part of the spectrum. Computed spectra for slowly rotating neutron stars are also presented. These results would be important for modelling the observed X-ray spectra of low-mass X-ray binaries containing fast-spinning neutron stars.  相似文献   

10.
Using a self-consistent dynamic theory of non-local convection, the atmospheric lithium-abundance depletion values of a series of stellar evolutionary models with     are calculated, and the results show that the general observed properties of lithium abundance in lower-main-sequence stars can be reproduced by overshooting mixing and gravitational settling. After a careful study of the mechanisms of lithium depletion in stars, it is concluded that overshooting mixing and microdiffusion induced by gravitational settling and radiative acceleration are the two primary depletion mechanisms: for warm stars with     (or     ) microdiffusion dominates, while it is the other way round for cooler objects with     (or     ).  相似文献   

11.
Recent spectropolarimetric observations of Ap and Bp stars with improved sensitivity have suggested that most Ap and Bp stars are magnetic with dipolar fields of at least a few hundred gauss. These new estimates suggest that the range of magnetic fluxes found for the majority of magnetic white dwarfs is similar to that of main-sequence Ap–Bp stars, thus strengthening the empirical evidence for an evolutionary link between magnetism on the main sequence and magnetism in white dwarfs. We draw parallels between the magnetic white dwarfs and the magnetic neutron stars and argue that the observed range of magnetic fields in isolated neutron stars  ( Bp ∼ 1011–1015 G)  could also be explained if their mainly O-type progenitors have effective dipolar fields in the range of a few gauss to a few kilogauss, assuming approximate magnetic flux conservation with the upper limit being consistent with the recent measurement of a field of   Bp ∼ 1100 G  for θ Orion C.
In the magnetic field–rotation diagram, the magnetic white dwarfs can be divided into three groups of different origin: a significant group of strongly magnetized slow rotators  ( P rot∼ 50 –100 yr)  that have originated from single-star evolution, a group of strongly magnetized fast rotators  ( P rot∼ 700 s)  , typified by EUVE J0317–853, that have originated from a merger, and a group of modest rotators ( P rot∼ hours–days) of mixed origin (single-star and CV-type binary evolution). We propose that the neutron stars may similarly divide into distinct classes at birth , and suggest that the magnetars may be the counterparts of the slowly rotating high-field magnetic white dwarfs.  相似文献   

12.
A     region along the celestial equator (Stripe 82) has been imaged repeatedly from 1998 to 2005 by the Sloan Digital Sky Survey (SDSS). A new catalogue of ∼4 million light-motion curves, together with over 200 derived statistical quantities, for objects in Stripe 82 brighter than   r ∼21.5  has been constructed by combining these data by Bramich et al. This catalogue is at present the deepest catalogue of its kind. Extracting ∼130 000 objects with highest signal-to-noise ratio proper motions, we build a reduced proper motion diagram to illustrate the scientific promise of the catalogue. In this diagram, disc and halo subdwarfs are well-separated from the cool white dwarf sequence. Our sample of 1049 cool white dwarf candidates includes at least eight and possibly 21 new ultracool H-rich white dwarfs  ( T eff < 4000 K)  and one new ultracool He-rich white dwarf candidate identified from their SDSS optical and UKIDSS infrared photometry. At least 10 new halo white dwarfs are also identified from their kinematics.  相似文献   

13.
We have examined the evolution of merged low-mass double white dwarfs that become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, on to a carbon–oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium-burning shell becomes small enough, the star evolves blueward almost horizontally in the Hertzsprung–Russell diagram. The theoretical models for the merger of a 0.6-M⊙ CO white dwarf with a 0.3-M⊙ He white dwarf agree very well with the observed locations of extreme helium stars in the  log  T eff–log  g   diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for  CO+He  white dwarf pairs, the evolutionary time-scales are roughly consistent with the observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with the observed values if carbon and oxygen produced in the helium shell during a previous asymptotic giant branch phase are assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the  CO+He  white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.  相似文献   

14.
Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B  ∼ 1015 − 1016 G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources.  相似文献   

15.
Short-period double degenerates (DDs) are close white dwarf–white dwarf binary stars which are the result of the evolution of interacting binary stars. We present the first definitive measurements of the mass ratio for two DDs, WD 0136+768 and WD 1204+450, and an improved measurement of the mass ratio for WD 0957−666. We compare the properties of the six known DDs with measured mass ratios to the predictions of various theoretical models. We confirm the result that standard models for the formation of DDs do not predict sufficient DDs with mass ratios close to 1. We also show that the observed difference in cooling ages between white dwarfs in DDs is a useful constraint on the initial mass ratio of the binary. A more careful analysis of the properties of the white dwarf pair WD 1704+481.2 leads us to conclude that the brighter white dwarf is older than its fainter companion. This is the opposite of the usual case for DDs and is caused by the more massive white dwarf being smaller and cooling faster. The mass ratio in the sense (mass of younger star)/(mass of older star) is then  1.43±0.06  rather than the value of  0.70±0.03  given previously.  相似文献   

16.
The properties of accretion discs around stars and brown dwarfs in the σ Ori cluster (age 3 Myr) are studied based on near-infrared (IR) time series photometry supported by mid-IR spectral energy distributions (SEDs). We monitor ∼30 young low-mass sources over eight nights in the J and K band using the duPont telescope at Las Campanas. We find three objects showing variability with J -band amplitudes  ≥0.5 mag  ; five additional objects exhibit low-level variations. All three highly variable sources have been previously identified as highly variable; thus, we establish the long-term nature of their flux changes. The light curves contain periodic components with time-scales of  ∼0.5–8 d  , but have additional irregular variations superimposed – the characteristic behaviour for classical T Tauri stars. Based on the colour variability, we conclude that hotspots are the dominant cause of variations in two objects (#19 and #33), including one likely brown dwarf, with spot temperatures in the range of 6000–7000 K. For the third one (#2), a brown dwarf or very low-mass star, inhomogeneities at the inner edge of the disc are the likely origin of variability. Based on mid-IR data from Spitzer , we confirm that the three highly variable sources are surrounded by circum-(sub)-stellar discs. They show typical SEDs for T Tauri-like objects. Using SED models, we infer an enhanced scaleheight in the disc for the object #2, which favours the detection of disc inhomogeneities in light curves and is thus consistent with the information from variability. In the σ Ori cluster, about every fifth accreting low-mass object shows persistent high-level photometric variability. We demonstrate that estimates for fundamental parameters in such objects can be significantly improved by determining the extent and origin of the variations.  相似文献   

17.
As part of a new southern sky survey for faint high proper motion stars based on Automatic Plate Measuring (APM) measurements of UK Schmidt Telescope plates, we have found a large number of previously unknown brighter objects. Spectroscopic follow-up observations with the European Southern Observatory New Technology Telescope of 15 of these new, relatively bright     high proper motion stars     show one-third of them to be nearby     . Among the nearby stars is an M6 dwarf with strong emission lines at a spectroscopic distance of about 11 pc and an M4 dwarf at about 13 pc. Coupled with earlier South African Astronomical Observatory spectroscopic observations of three similar bright high proper motion stars, the success rate of finding nearby stars     is about 45 per cent. All newly discovered nearby stars have disc kinematics confirmed by radial velocity measurements from our spectra. In addition there are several high-velocity stars with halo kinematics in the sample, mainly subdwarfs, at about 60 to 110 pc distance. These high-velocity stars are interesting targets for further study of the Galactic escape velocity. One of the detected nearby high proper motion stars was formerly thought to be an M giant in the Small Magellanic Cloud. The spectrum of one M3 star shows a strong blue continuum, which is likely to signify the presence of a hot companion. Spectroscopic follow-up observations of high proper motion stars are shown to be an effective tool in the search for the missing stars in the Solar neighbourhood. Candidates for more extensive trigonometric parallax determination can be selected on the basis of the spectroscopic distance estimates.  相似文献   

18.
One of the most promising space missions of the European Space Agency is the astrometric satellite Gaia , which will provide very precise astrometry and multicolour photometry, for all 1.3 billion objects to   V ∼ 20  , and radial velocities with accuracies of a few km s−1 for most stars brighter than   V ∼ 17  . Consequently, full homogeneous six-dimensional phase-space information for a huge number of stars will become available. Our Monte Carlo simulator has been used to estimate the number of white dwarfs potentially observable by Gaia . From this we assess the white dwarf luminosity functions that Gaia will obtain and discuss in depth the scientific returns of Gaia in the specific field of white dwarf populations. Scientifically attainable goals include, among others, a reliable determination of the age of the Galactic disc, a better knowledge of the halo of the Milky Way and the reconstruction of the star formation history of the Galactic disc. Our results also demonstrate the potential impact of a mission such as Gaia within the context of current understanding of white dwarf cooling theory.  相似文献   

19.
We report the discovery, in an Extreme Ultraviolet Explorer ( EUVE ) short-wavelength spectrum, of an unresolved hot white dwarf companion to the 5th magnitude B5Vp star HR 2875. This is the first time that a non-interacting white dwarf+B star binary has been discovered: previously, the earliest type of star known with a white dwarf companion was Sirius (A1V). As the white dwarf must have evolved from a main-sequence progenitor with a mass greater than that of a B5V star (≯6.0 M⊙), this places a lower limit on the maximum mass for white dwarf progenitors, with important implications for our knowledge of the initial–final mass relation. Assuming a pure-hydrogen atmospheric composition, we constrain the temperature of the white dwarf to be between 39 000 and 49 000 K. We also argue that this degenerate star is likely to have a mass significantly greater than the mean mass for white dwarf stars (≈0.55 M⊙). Finally, we suggest that other bright B stars (e.g. θ Hya) detected in the extreme ultraviolet surveys of the ROSAT Wide Field Camera and EUVE may also be hiding hot white dwarf companions.  相似文献   

20.
The white dwarf stars WD 1614+136 and WD 1353+409 are not sufficiently massive to have formed through single-star evolution. However, observations to date have not yet found any evidence for binarity. It has therefore been suggested that these stars are the result of a merger. In this paper we place an upper limit of ≈ 50 km s−1 on the projected rotational velocities of both stars. This suggests that, if these stars are the results of a merger, efficient angular momentum loss with accompanying mass loss must have occurred. If the same process occurs following the merging of more massive white dwarf stars, the predicted rate of Type Ia supernovae due to merging white dwarfs may have been greatly overestimated. Further observations to determine binarity in WD 1614+136 and WD 1353+409 are therefore encouraged.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号