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1.
The temperatures, radii, and masses of 81 He-rich white dwarfs are calculated from photometric data. It is shown that, on the average, they are less massive than DA white dwarfs: 70% of He-rich white dwarfs have masses<0.55M ⊙. Space density and birth-rate for different mass groups of H-rich and He-rich white dwarfs are obtained. Birth-rate is 1×10?12 pc?3 yr?1 and 1.5×10?12pc?3yr?1 for He-rich and H-rich white dwarfs, respectively. The mean mass of nascent white dwarfs is about 0.55M ⊙. It is shown thatV Tand its dispersion σ are correlated with the mass of white dwars, and from this progenitors' masses — of different mass groups of white dwarfs are estimated.  相似文献   

2.
We investigate the relation between the optical (g-band) and X-ray (0.5–10 keV) luminosities of accreting nonmagnetic white dwarfs. According to the present-day counts of the populations of star systems in our Galaxy, these systems have the highest space density among the close binary systems with white dwarfs. We show that the dependence of the optical luminosity of accreting white dwarfs on their X-ray luminosity forms a fairly narrow one-parameter curve. The typical half-width of this curve does not exceed 0.2–0.3 dex in optical and X-ray luminosities, which is essentially consistent with the amplitude of the aperiodic flux variability for these objects. At X-ray luminosities L x ～ 1032 erg s?1 or lower, the optical g-band luminosity of the accretion flow is shown to be related to its X-ray luminosity by a factor ～2–3. At even lower X-ray luminosities (L x ? 1030 erg s?1), the contribution from the photosphere of the white dwarf begins to dominate in the optical spectrum of the binary system and its optical brightness does not drop below M g ～ 13–14. Using the latter fact, we show that in current and planned X-ray sky surveys, the family of accreting nonmagnetic white dwarfs can be completely identified to the distance determined by the sensitivity of an optical sky survey in this region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity m g ～ 22.5, this distance is ～400–600 pc.  相似文献   

3.
The method of effective temperatureT eff and logg determination for DA-white dwarfs by KSW model atmospheres is analyzed. The existence of systematic errors in logg determination, leading to lower mass values forT eff>15000 K, is demonstrated. With due account to logg corrections, masses for 355 DA-dwarfs were evaluated. The influence of the effects of observational selection on mass distribution has been considered. These effects are connected with the fact that such selection on effects favour discovery of white dwarfs of low masses. The distribution obtained is characterized by the average mass of ～0.75M ⊙ and the distribution width of ～0.20M ⊙.  相似文献   

4.
In an investigation of the evolution of homogeneous, isentropic, stars through stages of diminishing entropy, Rakavy and Shaviv (1968) have recently found that stars of mass less thanM c (Chandrasekhar's limiting mass for white dwarfs) evolve into white dwarfs, while stars of mass greater thanM c approach a (singular) state of minimum entropy. An elementary explanation of these results is given and qualitative effects of general relativity are discussed. It is found that stars which are lighter than the Oppenheimer and Volkoff (1939) limit become white dwarfs, while heavier stars must become dynamically unstable at a finite stage in their evolution.  相似文献   

5.
A number of white dwarf models have been calculated which correspond to various radial and nonradial modes of vibration with eigenfrequencies in agreement with the observed pulsation frequencies of the X-ray sources Hercules X-1 and Centaurus X-3. Most of the white dwarf models have hot interiors, but for calculational purposes these were simplified so that the bulk of the interior was isothermal, and the surface layers were designed to produce an energy generation rate of 1037 erg s?1 and to transport this energy continuously to the surface by radiative transfer. Cold white dwarfs have a fairly large spread of masses corresponding to the different overtone modes with the given eigenfrequencies, but in the hot models this spread of masses is greatly reduced, for both radial and nonradial modes. It is concluded that if the pulsating X-ray sources are hot white dwarfs, the mass of Cen X-3 probably lies in the range 0.7–1.2M ⊙, and the mass of Her X-1 probably lies in the range 1.1–1.25M ⊙ (in accord with observation).  相似文献   

6.
We have monitored S Ori 45, a young, low-mass (20 M j up) brown dwarf of the σ Orionis cluster (～3 Myr, 352 pc), using optical and near-infrared filters. S Ori 45 (spectral type M8.5) is found to be multi-periodic with a dominant modulation at 2.5–3.5 h, and a short modulation at about 46 min. We ascribe the longer of these modulations to a rotation period. After comparing these results with observations of more massive cluster brown dwarfs and field brown dwarfs, we conclude that substellar objects present rotational and angular momentum evolution. We have also obtained intermediate-resolution near-infrared spectroscopy of S Ori 70, which is a T-class, free-floating planetary candidate member in the σ Orionis cluster. Its observed spectrum has been compared to data of field brown dwarfs of similar types and to theoretical spectra computed for different surface temperatures and gravities. We conclude that S Ori 70 has a significantly cool, low-gravity atmosphere. This supports the young age of this object and its membership in the cluster. From state-of-the-art evolutionary models, the mass of S Ori 70 is estimated at 3 times the Jovian mass (+5 ?2 M j up), challenging current stellar/substellar formation models. S Ori 70 remains the lowest mass object so far identified in any open cluster.  相似文献   

7.
It is known that intermediate and low-mass stars evolve finally to white dwarfs of mass characteristically centred around 0.6M . The observed luminosity distribution and the theoretical cooling curves of such white dwarfs are used in this work to estimate the rate of formation of these and, hence, of their progenitors (although not uniquely) in the solar neighbourhood as a function of time. It is found that the star formation rate has remained fairly constant over the past 10–12 billion years, and that the observed number density of the local white dwarfs match quite well with the one expected from the mass functions of the local stars.  相似文献   

8.
This is a study of the stability of strange dwarfs, superdense stars with a small self-confining core (M core  < 0.02 M⊙) containing strange quark matter and an extended crust consisting of atomic nuclei and degenerate electron gas. The mass and radius of these stars are of the same orders as those of ordinary white dwarfs. It is shown that any study of their stability must examine the dependence of the mass on two variables, which can, for convenience, be taken to be the rest mass (total baryon mass) of the quark core and the energy density ρ tr of the crust at the surface of the quark core. The range of variation of these quantities over which strange dwarfs are stable is determined. This region is referred to as the stability valley for strange dwarfs. The mass and radius from theoretical models of strange dworfs are compared with observational data obtained through the HIPPARCOS program and the most probable candidate strange dwarfs are identified.  相似文献   

9.
It is pointed out that, because of the large Faraday rotation an outlet of linear polarization from the photosphere of a white dwarf is hampered. In accordance with this fact it is proposed to distinguish two types of magnetic white dwarfs. The first type (its representative is Grw 70°8247) has a linear polarization which is comparable in magnitude with the circular one. Polarization of radiation from the white dwarfs of the first type cannot arise in the photosphere. It arises in the corona of the star either as a result of cyclotron emission of hot electrons (T～106 K) or as a result of scattering of slightly polarized emission from the photosphere in the corona. For the first type dwarfs such magnetic fields are required thatω B ωopt, i.e.B(1?3)×108G. The white dwarfs of the second type (its representative is G 99-37) have their linear polarization much smaller than the circular one. Polarization of these white dwarfs can arise as a result of the transfer of radiation in the nonisothermal photosphere. Magnetic fields required for the second type can be much smaller:B cos γ=(1?10)×106 G. It is shown that the photospheric model allows to obtain the quantitative accordance of the theory with all the observational data for G 99-37 and is not in accordance with the data for Grw 70°8247, at the same time the model with cyclotron emission from the corona explains the magnitude of both linear and circular polarization and their wavelength dependence for Grw 70°8247.  相似文献   

10.
This is a study of the stability of strange dwarfs, superdense stars with a small quark core (M 0core /M ⨀ < 0.017) and an extended crust consisting of atomic nuclei and a degenerate electron gas where the density may be two orders of magnitude greater than the maximum density for white dwarfs. For a given equation of state, the mass, total number of baryons, and radius of strange dwarfs are uniquely determined by the central energy density ρ c and the energy density ρ tr of the crust at the surface of the quark core. Thus, the entire range of variation of ρ c and ρ tr must be taken into account in studying the stability of these configurations. This can be done by examining a series of configurations with a fixed rest mass M 0 (total baryon number) of the quark core and different masses of the crust. In each series, ρ tr ranges from the value for white dwarfs to ρ drip = 4.3∙1011 g/cm3, at which free neutrons are created in the crust. According to the static criterion for stability, stability is lost in an individual series when the mass of the strange dwarf reaches a maximum as a function of ρ tr . Translated from Astrofizika, Vol. 52, No. 2, pp. 325–332 (May 2009).  相似文献   

11.
Radial velocity curves have been measured for nine sdB stars from high resolution optical spectra and are found to be sinusoidal indicating that they are binary stars with circular orbits. Their periods range from ≈12 h to more than 8 days. The companions are invisible in optical light. Minimum companion masses are derived from the mass functions, assuming the mass of the sdB primary to be half a solar mass. We argue that the companions to UVO 1735+22, Feige 108, HD 188112, and HD 171858 are white dwarfs, since their (statistically) most likely mass is above ≈0.7M ⊙. Five of the systems have not been investigated before, for the others our results agree with previously published ones.  相似文献   

12.
Molecular dissociation equilibrium calculations were done for the model atmospheres of DA and non-DA white dwarfs. Our calculations show that He 2 + and HeH+ appear as most abundant molecules in the atmospheres of non-DA white dwarfs while H2 and H 2 + are most abundant molecules in DA white dwarfs. It is suggested that these molecules should be searched for in the atmospheres of white dwarfs.  相似文献   

13.
The stars in the Main Sequence are seen as a hierarchy of objects with different massesM and effective dynamical radiiR eff=R/α given by the stellar radii and the coefficients for the inner structure of the stars. As seen in a previous work (Paper I), during the lifetime in the Main SequenceR eff(t) remains a near invariant when compared to the variation in the time ofR(t) and α(t). With such an effectiveR eff one obtains the amounts of actionA c(M), the effective densities ρeff(M)=ρ(M)α3(M), the densities of action and of energy (or mean presures in the stellar interior)a c(M),e c(M), and the potential energiesE p(M). The amounts of action areA c∝M k withk≈1.87 for the M stars,k≈5/3 for the KGF stars, andk≈1.83 for the A and earlier stars, representing very simples conditions for the other dynamical parameters. For instancek≈5/3 means a near invariant effective density αeff for the KGF stars, while for such stars the mean densities and coefficients α present the strongest variations with masses ρ(M)∝M ?1.81, α(M)∝M0.6. The cases for the M stars (e c(M)∝M ?1) and for the A and earlier stars (betweena c(M)=constant and αeff(M)∝M ?1) and also discussed. These conditions for the earlier stars also represent reasonable mean values for the whole stellar hierarchy in the range of masses 0.2M ⊙≤M≤25M ⊙. With all this, one can build ‘dynamical’ HR diagrams withA c(M), Ep(M), αeff M ?p , etc., whose characteristics are analogous to these in the photometrical HR diagram. A comparison is made betweenA c(M) from the models here and the HR diagram with the best known stars of luminosity classes IV, V, and white dwarfs. The comparison of the potential energiesE p(M)∝M ?p according to the stellar models used here and the observed frequency function ψ(M∝M ?q (number of stars in a given interval of masses) from different authors suggests the possibility that the productE p(M)ψ(M) is a constant, but this must be confirmed with further studies of the function ψ(M) and its fine structure. There are analogies between the formulation used here for the stellar hierarchy and other physical processes, for instance, in modified forms of the Kolmogorov law of turbulence and in the formulation used for the hierarchy of molecular clouds in gravitational equilibrium. Besides, the function of actionA c(M) for the stars has analogous properties to the relations of angular momenta and massesJ(M) for different types of objects. The cosmological implications of all this are discussed.  相似文献   

14.
The stability of strange dwarfs for quark cores with M 0core /M ⨀ = 10−4, has been studied by calculating, in each individual case, a series of strange dwarfs with configurations in which 5 ⋅ 10−4, 10−3, 5 ⋅ 10−3, 10−2, 1.31 ⋅ 10−2, 1.6 ⋅ 10−2, 1.7 ⋅ 10−2, 2 ⋅ 10−2, ranges from the values in white dwarfs to ρ drip = 4.3 ⋅ 1011 g/cm3, at which free neutrons are produced in the crust. For the series with M 0core /M ⨀ < 0.0131, stability is lost when ρ tr < ρ drip . For the series with M 0core /M ⨀ > 0.0131, the equality ρ tr = ρ drip is reached before the strange dwarf attains its maximum mass. Although the frequency of the radial pulsations in the fundamental mode obeys ω02 > 0 for these configurations, they are unstable with respect to transitions into a strange star state with the same total number of baryons and a radius on the order of that of neutron stars. An energy on the order of the energy in a supernova explosion is released during these transitions. It is shown that the gravitational red shift of white and strange dwarfs are substantially different for low and limiting (high) masses.  相似文献   

15.
We report the results of photometric observations of a number of magnetic white dwarfs in order to search for photometric variability in these stars. These V-band observations revealed significant variability in the classical highly magnetized white dwarf GRW+70?8247 with a likely period from several days to several dozen days and a half-amplitude of about 0. m 04. Our observations also revealed the variability of the well-known white dwarf GD229. The half amplitude of its photometric variability is equal to about 0. m 005, and the likely period of this degenerate star lies in the 10–20 day interval. This variability is most likely due to the rotation of the stars considered.We also discuss the peculiarities of the photometric variability in a number of other white dwarfs. We present the updated “magnetic field–rotation period” diagram for the white dwarfs.  相似文献   

16.
Carbon-oxygen white dwarfs may be the progenitors of type-I supernovae. Spherically-symmetric models of such dwarfs have been evolved from an artificial core incineration. The convectively unstable incinerated region was allowed to grow at a velocity prescribed by the mixing-length theory of convection. The mixing length can be varied to give different cases. In all the cases considered the dwarfs exploded and were totally disrupted. The calculations were stopped after the dwarf matter had gone into homologous expansion. The model with the best estimated mixing length incinerated 0.8M . The energy released in burning this amount of carbon-oxygen to56Ni provides a disrupted dwarf with velocities suitable for type-I supernovae.Research supported by the Natural Sciences and Engineering Research Council.  相似文献   

17.
Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (T e ff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research.  相似文献   

18.
The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling the evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than Mlim have lost an amount of mass equal to ΔM. The actual value of ΔM or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for ΔM in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6 × 10–11 M⊙/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
We investigate the possibility that gravity modes can be stochastically excited by turbulent convection in massive main-sequence (MS) stars. We build stellar models of MS stars with masses M=10?M ⊙,15?M ⊙, and 20?M ⊙. For each model, we then compute the power supplied to the modes by turbulent eddies in the convective core (CC) and the outer convective zones (OCZ). We found that, for asymptotic gravity modes, the major part of the driving occurs within the outer iron convective zone, while the excitation of low n order modes mainly occurs within the CC. We compute the mode lifetimes and deduce the expected mode amplitudes. We finally discuss the possibility of detecting such stochastically-excited gravity modes with the CoRoT space-based mission.  相似文献   

20.
We consider the evolutionary scenarios for close binaries that lead to the formation of semidetached systems in which a white dwarf can accumulate the Chandrasekhar mass through mass accretion from its companion, a main sequence star or a subgiant of mass M ～ 2M⊙. Such dwarfs probably explode as type-Ia supernovae or collapse to form a neutron star. The population synthesis method is used to analyze the dependence of the model rate of these events in the Galaxy on the common envelope parameter, the mass transfer rate, and the response of a main-sequence star to helium accretion at an intermediate evolutionary stage. The rate of explosions in semidetached systems of this type in the Galaxy was found to be no higher than ?0.2×10?3 yr?1, which is less than 10% of the lower level for the empirically estimated SNe Ia rate.  相似文献   

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