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1.
The Main-Sequence positions as well as the evolutionary behavior of Population III stars up to an evolution age of 2×1010 yr, taking this time as the age of the Universe, have been investigated in the mass range 0.2 and 0.8M . While Population III stars with masses greater than 0.3M develop a radiative core during the approach to the Main Sequence, stars with masses smaller than 0.3M reach the Main Sequence as a wholly convective stars. Population III stars with masses greater than 0.5M show a brightening of at most 2.2 in bolometric magnitude when the evolution is terminated as compared to the value which corresponds to zero-age Main Sequence. The positions of stars with masses smaller than 0.5M remain almost the same in the H-R diagram.If Population III stars have formed over a range of redshifts, 6相似文献   

2.
The High Energy Transient Experiment (HETE) will be able to perform multiwavelength observations of-ray and X-ray bursts. HETE will potentially be able to localize-ray bursts to a precision of 20 arc-minutes if significant X-ray flux is detected from the burst; a precision of 20 arc-seconds is possible if there is also significant UV radiation from the burst. HETE will broadcast information about bursts detected within seconds of their detection. This VHF-band broadcast will be received at a number of secondary ground stations (SGS) dedicated to HETE, and forwarded to a central distribution site at MIT, from which it is sent to interested observers via Internet.  相似文献   

3.
This paper considers the nonlinear effects of collective fluctuations about a slowly-varying solution of the collisionless Boltzmann equation appropriate for a system of stars with a distribution of masses. It is argued that these effects should be manifest on a time-scalet M intermediate between the violent and collisional relaxation times, inhibiting the natural progression towards equipartition induced by collisions, and favouring instead a well-mixed state, realized in numerical simulations, in which the mean square velocity is independent of the mass.  相似文献   

4.
Consideration is given to a search for relativistic objects in massive close binary systems without strong X-ray emission (L x <1034 erg s–1). It is pointed out that, according to the present-day theory on the evolution of massive close binaries, the number of neutron stars and black holes in non-X-ray binary systems must be 100 times the number of the known X-ray binaries comprising OB supergiant stars; that is why, in studying non-X-ray binary systems, the chances are to detect about a hundred of black holes in the Galaxy.Criteria are formulated for the relativistic nature of companions in the binary systems, such as high spatial velocity values and height Z over the galactic plane for OB stars (runaway stars) and for Wolf-Rayet stars. As reported by Tutukov and Yungelson (1973), as well as by van den Heuvel (1976), the presence of ring-type nebulae can serve as another indication of a relativistic nature of companions in the case of Wolf-Rayet stars.Data are collected on Wolf-Rayet stars with low-mass companions (Table I), which can be relativistic objects accreting within a strong stellar wind from Wolf-Rayet stars. Presented are new findings in respect of spectral examination of the runaway OB-stars (Table II), bringing together data on eight OB stars which can represent binary systems with relativistic companions (Table III).A list of 28 OB-stars (Table IV) which offer a good chance for finding relativistic companions is given.  相似文献   

5.
The nonequilibrium dynamical theory of thermonuclear reaction in the stars is used to analyse the stabilities of P-PI reactions in the solar core and 3 reactions of helium burning in the core of red giant stars. The constant stability of stars on the main sequence and the helium flash instability have been explained from a new point of view. Calculations show that the (g - T) term in the dynamical equation has a damaging effect on the stability of the thermonuclear reaction, but its intensity in the core of red giant stars is ~ 107 times that in the solar core. It may be intimately related to some instability of the stellar structure. The effect on the flash instability is especially analysed.  相似文献   

6.
Optically bright and very cool AFGL carbon stars have been analyzed in order to find a common evolutionary sequence according to indications in this sense derived from papers which have treated only the first or the second group of stars separately.An apparent discrepancy existing between the stellar parameters has been overcome following an inverse procedure which provides these values by means of the shell parameters.The results seem to indicate that AFGL stars, whose variability nature is still unknown, should be Mira or Mira-like according to the values of their dust shell temperatures.Dust shell masses have been estimated for both samples of stars finding an inverse dependence with the effective temperature,M d T * –9.1 .This may be interpreted in terms of evolutionary sequence in the sense that the cooler the stars the larger their shell masses.  相似文献   

7.
Edison is a proposed large-aperture, radiatively-cooled space observatory planned to operate at wavelengths between 2 and 130m or longer. Current estimates for the telescope allow an aperture of 1.7 m which will achieve a final equilibrium temperature of about 30 K, although use of cryocoolers may permit temperatures below 20 K.Edison will be a powerful tool to investigate our Solar System, as well as planetary material around distant stars. At near- and mid-infrared wavelengths, where planetary material emits most of its radiation,Edison will be the most sensitive photometric and spectroscopic observatory under current consideration by the space agencies. With its large aperture,Edison will be able both to resolve the structure in nearby circumstellar Vega disks and to discriminate faint IR emission in the crowded environment of the galactic plane. With its long lifetime,Edison will allow extensive follow-up observations and increase the likelihood of catching transient events. We proposeEdison as a precursor to elements of a future space-based IR interferometer.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.  相似文献   

8.
Evolution of massive stars losing mass with the rateM H L/V C is computed (for =1,2,7). It is shown that observed mass loss rates correspond to 0.3 and, therefore, mass loss by stellar wind cannot play any significant role in the evolution of normal massive stars. However, for several types of massive stars (WR, OH/IR, X-ray sources) enhanced mass loss explains their peculiar features. Computations of evolutionary sequences of massive stars with convective overshooting taken into account (as a formal increase of the convective core) show that a significant broadening of the hydrogen-burning band in the H-R diagram may be obtained.  相似文献   

9.
The stars in the Main Sequence are seen as a hierarchy of objects with different massesM and effective dynamical radiiR eff=R/ 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 (t).With such an effectiveR eff one obtains the amounts of actionA c(M), the effective densities eff(M)=(M)3(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 cM k withk1.87 for the M stars,k5/3 for the KGF stars, andk1.83 for the A and earlier stars, representing very simples conditions for the other dynamical parameters. For instancek5/3 means a near invariant effective density eff for the KGF stars, while for such stars the mean densities and coefficients present the strongest variations with masses (M)M –1.81, (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 eff(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 M25M .With all this, one can build dynamical HR diagrams withA c(M), Ep(M), eff 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 (MM –q (number of stars in a given interval of masses) from different authors suggests the possibility that the productE p(M)(M) is a constant, but this must be confirmed with further studies of the function (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   

10.
Relativistic, isentropic, homogeneous models are constructed by a method that automatically detects instabilities, and evolutionary tracks of central conditions are shown on a (T, ) diagram. Models heavier than 20M become unstable because of pair creation. Iron photodisintegration causes instability in the mass range between 1.5M and 20M . General relativistic effects bring about the onset of instability in models of 1.2–1.5M when the central density is about 1010 g/cm3. Lighter models become white dwarfs. It is pointed out that general relativistic instability will prevent the formation of neutron stars through hydrostatic evolution and may be relevant in setting off low-mass supernovae.  相似文献   

11.
C12 stars in the range 1.04–1.55M are evolved to simulate the core evolution of the possible precursors of planetary nebulae. The nuclear shell burning in stars above 1.2M advances to within about 0.2M of the surface, where the intense radiation interacts with the surface matter and causes mass loss. Comparison between our theoretical results and observations suggests that this may be a mechanism by which planetary nebulae are formed.Presented at the Trieste Colloquium on Mass Loss from Stars, September 12–16, 1968.  相似文献   

12.
A statistical model for the binaries in the solar neighbourhood is constructed, using schematic but plausible distribution functions for the semi-major axes, the mass-ratios, and the eccentricities. The model is calibrated to give correct numbers of observed visual binaries and is then used to study the (closer) binaries of relevance for the HIPPARCOS mission. One main purpose is to estimate the influence of astrometric binaries observed by their photocentres. It is found that some 500 such systems (out of the 100000 stars observed by HIPPARCOS) should show a detectably curved motion, while more than ten times more show an orbital proper motion bias greater than the HIPPARCOS accuracy 0.002 yr–1. Also, a non-negligible fraction (1%) of all stars will be binaries with periods close to 1 year causing problems for the parallax determination. The main contribution to these figures is from faint Main-Sequence stars. The period-distribution for the resolved HIPPARCOS binaries is also obtained, and for many of them the periods are fairly short (<100 yr). Such data as these are to be used as guidelines in the construction of reduction software for the HIPPARCOS observations.Communication presented at the International Conference on Astrometric Binaries, held on 13–15 June, 1984, at the Remeis-Sternwarte Bamberg, Germany, to commemorate the 200th anniversary of the birth of Friedrich Wilhelm Bessel (1784–1846).  相似文献   

13.
In this paper we adopt the method of relativistic fluid dynamics to examine the number density distribution of stars around a massive black hole in the core of stellar clusters. We obtain extensive results,n(r) r –a, 3/2a9/2, which include, respectively, then(r) r –7/4 power law obtained by Bahcall and Wolf and then(r) r –9/4 power law by Peebles. Sincen(r) is not an observable quantity for star clusters, we also consider general relativity effects, i.e., the consequence of the bending of light, in calculating the projected density of stars in such a system. As an example we employ a massive black hole 103 M inlaid in the center of a globular cluster and calculate various projected densities of stars. The results show that cusp construction occurs in all cases unless the central black hole massM=0, and the polytropic index does not affect at all the position of the capture radiusr a. The obvious differences in the surface density is only embodied in the interior of the capture radius. At the outer regions of the core, the surface density of stars declines rapidly with ar –5 power law in all cases. These results can be applied to cases of unequal-mass and non-steady state.  相似文献   

14.
Mass functions for samples of white dwarf stars and for a largeheterogeneous sample of nearby stars appear to have unexplained deficitsin the 0.70 M to 0.75 M range. The existence, ornon-existence, of this anomaly constitutes a definitive test of afractal cosmological model that inherently predicts a gap in stellarmass functions at 0.73 M .  相似文献   

15.
We present the fourteenth list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Sky Survey in the zone +13° +33° covering about 4736 sq. degrees. From 260 stars, 118 are newly detected objects: they are 19 carbon stars, 5 carbon star candidates, and 94 M-type stars. Among 118 detected objects 73( 57 PSC + 16 FSC ) are unclassified IRAS sources. Accurate positions, spectral classes, red magnitudes, color indices, and near-infrared J, H and K photometry are given, using several astronomical databases. Finding charts from DSS are given for the most interesting objects.  相似文献   

16.
According to the work of Truran and Cameron, and of others, on the chemical evolution of the Galaxy, the first generation of stars in the Galaxy contained principally massive objects. If big-bang nucleosynthesis was responsible for the formation of helium, the first generation of stars would contain about 80% hydrogen and 20% helium, to be consistent with the approximately 22% helium found in recent stellar evolutionary studies of the Sun. The present investigation has followed the pre-main sequence evolution and the main sequence evolution of stars of 5, 10, 20, 30, 100, and 200M . Normal stars in this entire mass range normally convert hydrogen into helium by the CN-cycle on the main sequence. the present hydrogen-helium stars of 5 and 10M must reach higher central temperatures in order to convert hydrogen to helium by the proton-proton chains. Consequently, the mean densities in the stars are greater, and the surface temperatures are higher than in normal stars. In the stars of 20M and larger, the proton-proton chains do not succed in supplying the necessary luminosity of the stars by the time the contraction has produced a central temperature near 108K. At that point triple-alpha reactions generate small amounts of C12, which then acts as a catalyst in the CN-cycle, the rate of which is then limited by the beta-decays occurring within the cycle. During the evolution of these more massive stars, the central temperature remains in the vicinity of 108 K, and the surface temperature on the main sequence approaches 105 K. The star of 200M becomes unstable against surface mass loss through radiation pressure in the later stages of its main sequence evolution, and these mass loss effects were not followed. Young galaxies containing these massive stars will have a very high luminosity, but if they have formed at one-tenth the present age of the universe or later, then the light from them will mainly reside in the visible or ultraviolet, rather than in the infrared as has been suggested by Partridge and Peebles.  相似文献   

17.
We have detected 1.1 mm continuum emission from 24 of 53 Herbig Ae/Be stars surveyed with the JCMT. Survival analysis shows that 1.1 mm luminosity is correlated with bolometric luminosity and with IRAS 25µm luminosity. For those stars that were also detected at 0.45 or 0.8 mm we find a typical flux dependence of the form S #x03BD; 3, which is steeper than that of most classical T Tauri stars.  相似文献   

18.
Chemically peculiar (CP) stars are mainly found among B and A stars. Present theories are not able to account fully for the observed spectrum anomalies, suggesting that in the atmospheres of stars in the range ofT e , where mass loss is no longer very large and convection is still not very important, the present theoretical approach needs further refinement. Moreover, the lack of laboratory data is responsible, at least partly, for the discrepancies between observed and computed stellar energy distributions.The choice for quick detection of CP stars among faint objects is reviewed; the measure of the flux depression at 5200 appears to be very powerful, and the identification of this complex feature is urgently needed.The need to separate the various subclasses of the He abnormal stars in a standard way is underlined and further analyses of the characteristics of the Boo stars are required.The problem of the determination ofT e and consequently of realistic atmospheric abundance anomalies of CP stars is discussed.  相似文献   

19.
A new sample of possibly massive early-type emission-line stars (METELS) based on the previous lists of peculiar Be stars is presented. It consists of 36 objects divided amongst supergiants, possible binaries, and candidates to the list. The central stars are probably more massive than 10M . Two new relations allowing idientification of possible binaries among the objects are proposed.  相似文献   

20.
Evolution, linear pulsation, and nonlinear pulsation codes were combined to produce nonlinear models of Delta-Scuti stars in an instability region extending over 3.8T e<3.95 and 0.6L/L <2.0. The linear analysis upheld the consensus that they are normal Population I stars of about 2M , in stages of evolution corresponding to central hydrogen burning and shell hydrogen burning. The growth rates were very slow; driving was due to an opacity mechanism in the second helium ionization region; periods and period ratios of the lowest modes of the models were in the same range as those observed. A wide range of nonlinear models was investigated. When eigenfunctions from the linear analysis were used as initial velocity profiles, it was found that the dominant peak in the periodogram of the light curve corresponded to the mode initiated. For a small subset of models, limiting amplitudes were identified, and were found to be in close agreement with observed light amplitudes.  相似文献   

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