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1.
This paper discusses the first all-sky surveys of cosmic extreme ultra-violet and soft X-ray sources, discovered by ROSAT. Details of the surveys are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification programs are described, and the major results summarized. We then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV observations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified as hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe and Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as standard candles in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. We finish with a discussion of the on-going ROSAT Galactic Plane Survey (RGPS) identification program.  相似文献   

2.
Strange quark stars with a crust and strange dwarfs consisting of a compact strange quark core and an extended crust are investigated in terms of a bag model. The crust, which consists of atomic nuclei and degenerate electrons, has a limiting density of cr=drip=4.3·1011 g/cm3. A series of configurations are calculated for two sets of bag model parameters and three different values of cr (109 g/cm3 cr drip) to find the dependence of a star's mass M and radius R on the central density. Sequences of stars ranging from compact strange stars to extended strange dwarfs are constructed out of strange quark matter with a crust. The effect of the bag model parameters and limiting crust density cr on the parameters of the strange stars and strange dwarfs is examined. The strange dwarfs are compared with ordinary white dwarfs and observational differences between the two are pointed out.  相似文献   

3.
A considerable amount of effort has been made towards obtaining a theoretical understanding of the collimated, optically detected outflows (Herbig-Haro objects) ejected by young stars. The most clear results have been obtained for the case of the Herbig-Haro jets, a loosely defined category which groups the Herbig-Haro (HH) objects with jet-like structures of aligned knots. In particular, it has recently been shown that at least some of the characteristics of the HH jets can be straightforwardly explained in terms of models of jets from variable sources. This paper presents a review of the properties of models of jet flows from sources with a variability in the ejection velocity, in the ejection direction, and with a general velocity+direction variability. Also, a comparison between the observational characteristics of HH jets and the predictions from variable source jet models is carried out.  相似文献   

4.
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   

5.
Summary In the solar neighborhood, approximately half of all intermediate mass main sequence stars with initially between 1 and about 5 Mbecome carbon stars with luminosities near 104 L for typically less than 106 years. These high luminosity carbon stars lose mass at rates nearly always in excess of 10–7 M yr–1 and sometimes in excess of 10–5 M yr–1. Locally, close to half of the mass returned into the interstellar medium by intermediate mass stars before they become white dwarfs is during the carbon star phase. A much greater fraction of lower metallicity stars become carbon-rich before they evolve into planetary nebulae than do higher metallicity stars; therefore, carbon stars are much more importan t in the outer than in the inner Galaxy.  相似文献   

6.
A comparison between theGeneral Catalogue of Cool Carbon Stars (CCS) and theAFGL Catalogue has been performed.Eighty-five stars have been found in common between the two lists. Eighty-four stars which were present in Baumert's comparison between CCS and the 2 Sky Survey have no counterpart in the AFGL. Four new tentative identifications are given. The analysis of the two colours diagrams K-[4.2 ] vs. I-K and I-[4.2 ] vs. [4.2 ]-[11 ] led to the conclusion that all the infrared emission from the sources seems to come from a single circumstellar shell.  相似文献   

7.
This article reviews observational data on cataclysmic variables (CVs) whose orbital periods fall within the so-called period gap between 2 and 3 hours. The orbital period distribution of cataclysmic variables and the filling of the period gap by various types of CVs are demonstrated. Roughly half the stars in the period gap are magnetic, of which the majority are polars, while the other half (except a few stars) are SU UMa-type stars characterized by superbursts and superhumps. It is also interesting to note that those intermediate polars whose periods fall within the gap are among the stars with constant superhumps. The height above the galactic plane is estimated for the nonmagnetic stars and it is shown that, contrary to predictions, the stars in the period gap do not belong (except for one) to the spherical component of the galaxy. Light curves, some characteristic times, burst amplitudes, etc. are given for a number of Su UMa-type stars observed by the authors. The evolution of superbursts from the stars NY Ser, V725 Aql, and Var73 Dra is described. Observational data are presented on the classical nova V Per, a star whose period lies right in the middle of the period gap but whose status (magnetic or nonmagnetic) has still unclear.  相似文献   

8.
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   

9.
By solving the Euler hydrodynamical equations we have obtained closed form solutions for the angular velocities and pressures of a three stratified non-confocal heterogeneous oblate spheroid. Limiting and particular solutions cases, such as a spheroid with N layers, a stratified spheroid with the same eccentricities, as well as confocal layered spheroids are also explicitly written down. As an application, we have numerically estimated planet Earth's outer and inner cores' ellipticities to be o=1/413.318 and i=1/424.616, respectively. These Earth's ellipticities values are in good agreement with those found in the literature.  相似文献   

10.
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   

11.
The final state of the primaries of binary systems with initial massesM 1i=10M to 15M is derived from the mass of their C/O-cores. The possibility of a second stage of mass transfer towards the secondary is considered. It turns out that the critical mass for the bifurcation is about 14M : stars with larger masses in this range are the progenitors of neutron stars, while the lower mass stars are the ancestors of white dwarfs.Research supported by the National Foundation of Collective Fundamental Research of Belgium (F.K.F.O.) under No. 10303.  相似文献   

12.
The twelfth list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Survey in zone -3° +1° covering about 1070 sq. degrees is presented. From 86 detected stars, 57 are newly discovered objects: they are 3 R-type carbon stars, 2 carbon star candidates and 52 M-type stars. Among the latter 34 (28 PSC + 6 FSC) are unclassified IRAS sources. Equatorial coordinates, spectral classes, color indices, and red magnitudes determined from the Palomar E-charts are presented.  相似文献   

13.
The present observational status of the Sct stars, Dor stars and roAp stars is discussed. The Sct stars are the most intensively observed of the three groups, but it has become clear that there are severe problems in extracting asteroseismic information from them. Dozens of frequencies are observed, but hundreds of frequencies are predicted from the models; unique matches of observation and theory still elude us. The Sct stars are observationally complex – some recent `best case' campaigns are discussed. It is possible that substantial observational advances for Sct stars may need to await upcoming satellite missions. New Dor stars are beingdiscovered frequently, and new behaviour is being found for them. They constitutean observationally young field. Their pulsational frequency range is being expanded, their position in the HR diagram is becoming better known (but is yet to be fully constrained), and the possibility exists of hybrid Dor – Sct stars that have greatasteroseismic promise, although it is clear such stars are rare, if they do exist. It has been observationally challenging to extract more than a fewfrequencies for any Dor star so far. Exciting spectroscopic discoveries of new behaviour in roAp stars promise unprecedented information about the structure of the peculiar atmospheres ofthose stars – pulsation amplitude and phase in 3D, magnetic field structurein 3D, abundance stratification in 3D, realistic T- for the most peculiarstars – as well as entirely new information about the interaction of pulsation,rotation and magnetic fields. Recent theoretical work has led to new understandingof the previously inexplicable frequency spacing of HR 1217 with new Whole Earth Telescope observations supporting this theory. An `improved oblique pulsator model' has been developed in which the pulsationaxis is not the magnetic axis; this model has passed several observationaltests and new ones are being devised to examine it further.  相似文献   

14.
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   

15.
We investigate the near-ultraviolet high-resolution LWR spectra of the stars Cas, And, Tau, Gem, Cru, Boo, and Peg, obtained with the aid of the International Ultraviolet Explorer Satellite. We have given here a list of the strongest and most prevalent emission lines in the near-ultraviolet spectra of Boo, KlIIIp, and Peg, M2.5II-III which have the same luminosity class and different spectral type. The near-ultraviolet continuum flux measurements and integrated emission line fluxes of these stars for the 2500–3200 Å region are presented in order to compare the variations in the appearance of the near-ultraviolet flux distribution with the temperature structure of their chromospheres for K and M giant stars. We also discuss differences between observed and calculated fluxes found from the Planck function.  相似文献   

16.
The eleventh list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Survey in zone -7° -3° and covering about 1000 square degrees is presented. From 126 detected stars, 88 are newly discovered objects: they are 6 carbon stars, 8 carbon star candidates, and 74 M-type stars; among the latter 38 (26 PSC + 12 FSC) are unclassified IRAS sources, and one object is an unclassified ROSAT source. Distances to the 6 newly discovered early-type carbon stars are estimated. Equatorial coordinates, red magnitudes, and spectral classes determined from the Palomar E-charts are provided. The lack of optical counterparts on Palomar O and E maps for two detected late M-type stars indicates a large variability in brightnesses of these objects (amplitude not smaller than 7.0 magnitude).  相似文献   

17.
We study the evolution of solid, CO white dwarfs after explosive carbon ignition at central densities around 1010 g cm–3 triggered by steady accretion in a close binary system, in order to elucidate whether these stars can collapse to form a neutron star. We show that as long as the velocity of the burning front remains below a critical value of 0.006c s (60 km s–1), gravitational collapse is the final fate. These calculations support the accretion-induced collapse (AIC) scenario for the origin of a fraction of low-mass X-ray binaries.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.  相似文献   

18.
The goal of asteroseismology is to discern the physical conditions of stars by comparing observed pulsations with models.To obtain this goal, the observed pulsation periods and the spherical harmonics (n, , and m) need to match the theoretical model.Typically the most difficult part in this process is the identification of the pulsation modes in the observations.Multicolour photometry is one method that has proven useful for identifying pulsation modes.By observing stars through various wavebands, and comparing the amplitudes and phases, it is possible to determine the spherical harmonics.This contribution will emphasize the work of Watson (1988), which has since been applied to many different types of variable stars including Scuti (Garrido et al., 1990), Doradus (Breger et al., 1997), Cepheid (Cugier et al., 1994), and EC 14026 (Koen, 1998) stars. I will also discuss the technique of summing spectra (especially UV) into various wavebands which are then used to identify modes as pioneered by Robinson, Kepler, and Nather (1982) and applied to white dwarf stars (Kepler et al., 2000).  相似文献   

19.
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   

20.
We report on our search for possible planetary system candidates in a volume-limited sample of 62 nearby A stars. Since the evolutionary lifetimes of A stars ( 109 yrs) roughly correspond to the era of planet formation and subsequent heavy bombardment in our solar system, our study could provide valuable insight into the origin of our own Solar System. From our ground-based visual and IUE high-resolution spectroscopy of all the northern nearby A stars, we have identified at least 12 stars with circumstellar gas. Combining these results with our previous IRAS survey we are probing the link between stars with circumstellar gas and those showing circumstellar dust disks. Our aim is not just to identify stars with gas, or stars with both gas and dust, but to identify systems with dynamic spectral activity similar to Pic, a well known proto-planetary system candidate. By measuring the gas dynamics in the disks of these Pic-like stars, we can begin to study the physics of accretion disks of young evolving systems.  相似文献   

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