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 31 毫秒
1.
Recent UV observations of the most massive Galactic globular clusters show a significant population of hot stars below the zero-age HB (“blue hook” stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the helium-core flash while descending the white dwarf cooling curve could populate this region. They should show higher temperatures than the hottest canonical HB stars and their atmospheres should be helium-rich and probably C/N-rich. We have obtained spectra of blue hook stars in ω Cen and NGC 2808 to test this possibility. Our analysis shows that the blue hook stars in these clusters reach effective temperatures well beyond the hot end of the canonical EHB and have higher helium abundances than canonical EHB stars. These results support the hypothesis that the blue hook stars arise from stars which ignite helium on the white dwarf cooling curve.  相似文献   

2.
We present the results of Monte Carlo mass-loss computations for hot low-mass stars, specifically for subdwarf B (sdB) stars. It is shown that the mass-loss rates on the Horizontal Branch (HB) computed from radiative line-driven wind models are not high enough to create sdB stars. We argue, however, that mass loss plays a role in the chemical abundance patterns observed both in field sdB stars, as well as in cluster HB stars. The derived mass loss recipe for these (extremely) hot HB stars may also be applied to other groups of hot low-mass stars, such as post-HB (AGB-manqué, UV-bright) stars, over a range in effective temperatures between ?10 000 and 50 000 K. Finally, we present preliminary spectral synthesis on the more luminous sdB stars for which emission cores in Hα have been detected (Heber, U., et al.: 2003, in:Stellar Atmosphere Modeling, ASP Conference Proceedings, p. 251). We find that these line profiles can indeed be interpreted as the presence of a stellar wind with mass loss on the order of 10?11?M ⊙ yr ?1.  相似文献   

3.
We present a progress report on an extensive survey to find and characterize all types of horizontal-branch stars in the nuclear bulge of the Galaxy. We have obtained wide, shallow imaging in UBV of ≈12 square degrees in the bulge, with follow-up spectroscopy for radial velocities and metal abundance determinations. We have discovered a number of metal-rich blue HB stars, whose presence in the bulge is expected by the interpretation of the extragalactic ultraviolet excess. Very deep images have been obtained in UBV and SDSS u along the bulge minor axis, which reveal a significant number of EHB candidates fainter than B=19, i.e., with the same absolute magnitudes as EHB stars in several globular clusters.  相似文献   

4.
The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest "UV upturns," while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al., 1994), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon. This revised version was published online in July 2006 with corrections to the Cover Date.  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.
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   

7.
I review the observational constraints on the stars responsible for the upturn in the UV spectra of ellipticals, ranging from galaxies in the local Universe to distant clusters. In nearby galaxies, this UV upturn is produced by a minority population of extreme horizontal branch (EHB) stars, with the large variations observed in the UV-to-optical flux ratio driven by variations in the number of EHB stars, and not the type of UV-bright stars. Deep UV images of the nearest elliptical galaxy, M32, show that it has a well-populated EHB, even though it has the weakest UV upturn of any known elliptical galaxy. However, M32 suffers from a striking dearth of the hot post-HB stars expected from canonical evolutionary theory. As we observe to larger lookback times in more distant galaxy clusters, the UV upturn fades, as predicted by theories of stellar and galactic evolution, but does so gradually. Because the EHB stars do not appear suddenly in the Universe, their presence is likely driven by a large dispersion in the parameters that govern HB morphology.  相似文献   

8.
In this contribution, we explore some open questions about Extreme Horizontal Branch (EHB) stars in globular clusters. In particular, we present the current status of the search for rapid pulsators, He-depleted stars, and close binaries, three kind of objects very common among field EHB stars, but which first surveys failed to detect in clusters. We also analyze how the lack of cluster EHB binaries can reconcile with theoretical expectations. We give special attention to the first close EHB binary discovered in a globular cluster, whose characteristics point to a very rare or even unique object. Finally, we analyze some recent puzzling results about spectroscopically derived masses, that could point to the presence of two distinct families of EHB’s in clusters. For all these topics, we present the recent advancement in knowledge, the results requiring more investigation, and what has still to be done to fix the unsolved problems, showing the main points of our studies, the aims of our works, and what we expect to obtain from them.  相似文献   

9.
I briefly review the method of population synthesis of binary stars and discuss the preliminary results of a study of the Galactic population of subdwarf B stars. In particular I focus on the formation of (apparently) single sdB stars and their relation to (apparently) single helium-core white dwarfs. I discuss the merits of mergers of two helium white dwarfs and interactions with sub-stellar companions for explaining these single objects. A preliminary conclusion is that the current observations suggest both mechanisms may contribute, but that the helium white dwarfs are likely formed in majority from interactions with sub-stellar companions.  相似文献   

10.
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   

11.
Assuming that soft X-ray sources in symbiotic stars result from strong thermonuclear runaways, and supersoft X-ray sources from weak thermonuclear runaways or steady hydrogen burning symbiotic stars, we investigate the Galactic soft and supersoft X-ray sources in symbiotic stars by means of population synthesis. The Galactic occurrence rates of soft X-ray sources and supersoft X-ray sources are from ～2 to 20 yr?1, and ～2 to 17 yr?1, respectively. The numbers of X-ray sources in symbiotic stars range from 2390 to 6120. We simulate the distribution of X-ray sources over orbital periods, masses and mass accretion rates of white dwarfs. The agreement with observations is reasonable.  相似文献   

12.
We propose a number of prospective observational programs for the ultraviolet space observatory WSO-UV, which seem to be of great importance to modern galactic astronomy. The programs include the search for binary Cepheids; the search and detailed photometric study and the analysis of radial distribution of UV-bright stars in globular clusters (“blue stragglers”, blue horizontal-branch stars, RR Lyrae variables, white dwarfs, and stars with UV excesses); the investigation of stellar content and kinematics of young open clusters and associations; the study of spectral energy distribution in hot stars, including calculation of the extinction curves in the UV, optical and NIR; and accurate definition of the relations between the UV-colors and effective temperature. The high angular resolution of the observatory allows accurate astrometric measurements of stellar proper motions and their kinematic analysis.  相似文献   

13.
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   

14.
The actual knowledge on Horizontal Branch stars of the metal-poor Globular Clusters of our Galaxy suggests that these stars evolve along the Horizontal Branch from the very blue stars towards lower effective temperature.From this behaviour it follows that a substantial mass loss occurs at the He-flash. On the basis of a gravitational thermalization of Horizontal Branch stars any recent observational data concerning NGC 7006 are re-examined. It is pointed out that today it is not necessary to leave the hypothesis of a strong correlation between the metal and helium content for our Galaxy.At present at the Osservatorio Astronomico, Bologna.  相似文献   

15.
A model of compact galactic nuclei in statistical equilibrium was developed in [L. Sh. Grigorian and G. S. Sahakian, Astrofizika (in press)]. It was shown that they should consist predominantly of neutron stars (pulsars) and white dwarfs. The problem of the energy reserves of galactic nuclei is discussed in terms of this concept. The mechanism of conversion of a white dwarf into a neutron star due to the accretion of interstellar matter is considered. This means that a galactic nucleus has an energy reserve of some 5·1060 N8 erg (N is the number of stars in the nucleus). It is shown that galactic nuclei are powerful sources of hard γ radiation [power L » 2·1044µ30N8(Ω/50)17/7 erg/sec, where µ is the magnetic moment and Ω is the angular rotation rate of a neutron star ] due to curvature radiation from relativistic electron fluxes flowing along channels of open magnetic field lines of pulsars. The x-ray and ultraviolet emission are due to synchrotron emission from the same electron fluxes in the magnetic field of the galactic nucleus (L » 1042-1044 erg/sec). The optical (visible and infrared) and radio emission are due to bremsstrahlung from electrons in the interstellar medium [L » 6·1046N 8 2 (5/Rpc)3 erg/sec, where R is the radius of the galactic nucleus]. An equation is obtained for the magnetic moment of a pulsar: µ ≈ 3.4·10-5LγP17/7, where P is the pulsar’s period and L03B3; is the luminosity of the pulsar’s y radiation.  相似文献   

16.
The consequences of gas-liquid phase transitions in the core of hot white dwarf stars are discussed. Expressions for the latent heat and the liquefaction curveT l =T l (Q) are obtained. Then amodel for a hot white dwarf is introduced and the corresponding liquefaction sequences are built on the H-R diagram; relations luminosity-central temperature and effective temperature-central temperature are also given for liquefying white dwarfs.Finally the cooling curves are obtained for such stars taking into account the effect of latent heat emission.Our results seem to suggest a possible identification of the central stars of planetary nebulae as hot liquefying white dwarfs.  相似文献   

17.
A possible scenario for the evolution of the universe following the big bang at t > 10-5 sec is considered. The necessary conditions that must be present for the formation of stars and stellar systems to be possible are formulated. As a condition for the formation of stars we take kTs≤ GMsmp(3R), and for stellar systems HR ? (GM/R)1/2, where Ts is the temperature of the cosmic plasma, mp is the mass of a proton, Ms is the mass of a star, M is the mass of a stellar cluster, R is the radius of these celestial bodies, and H is the bubble parameter for the corresponding time. In accordance with these criteria, we assume that in the course of cosmological expansion, neutron stars should have been formed first (times 2.10-4 ? t ? 1 sec, densities 0.07 ? ρB? 2.104 g/cm3) and then, in chronological order, appeared white dwarfs (t ≈ 102 sec, ρB ? 5.10-3 g/cm3), ordinary stars (t ≈ 4.106 sec, ?B ≈ 10-11 g/cm3), galactic nuclei (t ≈ 3.1011 sec, ?B ≈ 5.10-19 g/cm3, globular clusters (t ≈ 1013 sec, ?B ≈ 4.10-21 g/cm3), and galaxies (t ≈ 1015 sec, ?B ≈ 10-24 g/cm3), where ?B is the average density of ordinary (baryon) matter in the universe. It is shown that a galactic nucleus is a stellar system in statistical equilibrium and consists mainly of neutron stars and white dwarfs. The formation of some pulsars (neutron stars with angular rotation rates 1 < Ω < 200 sec-1) may occur in a galactic nucleus. Observed pulsars should therefore contain some fraction of neutron stars from the nucleus of the Galaxy that were able to escape it over the relaxation time (the tail of the Maxwell distribution, with star velocities v > v0, where v0 is the velocity corresponding to the work function 2GMMs/R, M being the mass and R the radius of the Galaxy’s nucleus.  相似文献   

18.
Spherically symmetric stellar clusters (compact galactic nuclei and globular clusters), far advanced toward the state of complete statistical equilibrium in the course of evolution, are investigated. The equation of state of such systems (a polytrope with an index k = 0.5) is derived and their main characteristics are calculated. It is shown that compact galactic nuclei must consist mainly of rapidly rotating neutron stars and white dwarfs. It is demonstrated that pulsars may be created by the evaporation of neutron stars from the nucleus of our Galaxy. The number of such pulsars is ~3.106. Translated from Astrofizika, Vol. 41, No. 1, pp. 41–50, January-March, 1998.  相似文献   

19.
We investigate whether the recently observed population of high-velocity white dwarfs can be derived from a population of binaries residing initially within the thin disc of the Galaxy. In particular, we consider binaries where the primary is sufficiently massive to explode as a Type II supernova. A large fraction of such binaries are broken up when the primary then explodes as a supernova, owing to the combined effects of the mass loss from the primary and the kick received by the neutron star on its formation. For binaries where the primary evolves to fill its Roche lobe, mass transfer from the primary leads to the onset of a common envelope phase during which the secondary and the core of the primary spiral together as the envelope is ejected. Such binaries are the progenitors of X-ray binaries if they are not broken up when the primary explodes. For those systems that are broken up, a large number of the secondaries receive kick velocities ∼100–200 km s−1 and subsequently evolve into white dwarfs. We compute trajectories within the Galactic potential for this population of stars and relate the birth rate of these stars over the entire Galaxy to those seen locally with high velocities relative to the local standard of rest (LSR) . We show that for a reasonable set of assumptions concerning the Galactic supernova rate and the binary population, our model produces a local number density of high-velocity white dwarfs compatible with that inferred from observations. We therefore propose that a population of white dwarfs originating in the thin disc may make a significant contribution to the observed population of high-velocity white dwarfs.  相似文献   

20.
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   

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