共查询到20条相似文献，搜索用时 62 毫秒
1.
Z. I. Peikov 《Astronomy Reports》2002,46(5):352-359
Lower limits for the percentages of stars with various luminosities in the cores of six globular clusters are derived using stellar spatial density distributions f(r) to deep limiting B magnitudes obtained earlier. For NGC 6535 and NGC 5466, the logarithmic density range and Kholopov parameters Df and Dr are also determined. These two parameters are correlated with the mean masses of stars of various subsystems and the total mass (number) of stars in the cluster. 相似文献
2.
V. F. Gopka A. V. Yushchenko T. V. Mishenina Chulhee Kim F. A. Musaev A. V. Bondar’ 《Astronomy Reports》2004,48(7):577-587
The atmospheric abundances of 30 chemical elements in the halo star HD 221170 are analyzed by fitting synthetic spectra to observed spectra (i) with a resolution of 60 000 and signal-to-noise ratios of about 200 taken with the 1.93-m telescope of the Observatoire de Haute Provence and (ii) with a resolution of 35 000 and signal-to-noise ratios of more than 100 taken with the 2-m telescope of the Terskol Peak Observatory. The derived parameters of the stellar atmosphere are Teff=4475 K, log g=1.0, [Fe/H]=?2.03, Vmicro=1.7 km/s, and Vmacro=4 km/s. The parameters Teff, log g, [Fe/H], and Vmicro can be determined by analyzing the variations of the rms error of the mean iron abundance derived using different model atmospheres. The chemical composition of the star’s atmosphere is analyzed. The abundances of a total of 35 elements in HD 221170 have been derived in this paper and in previous studies. Overall, the abundances of elements lighter than praseodymium are consistent with the elemental abundances in the atmospheres of stars with similar metal deficits. Copper and manganese are underabundant by ?2.9 and ?2.6 dex, respectively, relative to the Sun (when the analysis includes the effects of hyperfine structure). Heavy r-process elements (starting from praseodymium) are overabundant compared to iron-group elements. This can be explained by an enrichment in r-process elements of the material from which the star was formed. 相似文献
3.
The first high-accuracy CCD UBV RI(RI)C light curves for the recently discovered eclipsing binary V798 Cep (P = 16 d .08, V = 11 m . 8) are presented; this star is included in our program of eclipsing systems with considerable eccentricities. A photometric solution for the light curves and physical characteristics of the component stars are derived. The orbital eccentricity is quite high, e = 0.437. The longitude of periastron is close to 180°, making studies of the apsidal motion difficult. V798 Cep may be a hierarchical system. 相似文献
4.
Yu. A. Fadeev 《Astronomy Reports》2008,52(8):645-655
The evolution of Population I stars with initial masses 60 M ⊙ ≤ M ZAMS ≤ 120 M ⊙ is computed up to the Wolf-Rayet stage, when the central helium abundance decreases to Y c ≈ 0.05. Several models from evolutionary sequences in the core helium-burning stage were used as initial conditions when solving the equations of radiative hydrodynamics for self-exciting stellar radial pulsations. The low-density envelope surrounding the compact core during the core helium burning is unstable against radial oscillations in a wide range of effective temperatures extending to T eff ～ 105 K. The e-folding time of the amplitude growth is comparable to the dynamical time scale of the star, and, when the instability ceases growing, the radial displacement of the outer layers is comparable to the stellar radius. Evolutionary changes of the stellar radius and luminosity are accompanied by a decrease in the amplitude of radial pulsations, but, at the effective temperature T eff ≈ 105 K, the stellar oscillations are still nonlinear, with a maximum expansion velocity of the outer layers of about one-third the local escape velocity. The period of the radial oscillations decreases from 9 hr to 4 min as stellar mass decreases from M = 28 M ⊙ to M = 6 M ⊙ in the course of evolution. The nonlinear oscillations lead to a substantial increase of the radii of the Lagrangian mass zones compared to their equilibrium radii throughout the instability region. The instability of Wolf-Rayet stars against radial oscillations is due to the action of the κ mechanism in the iron-group ionization zone, which has a temperature of T ～ 2 × 105 K. 相似文献
5.
We have determined the main parameters of the old precataclysmic variable stars MS Peg and LM Com. The radial velocities of the components, reflection effects in the spectra, and light curves of the systems are studied based on model stellar atmospheres subject to external irradiation. Forty-seven moderate-resolution spectra for MS Peg and 57 for LM Com obtained with the 6-m telescope of the Special Astrophysical Observatory are used to derive the refined orbital periods of 0.1736660 days and 0.2586873 days, respectively; the orbital eccentricities do not exceed e=0.04. The mass (M w =0.49e⊙) and radius (e w =0.015R⊙) of the MS Peg primary calculated using the gravitational redshift correspond to those for a cooling carbon white dwarf with a thin hydrogen envelope. The parameters of the red dwarf (M r =0.19M⊙, Teff=3560 K, R r =0.18R⊙) are close to those derived from evolutionary tracks for main-sequence M stars with solar chemical composition. The radius (R r =0.22R⊙) and temperature (Teff=3650 K) of the LM Com secondary exceed theoretical estimates for main-sequence stars with masses of M r =0.17M⊙. The luminosity excess of the red dwarf in LM Com can be explained by a prolonged (T>5×106 yrs) relaxation of the M star to its normal state after the binary leaves the common-envelope stage. For both systems, theoretical U, B, V, and R light curves and spectra calculated using the adopted sets of parameters are generally consistent with the observations. This confirms the radiative origin of the hot spots, the unimportance of horizontal radiative transport, and the absence of large-scale velocity fields with high values (Vtrans>50 km/s) at the surfaces of the secondaries. Most of the emission lines in the spectra of these objects are formed under conditions close to thermalization, enabling modeling of their pro files in an LTE approximation. A strong λ3905 Å emission line has been identified as the 3s23p4s 1P0-3s23p2 1S SiI λ3905.52 Å line formed in the atmosphere of the hot spot. The observed intensity can be explained by non-LTE “superionization” of SiI atoms by soft UV radiation from the white dwarf. We suggest a technique for identifying binaries whose cool components are subject to UV irradiation based on observations of λ3905 Å emission in their spectra. 相似文献
6.
I. F. Malov 《Astronomy Reports》2001,45(5):389-394
Various mechanisms for the loss of angular momentum of neutron stars are analyzed. Theoretical predictions about the evolution of the period are compared with the observed distribution of pulsars on the log\(\dot P\)log(P) diagram. Pulsars with short periods (P≤0.1 s) cannot be fit well by any of the models considered. Their braking index is n=?1, which requires the development of a new braking mechanism. The evolution of pulsars with P>1.25 s is described by the law \(\dot P \propto P^2\), probably due to processes internal to the neutron stars. The observational data for pulsars with 0.1<P≤1.25 s can be fit with a hybrid model incorporating internal processes and magnetic-dipole losses. The magnetic fields in pulsar catalogs should be recomputed in accordance with the results obtained. For example, the magnetic fields obtained for two magnetars with P=5.16 s and P=7.47 s are B s =1.7×1013 and 2.9×1013 G, which are lower than the critical field Bcr=4.4×1013 G. For a substantial fraction of pulsars, their characteristic ages \(\tau = P/2\dot P\) cannot serve as measures of their real ages. 相似文献
7.
We consider the formation of massive stars under the assumption that a young star accretes material from the protostellar cloud through its accretion disk while losing gas in the polar directions via its stellar wind. The mass of the star reaches its maximum when the intensity of the gradually strengthening stellar wind of the young star becomes equal to the accretion rate. We show that the maximum mass of the forming stars increases with the temperature of gas in the protostellar cloud T 0, since the rate at which the protostellar matter is accreted increases with T 0. Numerical modeling indicates that the maximum mass of the forming stars increases to ～900 M ⊙ for T 0 ～ 300 K. Such high temperatures of the protostellar gas can be reached either in dense star-formation regions or in the vicinity of bright active galactic nuclei. It is also shown that, the lower the abundance of heavy elements in the initial stellar material Z, the larger the maximum mass of the star, since the mass-loss rate due to the stellar wind decreases with decreasing Z. This suggests that supermassive stars with masses up to 106 M ⊙ could be formed at early stages in the evolution of the Universe, in young galaxies that are almost devoid of heavy elements. Under the current conditions, for T 0 = (30–100) K, the maximum mass of a star can reach ～100M ⊙, as is confirmed by observations. Another opportunity for the most massive stars to increase their masses emerges in connection with the formation and early stages of evolution of the most massive close binary systems: the most massive stars can be produced either by coalescence of the binary components or via mass transfer in such systems. 相似文献
8.
We have obtained the stellar velocity dispersion in three mutually perpendicular directions in the halos and cores of clusters as a function of time for several non-stationary open-cluster models. During the dynamical evolution of the open-cluster models, the velocity dispersions undergo oscillations that do not decay during 5–10 violent-relaxation timescales, τ vr . We estimated the time for synchronization of the rotation of the open-cluster models and their motion around the center of the Galaxy, t s , which, depending on the model parameters, is t s ? (5–27)τ vr . Synchronization mechanisms for the models are discussed. The disruption of the systems in the force field of the Galaxy is strongly affected by tidal friction. We have also estimated the time for the formation of a spherical stellar-velocity distribution in the cluster models, t σ ? (6 ? 25)τ vr . The impact of instability in the stellar motions in a cluster on the formation of a spherical velocity distribution in the open-cluster models is discussed. We have noted a tendency for a weakening of the dependence of the coarse phase density of the cluster on small initial perturbations of the stellar phase coordinates in the model cluster cores for times about five times longer than the violent-relaxation time. 相似文献
9.
Evolutionary connection between low-mass detached binaries,contact W UMa systems,and blue stragglers
We analyze the distribution of close binary stars in the orbital semimajor axis—primary mass plane. The reduced spatial density of stars with semimajor axes below 10R⊙ is confirmed. We identify the area in this plane occupied by precursors of W UMa stars, assuming that the driving force causing the components to approach each other is their magnetic stellar wind. This scenario enables us to estimate the rate of formation (0.02/year) and lifetime (108 yr) of W UMa stars. We derive a theoretical estimate of the ratio of the number of blue stragglers, N BS , and of horizontal-branch stars, N HB , in globular clusters based on the hypothesis that all blue stragglers are the result of component mergers in W UMa contact binaries. This ratio is N BS /N HB =0.4, close to the observed value for 62 Galactic globular clusters. We discuss possible reasons for the considerable dispersion of the observed estimates of this ratio for different clusters in our Galaxy. 相似文献
10.
Using atmosphere models based on high-resolution spectra, we have derived the abundances of chemical elements in the atmospheres of seven classical barium stars and compared them with the elemental abundances of moderate barium stars and normal red giants. The behavior of elements up to the iron peak is the same in all three groups of giants, providing evidence that they have a common origin. The dependence of the anomalous abundances of s-process elements on stellar mass and metallicity is qualitatively similar for all three groups, probably indicating that a substantial role is played by the evolutionary phase of the stars. We conclude that the barium-star phenomenon and the overabundances of s-process elements in barium stars cannot be explained as a consequence of binarity alone. The extent to which the s-process elements are overabundant is affected by the mass, metallicity, and evolutionary phase of the given star, and any of these parameters may prove to be important in a specific object. 相似文献
11.
A mechanism for the separation of chemical elements and isotopes in the atmospheres of chemically peculiar (CP) stars due to light-induced drift (LID) of ions is discussed. The efficiency of separation due to LID is proportional to the relative difference of the transport frequencies for collisions of ions of heavy elements located in the excited state (collision frequency ν e ) and ground state (collision frequency ν g ) with neutral buffer particles (hydrogen and helium), (ν e ? ν g )/ν g . The known interaction potentials are used to numerically compute the relative difference (ν e H ? ν g H )/νg H for collisions between the ions Be+, Mg+, Ca+, Sr+, Cd+, Ba+, Al+, and C+ and hydrogen atoms. These computations show that, at the temperatures characteristic of the atmospheres of CP stars, T = 7000?20 000 K, values of |ν e H ?ν g H |/ν g H ≈ 0.1?0.4 are obtained. With such relative differences in the transport collision frequencies, the LID rate of ions in the atmospheres of coolCP stars (T < 10000 K) can reach ~0.1 cm/s,which exceeds the drift rate due to light pressure by an order of magnitude. This means that, under these conditions, the separation of chemical elements under the action of LID of ions could be an order of magnitude more efficient than separation due to light pressure. Roughly the same manifestations of LID and light pressure are also expected in the atmospheres of hotter stars (20 000 > T > 10 000 K). LID of heavy ions is manifest only weakly in very hot stars (T > 20 000 K). 相似文献
12.
13.
A. I. Bogomazov 《Astronomy Reports》2005,49(9):709-713
Observations of eclipses of the radio pulsar B1259-63 by the disk of its Be-star companion SS 2883 provide an excellent opportunity to study the winds of stars of this type. The eclipses lead to variations in the radio flux (due to variations in the free-free absorption), dispersion measure, rotation measure, and linear polarization of the pulsar. We have carried out numerical modeling of the parameters of the Be-star wind and compared the results with observations. The analysis assumes that the Be-star wind has two components: a disk wind in the equatorial plane of the Be star with a power-law fall-off in the electron density n e with distance from the center of the star \(\rho (n_e \sim \rho ^{ - \beta _o } )\), and a spherical wind above the poles. The parameters for a disk model of the wind are estimated. The disk is thin (opening angle 7.5°) and dense (electron density at the stellar surface n0e ～ 1012 cm?3, β0 = 2.55). The spherical wind is weak (n0e ? 109 cm?3, β0 = 2). This is the first comparison of calculated and observed fluxes of the pulsating radio emission. 相似文献
14.
S. A. Lamzin 《Astronomy Reports》2003,47(6):498-510
We have calculated profiles of the CIV 1550, NV 1240, OVI 1035, and SiIV 1400 resonance doublets for a plane-parallel shock viewed at various angles. Calculations were performed for the range of preshock gas velocities V0 and gas densities ρ0 appropriate for classical T Tauri stars. The parameters of accretion shocks in young stars can be determined by comparing the calculated and observed profiles of the studied lines and their relative intensities. It is not possible to derive the parameters of the accreting gas from the line profiles without knowing the geometry of the accretion zone. The relation I v (µ,V0,ρ0) for a plane shock, where I v is the intensity μ=cosθ, can be used to determine the accretion parameters by either choosing a geometry for the radiating region or using a technique similar to Doppler tomography. The results obtained for DR Tau, T Tau, and RY Tau indicate that, in contrast to current concepts, the inner regions of the accretion disk are not disrupted by the magnetic field of the star, and the disk reaches the stellar surface. As a result, only a small fraction of the accreted matter passes through the shock and falls onto the star. 相似文献
15.
A. N. Gerashchenko 《Astronomy Reports》2007,51(7):542-548
A CCD BV R photometric study of the central region (15″ ≤ r ≤ 100″) of the globular cluster NGC 7006 based on color-magnitude diagrams is presented. We find for the main parameters of the cluster [Fe/H] = ?1.62, Y = 0.21, E B?V = 0.15 m , V HB = 18.84 m , M V HB =+0.56 m , R ⊙ = 37.1 kpc). Two previously unknown RR Lyr variables were discovered in the central region of the cluster. The morphological index of the horizontal branch for the entire region studied indicates that the red stellar population dominates, consistent with previous studies: HB mi = ?0.13. Such anomalously negative morphological indices are possessed by a whole group of Ool clusters with intermediate metallicities, which also display a characteristic distribution of stars along the horizontal branch. There is a radial dependence for the horizontal-branch morphology, with the color becoming primarily blue with approach toward the cluster center. One possible origin for this behavior could be the effect of inner dynamical processes on the spatial distribution of hot stars. 相似文献
16.
We have obtained high-accuracy photoelectric measurements of ES Lac, an eclipsing binary with an elliptical orbit (B9III + B9III; P = 4.459d, e = 0.198) in 1985–2004 at the Sternberg Astronomical Institute’s Tien Shan High-Altitude Observatory. Our detailed analysis of the 19-year uniform series of measurements has yielded the first photometric elements for this system, as well as a self-consistent set of physical and geometrical parameters for the binary. The virtually identical components (M 1 = M 2 = 3.0 M ⊙; R 1 = R 2 = 4.12 R ⊙) are appreciably separated from the main sequence, and are located on the giant branch: their age is t = (3.5 ± 0.2) × 108 yrs. An analysis of our observations together with previously published times of minima has enabled a considerable refinement of the period of the apsidal motion, U = 355 ± 20 years, and a first determination of the apsidal parameter reflecting the radial density distributions for the components stars: k 2 obs = 0.00213(18). This value is in a good agreement with the value expected theoretically for current evolutionary models of such stars: k 2 th = 0.00257(15). 相似文献
17.
It is shown that the approximation of the complex, tidally distorted shape of a star as a circular disc with local line profiles and a linear limb-darkening law, which is usually applied when deriving equatorial stellar rotation velocities from line profiles, leads to overestimation of the equatorial velocity V rot sin i and underestimation of the component mass ratio q = M x /M v . A formula enabling correction of the effect of these simplifying assumptions on the shape of a star is used to re-determine the mass ratios q and the masses of the black holes M x and visual components M v in low-mass X-ray binary systems containing black holes. Taking into account the tidal–rotational distortion of the stellar shape can significantly increase the mass ratios q = M x /M v , reducing M v , while M x changes only slightly. The resulting distribution of M v attains its maximum near M v ? 0.35M ⊙, in disagreement with the results of population synthesis computations realizing standard models for Galactic X-ray novae with black holes. Possible ways to overcome this inconsistency are discussed. The derived distribution of M x also differs strongly from the mass distribution for massive stars in the Galaxy. 相似文献
18.
The He/H abundances in HII regions in Blue Compact Dwarf Galaxies are redetermined using the new recombination coefficients of Benjamin et al. The electron number density n e in the He+ region, optical depth τ3889 in the HeI λ3889 Å line, and coefficient of underlying stellar absorption aHeI are determined using the self-consistent method of Olive and Skillman. The primordial helium abundance and its enrichment are found using the helium abundances obtained in this paper and heavy-element abundances from our recent paper I: Y p = 0.244 ± 0.004 and dY/dZ = 8.8 ± 4.6. 相似文献
19.
Observations of the K2 continuation of Kepler Space Telescope program are used to estimate the spot coverage S (the fractional spotted area on the surface of an active star) for stars of the Pleiades cluster. The analysis is based on data on photometric variations of 759 confirmed clustermembers, together with their atmospheric parameters, masses, and rotation periods. The relationship between the activity (S) of these Pleiades stars and their effective temperatures shows considerable change in S for stars with temperatures T eff less than 6100 K (this can be considered the limiting value for which spot formation activity begins) and a monotonic increase in S for cooler objects (a change in the slope for stars with Teff ~ 3700 K). The scatter in this parameter ΔS about its mean dependence on the (V ?Ks)0 color index remains approximately the same over the entire (V?K s )0 range, including cool, fully convective dwarfs. The computated S values do not indicate differences between slowly rotating and rapidly rotating stars with color indices 1.1 < (V?K s )0 < 3.7. The main results of this study include measurements of the activity of a large number of stars having the same age (759 members of the Pleiades cluster), resulting in the first determination of the relationship between the spot-forming activity and masses of stars. For 27 stars with masses differing from the solarmass by nomore than 0.1M⊙, themean spot coverage is S = 0.031±0.003, suggesting that the activity of candidate young Suns is more pronounced than that of the present-day Sun. These stars rotate considerably faster than the Sun, with an average rotation period of 4.3d. The results of this study of cool, low-mass dwarfs of the Pleiades cluster are compared to results from an earlier study of 1570 M stars. 相似文献
20.
We have performed speckle interferometry with the 6-m telescope of the Special Astrophysical Observatory and spectroscopy (at 3700–9200 Å) with the 2-m telescope at Peak Terskol of the spectroscopic and interferometric binary 9 Cyg, which is a composite-spectrum star with an orbital period of 4.3 yrs. The atmosphere of the system’s primary component is analyzed in detail. The luminosities of both components estimated to be L 1 = 103.8 L ⊙, L 2 = 55.2 L ⊙, where L ⊙ is the solar luminosity, and their effective temperatures to be T e (1) = 5300 K and T e (2) = 9400 K. The abundances of C, N, O, Fe, and other elements in the primary’s atmosphere have been derived. The chemical composition shows signatures of mixing of material from its atmosphere and the region of nuclear reactions. The evolutionary status of 9 Cyg has been determined. The binary’s age is about 400 million years; the brighter star is already in the transition to becoming a red giant, while the secondary is still in the hydrogen-burning stage near the zero-age main sequence. We suggest an evolutionary model for the binary’s orbit that explains the high eccentricity, e = 0.79. 相似文献