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1.
The excess of the rate of type I X-ray bursts over that expected when the matter fallen between bursts completely burns out in a thermonuclear explosion which is observed in bursters with a high persistent luminosity (4 × 1036 ? LX ? 2 × 1037 erg s?1) is explained in terms of the model of a spreading layer of matter coming from the accretion disk over the neutron star surface. In this model the accreting matter settles to the stellar surface mainly in two high-latitude ring zones. Despite the subsequent spreading of matter over the entire star, its surface density in these zones turns out to be higher than the average one by 2–3 orders of magnitude, which determines the predominant ignition probability. The multiple events whereby the flame after the thermonuclear explosion in one ring zone (initial burst) propagates through less densematter to another zone and initiates a second explosion in it (recurrent burst) make a certain contribution to the observed excess of the burst rate. However, the localized explosions of matter in these zones, after which the burning in the zone rapidly dies out without affecting other zones, make a noticeably larger contribution to the excess of the burst rate over the expected one. 相似文献
2.
We analyze the spectra of DR Tau in the wavelength range 1200 to 3100 Å obtained with the GHRS and STIS spectrographs from the Hubble Space Telescope. The profiles for the C IV 1550 and He II 1640 emission lines and for the absorption features of some lines indicate that matter falls to the star at a velocity ～300 km s?1. At the same time, absorption features were detected in the blue wings of the N I, Mg I, Fe II, Mg II, C II, and Si II lines, suggesting mass outflow at a velocity up to 400 km s?1. The C II, Si II, and Al II intercombination lines exhibit symmetric profiles whose peaks have the same radial velocity as the star. This is also true for the emission features of the Fe II and H2 lines. We believe that stellar activity is attributable to disk accretion of circumstellar matter, with matter reaching the star mainly through the disk and the boundary layer. At the time of observations, the accretion luminosity was Lac ? 2L⊙ at an accretion rate ?10?7M⊙ yr?1. Concurrently, a small (<10%) fraction of matter falls to the star along magnetospheric magnetic field lines from a height ～R*. Within a region of size ?3.5R*, the disk atmosphere has a thickness ～0.1R* and a temperature ?1.5 × 104 K. We assume that disk rotation in this region significantly differs from Keplerian rotation. The molecular hydrogen lines are formed in the disk at a distance <1.4 AU from the star. Accretion is accompanied by mass outflow from the accretion-disk surface. In a region of size <10R*, the wind gas has a temperature ～7000 K, but at the same time, almost all iron is singly ionized by H I L α photons from inner disk regions. Where the warm-wind velocity reaches ?400 km s?1, the gas moves at an angle of no less than 30° to the disk plane. We found no evidence of regions with a temperature above 104 K in the wind and leave open the question of whether there is outflow in the H2 line formation region. According to our estimate, the star has the following set of parameters: M* ? 0.9M⊙, R* ? 1.8R⊙, L* ? 0.9L⊙, and \(A_V \simeq 0\mathop .\limits^m 9\). The inclination i of the disk axis to the line of sight cannot be very small; however, i≤60°. 相似文献
3.
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
4.
We investigate the combined effect of neutron and proton superfluidities on the cooling of neutron stars whose cores consist of nucleons and electrons. We consider the singlet state paring of protons and the triplet pairing of neutrons in the cores of neutron stars. The critical superfluid temperatures T c are assumed to depend on the matter density. We study two types of neutron pairing with different components of the total angular momentum of a Cooper pair along the quantization axis (|m J |=0 or 2). Our calculations are compared with the observations of thermal emission from isolated neutron stars. We show that the observations can be interpreted by using two classes of superfluidity models: (1) strong proton superfluidity with a maximum critical temperature in the stellar core T c max ?4×109 K and weak neutron superfluidity of any type (T c max ?2×108 K); (2) strong neutron superfluidity (pairing with m J =0) and weak proton superfluidity. The two types of models reflect an approximate symmetry with respect to an interchange of the critical neutron and proton pairing temperatures. 相似文献
5.
The following conclusions about the kinematics and parameters of the gas in the vicinity of TW Hya have been drawn from an analysis of optical and ultraviolet line profiles and intensities. The accreting matter rises in the magnetosphere to a distance z>R* above the disk plane and falls to the star near its equator almost perpendicular to its plane. The matter outflows from a disk region with an outer radius of ≤0.5 AU. The [OI], [SII], and H2 lines originate in the disk atmosphere outside the outflow region, where the turbulent gas velocity is close to the local speed of sound. In the formation region of the forbidden lines, T?8500 K and Ne?5×106 cm?3, and the hydrogen is almost neutral: xe<0.03. The absorption features observed in the blue wings of some of the ultraviolet lines originate in the part of the wind that moves almost perpendicular to the disk plane, i.e., in the jet of TW Hya. The V z gas velocity component in the jet decreases with increasing distance from the jet axis from 200 to 30 km s?1. The matter outflowing from the inner disk boundary, moves perpendicular to the disk plane in the formation region of blue absorption line components, at a distance of ～0.5 AU from the axis of symmetry of the disk. This region of the wind is collimated into the jet at a distance of <3 AU from the disk plane. The gas temperature in the formation region of absorption components is ?2×104 K, and the gas density is <3×106 cm?3. This region of the jet is on the order of several AU away from the disk plane, while free recombination in the jet begins even farther from the disk. The mass-loss rate for TW Hya is \(\dot M_w < 7 \times 10^{ - 10} M_ \odot yr^{ - 1}\), which is a factor of 3lower than the mean accretion rate. The relative abundance of silicon and aluminum in the jet gas is at least an order of magnitude lower than its standard value. 相似文献
6.
We consider an equation of state that leads to a first-order phase transition from the nucleon state to the quark state with a transition parameter λ>3/2 (λ=ρQ/(ρN+P0/c2)) in superdense nuclear matter. Our calculations of integrated parameters for superdense stars using this equation of state show that on the stable branch of the dependence of stellar mass on central pressure dM/dPc>0) in the range of low masses, a new local maximum with Mmax=0.082⊙ and R=1251 km appears after the formation of a toothlike kink (M=0.08M⊙, R=205 km) attributable to quark production. For such a star, the mass and radius of the quark core are Mcore=0.005M⊙ and Rcore=1.73 km, respectively. In the model under consideration, mass accretion can result in two successive transitions to a quark-core neutron star with energy release similar to a supernova explosion: initially, a low-mass star with a quark core is formed; the subsequent accretion leads to configurations with a radius of ～1000 km; and, finally, the second catastrophic restructuring gives rise to a star with a radius of ～100 km. 相似文献
7.
I. F. Malov 《Astronomy Letters》2008,34(10):699-703
At present, it is widely believed that anomalous X-ray pulsars (AXPs), soft gamma-ray repeaters (SGRs), rotational radio transients (RRATs), compact central objects (CCOs) in supernova remnants, and X-ray dim isolated neutron stars (XDINSs) belong to different classes of anomalous objects in which the central bodies are isolated neutron stars. Previously, we have shown that AXPs and SGRs can be described in terms of the drift model for parameters of the central neutron star typical of radio pulsars (rotation periods P ～ 0.1–1 s and surface magnetic fields B ～ 1011–1013 G). Here, we show that some of the peculiarities of the sources under consideration can be explained by their geometry (in particular, by the angle β between the rotation axis and the magnetic moment). If β ? 10° (an aligned rotator), the drift waves in the outer layers of the neutron star magnetosphere can account for the observed periodicity in the radiation. For large β (a nearly orthogonal rotator), the observed modulation of the radiation and its short bursts can be explained by mass accretion from the ambient medium (e.g., a relic disk). 相似文献
8.
The dependence of the spin frequency derivative \(\dot \nu \) of accreting neutron stars with a strongmagnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, Lbol) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence \(\dot \nu \) (Lbol) can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation \(\dot \nu \) ～ L6/7bol. Hysteresis in the dependence \(\dot \nu \) (Lbol) has been confirmed in the system V 0332+53 and has been detected for the first time in the systems KS 1947+300, GRO J1008-57, and 1A 0535+26. Estimates for the radius of the neutron star magnetosphere in all of the investigated systems have been obtained. We show that this quantity varies from pulsar to pulsar and depends strongly on the analytical model and the estimates for the neutron star and binary system parameters. 相似文献
9.
We studied the intermediate polar TVCol during and after its flare in November 1982 observed in the ultraviolet range with the International Ultraviolet Explorer. Two spectra revealing the variations of emission lines at different times are presented. We have estimated a new value of the reddening from the 2200 Å absorption feature, E (B ? V ) = 0.12 ± 0.02, and calculated the line fluxes of C IV and He II emission lines produced in the outer accretion disk. The average ultraviolet luminosity of emitting region during and after the flare is approximately 4 × 1032 erg s?1 and 9 × 1030 erg s?1, the corresponding average mass accretion rate is nearly 3 × 1015 erg s?1 (4.76 × 10?11M⊙ yr?1) and 5 × 1013 erg s?1 (7.93 × 10?13M⊙ yr?1), and the average temperature of the emitting region during and after flare is estimated to be of about 3.5 × 103 K and 2 × 103 K. We attribute this flare to a sudden increase in the mass accretion rate leading to the outburst activity. 相似文献
10.
We present the results of JHKLM-photometry for the symbiotic Mira star candidate V 335 Vul. Based on the average flux data, supplemented by IRAS, MSX, AKARI, and WISE mid-IR observations, we calculated a model of a spherically symmetric dust envelope of the star, made up of amorphous carbon and silicon carbide particles. The optical depth of the envelope in the visible range with a dust temperature at the inner boundary of T1 = 1300 K is τ V = 0.58. For an envelope expansion velocity of 26.5 km s?1, the estimated mass loss rate is equal to 5.7 × 10?7M⊙ yr?1. 相似文献
11.
V. P. Grinin A. A. Arkharov O. Yu. Barsunova S. G. Sergeev L. V. Tambovtseva 《Astronomy Letters》2009,35(2):114-120
We present the results of our infrared JHK photometry for the unusual UX Ori star V1184 Tau. Comparison with previous observations performed before the catastrophic decline in its optical brightness in 2004 (when the star faded approximately by a factor of 100) has shown the following: the star faded approximately by 2 m and 1 m in the J and H bands, respectively, while its K brightness remained almost constant. This pattern of infrared variability seems incompatible with the mechanism of variable circumstellar extinction responsible for the dramatic decline in the star’s optical brightness. However, if this mechanism is considered in the context of an accretion disk model with a puffed-up inner rim in the dust sublimation zone and with a disk wind producing an expanding gas-dust atmosphere above the disk surface, then the paradox can be resolved. In this model, the photometric activity of V1184 Tau in both visible and near-infrared spectral ranges, including the sharp brightness decline in 2004, can be explained by an increase in the geometric thickness of the disk in the dust sublimation zone caused by enhanced accretion of circumstellar matter onto the star. There is reason to believe that such events occur periodically and result from the presence of a companion to V1184 Tau moving in a highly eccentric orbit. The offered interpretation of the photometric activity of V1184 Tau allows this object to be classified as an UX Ori star based on the observed photometric effect and, at the same time, as a FU Ori star based on the pattern of the physical process that produced this effect. 相似文献
12.
We analyze the encounters of the neutron star (pulsar) Geminga with open star clusters in the OB association Ori OB1a through the integration of epicyclic orbits into the past by taking into account the errors in the data. The open cluster ASCC21 is shown to be the most probable birthplace of either a single progenitor star for the Geminga pulsar or a binary progenitor system that subsequently broke up. Monte Carlo simulations of Geminga-ASCC21 encounters with the pulsar radial velocity V r = ?100±50 km s?1 have shown that close encounters could occur between them within ≤10 pc at about t = ?0.52 Myr. In addition, the trajectory of the neutron star Geminga passes at a distance of ≈25 pc from the center of the compact OB association λ Ori at about t = ?0.39 Myr, which is close to the age of the pulsar estimated from its timing. 相似文献
13.
During the GRIF experiment onboard the Mir orbiting station, the sky was monitored with a PX-2 wide-field (～1 sr) scintillation X-ray spectrometer to detect bursts in the photon energy range 10–300 keV. Because of the comprehensive instrumentation, which, apart from the X-ray and gamma-ray instruments, also included charged-particle detectors, the imitations of astrophysical bursts by magnetospheric electron precipitations and strongly ionizing nuclei were effectively filtered out. It was also possible to separate solar and atmospheric events. Several tens of bursts interpreted as being astrophysical were detected in the experiment at sensitivity levels S～10?7 erg cm?2 (for bursts whose spectra were characterized by effective temperatures kT～100 keV) and S～3×10?8 erg cm?2 (for bursts with kT～25 keV). Some of the soft gamma-ray or hard X-ray bursts with kT～10–50 keV were identified with the bursting pulsar GRO J1744-28. Our estimate of the detection rate for cosmological soft gamma-ray or hard X-ray bursts from the entire sky suggests that the distributions of long-duration (>1 s) gamma-ray bursts (GRBs) in characteristic energy kT and duration are inconsistent with the steady-state cosmological model in which the evolution of burst sources is disregarded. Based on GRIF and BATSE/CGRO data, we conclude that most of the GRB sources originate at redshifts 1<z<5. 相似文献
14.
Yu. A. Fadeyev 《Astronomy Letters》2007,33(10):692-698
The evolution of a Population-I star with an initial mass M ZAMS = 60 M ⊙ has been calculated. At the stage when a red giant turns into an early-type helium star, the vast bulk of the stellar mass is concentrated in a compact core surrounded by an extended envelope that is unstable with respect to radial oscillations. The range of effective temperatures within which the instability arises extends to T eff ? 105 K. For the models corresponding to the Wolf-Rayet evolutionary stage (5 × 104 K ≤ T eff ≤ 1.05 × 105 K), hydrodynamic calculations of self-exciting radial stellar pulsations have been performed. The pulsational instability develops in a time interval comparable to the dynamic timescale. Once the amplitude has ceased to grow, the pulsational motions are nonlinear traveling waves propagating from the core boundary to the stellar surface. The velocity amplitude of the outer layers is 500 km s?1 < ΔU < 103 km s?1, depending on the effective temperature. During the evolution of a helium star, the mean ratio of the maximum expansion velocity of the outer layers to the local escape velocity decreases and lies within the range 0.25 < U max/v esc < 0.6 for the models considered. The nonlinearity of the stellar pulsations is responsible for the increase in the mean radius \(\bar r\) of the Lagrangian layers compared to the equilibrium radius r eq. The effect of the increase in mean radius decreases with rising effective temperature from\(\bar r\)/r ～ 10 at T eff = 7 × 104 K to \(\bar r\)/r ≈ 2 at T eff = 105 K. The radial pulsation periods for the models considered lie within the range 0.1 day ≤ Π ≤ 1.6 day and the amplitude of the bolometric magnitude variations does not exceed 0 . m 2. 相似文献
15.
Infrared observations of the unique symbiotic system CH Cyg in 2003–2006 are presented. Analysis of the observations has shown
that a fairly dense dust structure (a cloud or a shell) appeared on the line of sight in August–November 2006. The dust grains
in the new shell are similar in optical properties to graphite ones and their sizes are mostly within the range 0.14–0.16
μm. The dust shell is optically thick and its optical depth at 2.2 μm is τ(2.2) ≈ 0.97. The dust shell mass is M
d(06) ≈ 8 × 10−6
M
⊙ and the rate of matter flow into the shell has reached ∼2 × 10−5
M
⊙ yr−1.
Original Russian Text ? O.G. Taranova, V.I. Shenavrin, 2007, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2007, Vol. 33,
No. 8, pp. 598–603. 相似文献
16.
É. A. Vitrichenko 《Astronomy Letters》2002,28(12):843-846
We measured the radial velocity of the star θ1 Ori D from IUE spectra and used published observations. Based on these data, we determined the period of its radial-velocity variations, P=20.2675±0.0010 days, constructed the phase radial-velocity curve, and solved it by least squares. The spectroscopic orbital elements were found to be the following: the epoch of periastron passage Ep=JD 2430826.6±0.1, the system's center-of-mass velocity /Gg=32.4±1.0 km s?1, K=14.3±1.5 km s?1, Ω=3.3±0.1 rad, e=0.68±0.09, a1 sin i = 3 × 1010 km, and f1 = 0.0025M⊙. Twice the period, P=40.528±0.002 days, is also consistent with the observations. 相似文献
17.
During the period 1979–1999, we investigated the hyperfine structure of the H2O supermaser region located in the core of the molecular cloud OMC-1 in Orion KL. The angular resolution is 0.1 mas, which corresponds to 0.045 AU. The detected structure, which consists of a central object, an accretion disk, a bipolar outflow, and an envelope, corresponds to the initial formation stage of a low-mass star. The accretion disk is at the stage of separation into groups of concentric rings. The bipolar outflow is a neutral, highly collimated jet of accreted material that includes H2O molecules and dust grains in the icy envelope. The injector is a bright compact source with a size <0.05 AU and a brightness temperature Tb≈1017 K. The velocity of the bipolar outflow is v≈10 km s?1. The rotation velocity of the jet is vrot≈1.5 km s?1. The jet has the shape of a conical helix due to the precession of the rotation axis. Occasionally, dense blobs (comet-shaped bullets) are ejected. The envelope amplifies the radio emission from the structures in a ～0.5 km s?1 maser window band with velocities v≈7.65 km s?1 by more than two orders of magnitude. 相似文献
18.
The accretion of hot slowly rotating gas onto a supermassive black hole is considered. The important case where the velocities of turbulent pulsations at the Bondi radius r B are low, compared to the speed of sound c s, is studied. Turbulence is probably responsible for the appearance of random average rotation. Although the angular momentum at r B is low, it gives rise to the centrifugal barrier at a depth r c = l 2 /GM BH ? r B, that hinders supersonic accretion. The numerical solution of the problem of hot gas accretion with finite angular momentum is found taking into account electron thermal conductivity and bremsstrahlung energy losses of two temperature plasma for density and temperature near Bondi radius similar to observed in M87 galaxy. The saturation of the Spitzer thermal conductivity was also taken into account. The parameters of the saturated electron thermal conductivity were chosen similar to the parameters used in the numerical simulations of interaction of the strong laser beam radiation with plasma targets. These parameters are confirmed in the experiments. It is shown that joint action of electron thermal conductivity and free-free radiation leads to the effective cooling of accreting plasma and formation of the subsonic settling of accreting gas above the zone of a centrifugal barrier. A toroidal condensation and a hollow funnel that separates the torus from the black hole emerge near the barrier. The barrier divides the flow into two regions: (1) the settling zone with slow subKeplerian rotation and (2) the zone with rapid supersonic nearly Keplerian rotation. Existence of the centrifugal barrier leads to significant decrease of the accretion rate ? in comparison with the critical Bondi solution for γ = 5/3 for the same values of density and temperature of the hot gas near Bondi radius. Shear instabilities in the torus and related friction cause the gas to spread slowly along spirals in the equatorial plane in two directions.As a result, outer (r > r c) and inner (r < r c) disks are formed. The gas enters the immediate neighborhood of the black hole or the zone of the internal ADAF flow along the accretion disk (r < r c). Since the angular momentum is conserved, the outer disk removes outward an excess of angular momentum along with part of the matter falling into the torus. It is possible, that such outer Keplerian disk was observed by Hubble Space Telescope around the nucleus of the M87 galaxy in the optical emission lines. We discuss shortly the characteristic times during which the accretion of the gas with developed turbulence should lead to the changes in the orientation of the torus, accretion disk and, possibly, of the jet. 相似文献
19.
Observations of the circumstellar maser emission from the long-period variable star Y Cas in the 1.35-cm water-vapor line are presented. The observations were performed with the RT-22 radio telescope at the Pushchino Radio Astronomy Observatory (Astrospace Center, Lebedev Physical Institute, Russian Academy of Sciences) in the period 1982–2005. The variations in the integrated flux Fint in the H2O line correlate with the visual light curve of the star. The phase delay Δ? between the Fint variations and the light curve is 0.2–0.4P (P is the period of the star). The H2O maser Y Cas belongs to transient sources: peaks of high maser activity alternate with intervals of a low emission level when the H2O-line flux does not exceed (0.1–0.5) × 10?20 W m?2. A “superperiod” of ～5.7 yr was found in the occurrence of activity peaks. A particularly strong maximum of maser radio emission took place at the end of 1997, when the flux Fint reached 15.6 × 10?20 W m?2. A model for the H2O maser variability in Y Cas is discussed. The variability is caused by a periodic action of shock waves driven by stellar pulsations. The H2O maser flares may be associated with short-lived episodes of enhanced mass loss by the star or with the propagation of a particularly strong shock wave when a planet orbiting the star passes through its periastron. 相似文献
20.
V. P. Arkhipova V. F. Esipov N. P. Ikonnikova G. V. Komissarova R. I. Noskova 《Astronomy Letters》2011,37(5):343-357
Long-term photometric and spectroscopic observations of the yellow symbiotic star LT Del are analyzed. UBV light curves are presented. Based on the observations of 20 cycles, we have refined the orbital period of the star, P = 476 · d 0 ± 1 · d 0. The brightness has been found to be unstable at some orbital phases with an amplitude up to 0 · m 3. We have measured the fluxes in hydrogen and helium emission lines and in continuum and investigated their relationship to the orbital period. The fluxes in hydrogen and HeI lines follow the UBV light curves in phase; the He II 4686 Å flux does not depend on the phase and is constant within the accuracy of our measurements. The intensity ratio of the 4686 Å andHβ lines changes from 0.2 to 0.9 over the period. We interpret the spectroscopic observations based on the hypothesis of heating and ionization of the stellar wind from a cool component by high-frequency radiation from a hot star with a temperature of 105 K. We have estimated the spectral type of the cool star from our photometry and its continuum energy distribution as a bright K2–4 red giant branch halo star. The bolometric luminosity and mass loss rate have been estimated for the K component to be L bol ～ 700L ⊙ and \(\dot{M}\) ～ 10?8 M ⊙ yr?1, respectively. 相似文献