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1.
In a series of papers, we developed a technique for estimating the inner eccentricity in hierarchical triple systems, with the inner orbit being initially circular. However, for certain combinations of the masses and the orbital elements, the secular part of the solution failed. In this paper, we derive a new solution for the secular part of the inner eccentricity, which corrects the previous weakness. The derivation applies to hierarchical triple systems with coplanar and initially circular orbits. The new formula is tested numerically by integrating the full equations of motion for systems with mass ratios from 10−3 to 103. We also present more numerical results for short-term eccentricity evolution, in order to get a better picture of the behaviour of the inner eccentricity.  相似文献   

2.
The inner product provides a conceptually and algorithmically simple method for calculating the comoving distance between two cosmological objects given their redshifts, right ascension and declination, and arbitrary constant curvature. The key to this is that just as a distance between two points 'on' the surface of the ordinary 2-sphere 2 is simply an arc-length (angle multiplied by radius) in ordinary Euclidean 3-space ℰ3, the distance between two points 'on' a 3-sphere 3 (a 3-hyperboloid ℋ3) is simply an 'arc-length' in Euclidean 4-space ℰ4 (Minkowski 4-space ℳ4), i.e. an 'hyper-angle' multiplied by the curvature radius of the 3-sphere (3-hyperboloid).  相似文献   

3.
Invisible plasma content in blazar jets such as protons and/or thermal electron–positron ( e ±) pairs is explored through combined arguments of dynamical and radiative processes. By comparing physical quantities required by the internal shock model with those obtained through the observed broad-band spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair of cold shells resides in about 2 ∼ 20, which implies that the shocks are at most mildly relativistic. Using the obtained Lorentz factors, the total mass density ρ in the shocked shells is investigated. The upper limit of ρ is obtained from the condition that thermal bremsstrahlung emission should not exceed the observed γ-ray luminosity, whilst the lower limit is constrained from the condition that the energy density of non-thermal electrons is smaller than that of the total plasma. Then, we find ρ is 102–103 times heavier than that of non-thermal electrons for pure   e ±  pairs, while 102–106 times heavier for pure electron–proton ( e / p ) content, implying the existence of a large amount of invisible plasma. The origin of the continuous blazar sequence is briefly discussed and we speculate that the total mass density and/or the blending ratio of e ± pairs and e / p plasma could be new key quantities for the origin of the sequence.  相似文献   

4.
On a time-symmetric Hermite integrator for planetary N-body simulation   总被引：2，自引：0，他引：2  
We describe a P(EC) n Hermite scheme for planetary N -body simulation. The fourth-order implicit Hermite scheme is a time-symmetric integrator that has no secular energy error for the integration of periodic orbits with time-symmetric time-steps. In general N -body problems, however, this advantage is of little practical significance, since it is difficult to achieve time-symmetry with individual variable time-steps. However, we can easily enjoy the benefit of the time-symmetric Hermite integrator in planetary N -body systems, where all bodies spend most of the time on nearly circular orbits. These orbits are integrated with almost constant time-steps even if we adopt the individual time-step scheme. The P(EC) n Hermite scheme and almost constant time-steps reduce the integration error greatly. For example, the energy error of the P(EC)2 Hermite scheme is two orders of magnitude smaller than that of the standard PEC Hermite scheme in the case of an N  = 100,  m  = 1025 g planetesimal system with the rms eccentricity 〈 e 2〉1/2 ≲0.03.  相似文献   

5.
Ultraluminous X-ray sources (ULXs) with   L x > 1039 erg s−1  have been discovered in great numbers in external galaxies with ROSAT , Chandra and XMM-Newton . The central question regarding this important class of sources is whether they represent an extension in the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes (BHs), or a new class of objects, e.g. systems containing intermediate-mass BHs  (100–1000 M⊙)  . We have carried out a theoretical study to test whether a large fraction of the ULXs, especially those in galaxies with recent star formation activity, can be explained with binary systems containing stellar-mass BHs. To this end, we have applied a unique set of binary evolution models for BH X-ray binaries, coupled to a binary population synthesis code, to model the ULXs observed in external galaxies. We find that for donor stars with initial masses  ≳10 M⊙  the mass transfer driven by the normal nuclear evolution of the donor star is sufficient to potentially power most ULXs. This is the case during core hydrogen burning and, to an even more pronounced degree, while the donor star ascends the giant branch, although the latter phases last only ∼5 per cent of the main-sequence phase. We show that with only a modest violation of the Eddington limit, e.g. a factor of ∼10, both the numbers and properties of the majority of the ULXs can be reproduced. One of our conclusions is that if stellar-mass BH binaries account for a significant fraction of ULXs in star-forming galaxies, then the rate of formation of such systems is  ∼3 × 10−7 yr−1  normalized to a core-collapse supernova rate of 0.01 yr−1.  相似文献   

6.
One of the predictions of the standard cold dark matter model is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant-density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark-matter-dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well-defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting cold dark matter as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo centre surrounded by a Navarro, Frenk & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross-section ( σ ) which has an explicit dependence with the halo dispersion velocity ( v ). If m x is the mass of the dark matter particle: σ m x ≈4×10−25 (100 km s−1  v −1) cm2 GeV−1.  相似文献   

7.
We employ spectra of resolution 20–35000 of seven SC stars, four S stars, two Ba stars and two K–M stars to derive abundances of a variety of elements from Sr to Eu relative to iron. Special attention is paid to Rb and Tc, and to the ratio of the heavy s-process species to the light s-process elements. Abundances are derived in LTE, both by using model atmospheres in which the carbon and oxygen abundances are nearly equal and by using curves of growth. Spectrum synthesis is used for critical lines such as the 5924-Å line of Tc and the 7800-Å line of Rb. For most of the heavy-element stars the enhancement of the s-process elements is about a factor of 10. The ratio of the heavy to light s-process species is not far from solar, except for RR Her for which the same ratio is +0.45 dex. For Tc the blending by other lines is severe. While we have probably detected the 5924-Å line, we can only present abundances in the less-than-or-equal-to category. For Rb, whose abundance is sensitive to the 85Rb/87Rb ratio and hence to the neutron density during s-process production, we find a considerable range of abundances, indicating a neutron density from 106 to ≳108 cm−3 for the SC stars. For the four S stars the range is from 107 to ≳108 cm−3. Recent calculations by Gallino et al. show that neutron densities near 107 cm−3 favour the 13C source for neutrons, while densities greater than 108 cm−3 may be associated with neutrons from the 22Ne source.  相似文献   

8.
We report here on multifrequency radio observations of the pulsed emission from PSR B1259−63 around the time of the closest approach (periastron) to its B2e companion star. There was a general increase in the dispersion measure (DM) and scatter-broadening of the pulsar, and a decrease in the flux density towards periastron although fluctuation in these parameters were seen on time-scales as short as minutes. The pulsed emission disappeared 16 d prior to periastron and remained undetectable until 16 d after periastron.
The observations are used to determine the parameters of the wind from the Be star. We show that a simple model, in which the wind density varies with radius as r −2, provides a good fit to the data. The wind is highly turbulent with an outer scale of ≤1010 cm and an inner scale perhaps as small as 104 cm, a mean density of ∼106 cm−3 and a velocity of ∼2000 km s−1 at a distance of ∼50 stellar radii. We find a correlation between DM variations and the pulse scattering times, suggesting that the same electrons are responsible for both effects.  相似文献   

9.
We present results of a Chandra survey of the ultraluminous X-ray sources (ULX) in 13 normal galaxies, in which we combine source detection with X-ray flux measurement. 22 ULX were detected, i.e. with   L x > 1 × 1039 erg s−1 ( L 10)  and 39 other sources were detected with   L x > 5 × 1038 erg s−1 ( L 5)  . We also use radial intensity profiles to remove extended sources from the sample. The majority of sources are not extended, which for a typical distance constrains the emission region size to less than 50 pc. X-ray colour–colour diagrams and spectral fitting results were examined for indicators of the ULX nature. In the case of the brighter sources, spectral fitting generally requires two-component models. In only a few cases do colour–colour diagrams or spectral fitting provide evidence of a black hole nature. We find no evidence of a correlation with stellar mass, however, there is a strong correlation with star formation as indicated by the 60-μm flux as found in previous studies.  相似文献   

10.
We propose a model for the source of the X-ray background (XRB) in which low-luminosity active nuclei ( L  ∼ 1043 erg s−1) are obscured ( N  ∼ 1023 cm−2) by nuclear starbursts within the inner ∼ 100 pc. The obscuring material covers most of the sky as seen from the central source, rather than being distributed in a toroidal structure, and hardens the averaged X-ray spectrum by photoelectric absorption. The gas is turbulent with velocity dispersion ∼ few × 100 km s−1 and cloud–cloud collisions lead to copious star formation. Although supernovae tend to produce outflows, most of the gas is trapped in the gravity field of the star-forming cluster itself and the central black hole. A hot ( T  ∼ 106 − 107 K) virialized phase of this gas, comprising a few per cent of the total obscuring material, feeds the central engine of ∼ 107 M⊙ through Bondi accretion, at a sub-Eddington rate appropriate for the luminosity of these objects. If starburst-obscured objects give rise to the residual XRB, then only 10 per cent of the accretion in active galaxies occurs close to the Eddington limit in unabsorbed objects.  相似文献   

11.
We study the evolution of supernova remnants in a low-metallicity medium   Z /Z⊙= 10−4 to 10−2  in the early universe, using one-dimensional hydrodynamics with non-equilibrium chemistry. Once a post-shock layer is able to cool radiatively, a dense shell forms behind the shock. If this shell becomes gravitationally unstable and fragments into pieces, next-generation stars are expected to form from these fragments. To explore the possibility of this triggered star formation, we apply a linear perturbation analysis of an expanding shell to our results and constrain the parameter range of ambient density, explosion energy and metallicity where fragmentation of the shell occurs. For the explosion energy of  1051 erg (1052 erg)  , the shell fragmentation occurs for ambient densities higher than  ≳102 cm−3 (10 cm−3  ), respectively. This condition depends little on the metallicity in the ranges we examined. We find that the mode of star formation triggered occurs only in massive  (≳108 M⊙)  haloes.  相似文献   

12.
In this paper we present numerical results on the decay of small stellar systems under different initial conditions (multiplicity 3 ≤  N  ≤ 10, and various mass spectra, initial velocities and initial configurations). The numerical treatment uses the CHAIN1 code (Mikkola &38; Aarseth). Particular attention is paid to the distribution of high-velocity escapers: we define these as stars with velocity above 30 km s−1. These numerical experiments confirm that small N -body systems are dynamically unstable and produce cascades of escapers in the process of their decay. It is shown that the fraction of stars that escape from small dense stellar systems with an escape velocity greater than 30 km s−1 is ∼1 per cent for all systems treated here. This relatively small fraction must be considered in relation to the rate of star formation in the Galaxy in small groups: this could explain some moderately high-velocity stars observed in the Galactic disc and possibly some young stars with relatively high metallicity in the thick disc.  相似文献   

13.
Effective collision strengths for electron-impact excitation of the N-like ion Ne  iv are calculated in the close-coupling approximation using the multichannel R-matrix method. Specific attention is given to the 10 astrophysically important fine-structure forbidden transitions among the 4So, 2Do and 2Po levels in the 2s22p3 ground-state configuration. The expansion of the total wavefunction incorporates the lowest 11 LS eigenstates of Ne  iv , consisting of eight n  = 2 terms with configurations 2s22p3, 2s2p4 and 2p5, together with three n  = 3 states of configuration 2s22p23s. We present in graphical form the effective collision strengths obtained by thermally averaging the collision strengths over a Maxwellian distribution of velocities, for all 10 fine-structure transitions, over the range of electron temperatures log T (K) = 3.6 to log T (K) = 6.1 (the range appropriate for astrophysical applications). Comparisons are made with the earlier, less sophisticated close-coupling calculation of Giles, and excellent agreement is found in the limited temperature region where a comparison is possible [log T (K) = 3.7 to log T (K) = 4.3]. At higher temperatures the present data are the only reliable results currently available.  相似文献   

14.
Effective collision strengths for the 10 astrophysically important fine-structure forbidden transitions among the 4So, 2Do and 2Po levels in the 3s23p3 configuration of Cl  iii are presented. The calculation employs the multichannel R-matrix method to compute the electron-impact excitation collision strengths in a close-coupling expansion, which incorporates the lowest 23 LS target eigenstates of Cl  iii . These states are formed from the 3s23p3, 3s3p4, 3s23p23d and 3s23p24s configurations. The Maxwellian-averaged effective collision strengths are presented graphically for all 10 fine-structure transitions over a wide range of electron temperatures appropriate for astrophysical applications [log  T (K)=3.3−log  T (K)=5.9]. Comparisons are made with the earlier seven-state close-coupling calculation of Butler & Zeippen, and in general excellent agreement is found in the low-temperature region where a comparison is possible [log  T (K)=3.3−log  T (K)=4.7]. However, discrepancies of up to 30 per cent are found to occur for the forbidden transitions which involve the 4So ground state level, particularly for the lowest temperatures considered. At the higher temperatures, the present data are the only reliable results currently available.  相似文献   

15.
One of the methods discussed in deflecting the orbit of an Earth-colliding asteroid is the use of nuclear explosives. In assessing its feasibility, apart from political considerations, it is important to quantify how effective it is in orbit deflection. The transfer of radiation incident at the surface is governed by a non-linear diffusion equation. For low-yield explosions with a slab geometry ( S 0≃108 kJ μs−1), the temperature at depth x and time t is well approximated by a similarity solution of the form T ( x , t )= T 0 f (ξ), ξ= x/ ( T n 0 t )1/2, with T 0 given by ( S 0/σ)1/4, where σ is the Stefan–Boltzmann constant, n is an index that specifies the radiation transfer and f (ξ) is the solution of a non-linear differential equation subject to the condition f (0)=1 and limξ→∞ f (ξ)=0. For high-yield explosions ( S 0≃1010 kJ μs−1), numerical solutions to the non-linear diffusion equation can be obtained. These solutions have properties similar to the case of low-yield explosions. If the duration of the explosion is d ×10−8 s, where d is close to 3, the fraction of energy absorbed by the surface is found to be 7, 12 and 23 per cent for S 0=108, 109 and 1010 kJ μs−1 respectively.  相似文献   

16.
The wavelength and Einstein A coefficient are calculated for all rotation–vibration transitions of  4He1H+, 3He1H+, 4He2H+  and  3 He2H+  , giving a complete line list and the partition function for  4HeH+  and its isotopologues. This opacity is included in the calculation of the total opacity of low-metallicity stars and its effect is analysed for different conditions of temperature, density and hydrogen number fraction. For a low helium number fraction (as in the Sun), it is found that HeH+ has a visible but small effect for very low densities  (ρ≤ 10−10 g cm−3)  , at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH+ becomes important for higher densities  (ρ≤ 10−6 g cm−3)  , its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented.  相似文献   

17.
V -band polarimetric observations of HD 108 were obtained during 1994 August and September. This is the first time that the temporal polarimetric variability of this star has been investigated. Its percentage polarization and position angle vary randomly, and no evidence of its proposed binary nature is detected. The random variability is consistent with that seen for previously observed Wolf–Rayet stars and OB supergiants. Nightly variations do show some systematic behaviour consistent with the blob ejection model of Underhill & Fehey (1984). From the data it is estimated that the mass-loss rate resulting from blobs is ∼1-10−7 M⊙ yr−1. The total mass-loss rate of the star is estimated to be ∼1-10−5 M⊙ yr−1. The stellar rotation rate is estimated to be V rot∼400 km s−1 with an inclination of i <20c. The data are also interpreted in terms of perturbations occurring in an equatorial disc plus bipolar jets viewed equatorially (Underhill 1994). It is found that the perturbation mass-loss rate is about ∼3-10−7 M⊙ yr−1, giving a total mass-loss rate for the star of 13-10−5 M⊙ yr−1. Both models are consistent with HD 108 being an OB supergiant or of the class Ofpe/WR9.  相似文献   

18.
Extensive photometric and spectroscopic observations of SN 1994aj until 540 d after maximum light have been obtained. The photometry around maximum suggests that the SN belongs to the Type II Linear class, with a peak absolute magnitude of M V∼−17.8 (assuming H 0=75 km s−1 Mpc−1). The spectra of SN 1994aj were unusual, with the presence of a narrow line with a P Cygni profile on top of the broad Balmer line emission. This narrow feature is attributed to the presence of a dense superwind surrounding the SN. At 100–120 d after maximum light the SN ejecta start to interact with this circumstellar material. The SN luminosity decline rates slowed down [γ R =0.46 mag (100 d)−1], becoming less steep than the average late luminosity decline of normal SN II [∼1 mag (100 d)−1]. This dense ( ˙M / u W∼1015 g cm−1) wind was confined to a short distance from the progenitor ( R out=∼5×1016 cm), and results from a very strong mass-loss episode ( ˙M =10−3 M⊙ yr−1), which terminated shortly before explosion (∼5–10 yr).  相似文献   

19.
We have used the Very Large Array (VLA) to search for the H92α radio recombination line (RRL) in four starburst galaxies. In NGC 660, the line was detected over a 17Å‐8 arcsec2 region near its starburst nucleus. The line and continuum emission indicate that the RRL-emitting gas is most likely in the form of a cluster of H ii regions with a small filling factor. Using a simple model we find that the total ionized mass in the nuclear region is in the range 2–8Å‐104 M⊙ and the rate of production of UV photons N Lyc∼1–3Å‐1053 s−1. The ratio of H92α and Brγ line intensities in NGC 660 indicates that extinction is significant even at λ=2 μm. The velocity field of the ionized gas is consistent with a rotating disc with an average velocity gradient of ∼15 km s−1 arcsec−1. The dynamical mass within the central 500 pc is ∼4Å‐108 M⊙ and may be about ∼6Å‐107 M⊙ within the central 120 pc. No line was detected in the other galaxies (NGC 520, NGC 1614 and NGC 6946) to a 3σ limit of 300 μJy. In the starburst galaxies in which RRLs have been detected, we find that there is a rough correlation between the integrated H92α line flux density and both the total far-infrared flux density and the radio continuum emission from the central region.  相似文献   

20.
We present a sample of 33 damped Lyman α systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS) whose absorption redshifts ( z abs) are within 6000 km s−1 of the quasi-stellar object's (QSO) systemic redshift ( z sys). Our sample is based on  731 2.5 < z sys < 4.5  non-broad absorption line (non-BAL) QSOs from Data Release 3 (DR3) of the SDSS. We estimate that our search is ≈100 per cent complete for absorbers with N (H  i )  ≥ 2 × 1020 cm−2  . The derived number density of DLAs per unit redshift, n ( z ), within  Δ v < 6000 km s−1  is higher (3.5σ significance) by almost a factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e. there is evidence for an overdensity of galaxies near the QSOs. This provides a physical motivation for excluding DLAs at small velocity separations in surveys of intervening 'field' DLAs. In addition, we find that the overdensity of proximate DLAs is independent of the radio-loudness of the QSO, consistent with the environments of radio-loud and radio-quiet QSOs being similar.  相似文献   

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