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1.
An exact solution of Einstein's equations for a static isentropic perfect fluid sphere is examined in detail. The analysis yields a strong indication that the model isstable with respect to infinitesimal radial pulsations. This means that the temperature is decreasing outwards. We prove that the adiabatic speed of sound is everywhere less than the speed of light if and only if the radius of the sphere is larger than 1.61 times its Schwarzschild radius. We further show that the strong energy condition is fulfilled everywhere if and only if the radius is larger than 1.76 times the Schwarzschild radius. The necessary and sufficient condition for the speed of sound to be decreasing outwards is given, and we find that this criterion is fulfilled if the fluid is causal. Taking the values of the pressure and the density to be somewhere given by the maximum values from Baymet al.'s equation of state, i.e., 0=5.1×1014 g cm–3 andp 0=7.4×1033 dyne cm–2, we find the maximum mass of the fluid sphere to be 2.5 solar masses.Dedicated to the memory of the late George Cunliffe McVittie (1904–1988).  相似文献   

2.
Strong absorption satellite lines of CaI 6572 were found on spectrograms taken on three successive days just after the fourth contact of the 1971–72 eclipse of Zeta Aurigae. The radial velocities of the satellite lines are –88 km s–1, –74 km s–1, and –180 km–1, respectively, relative to the K-type primary star (K4 Ib). These absorptions should be due to a circumstellar cloud in which the column density of neutral calcium atoms is 1×1017 cm–2 and the turbulent velocities come to 20–50 km s–1. It is suggested that the cloud may be formed by the rocket-effect of the Lyman quanta of the B-type component (B6 V). We estimate the density in the cloud to be 2×1011 atoms cm–3 fors=10R K and 2×1010 atoms cm–3 fors=102 R K, wheres denotes the distance of the cloud from the K star andR K the K star's radius. The mass loss rate of the K-type component is also estimated to be about 10–7 M yr–1, assuming that the expansion of the K star occurs isotropically.  相似文献   

3.
The detailed evolution of low-mass main-sequence stars (M < 1M ) with a compact companion is studied. For angular momentum loss associated with magnetic braking it is found that about 10–11–10–12 M yr–1 in stellar wind loss would be required. This wind is 102–103 times stronger than the solar wind, so we believe here magnetic stellar wind is insufficient. It is well known that there is mass outflow in low-mass close binary systems. We believe here that these outflows are centrifugal driven winds from the outer parts of the accretion disks. The winds extract angular momentum from these systems and therefore drive secular evolution. Disk winds are preferred to winds from the secondary, because of the lower disk surface gravity.  相似文献   

4.
On the basis of empirical (D)-dependency at the frequency of 5 GHz constructed using 15 planetary nebulae with the independently measured distances (10–171×10–20 W m–2 Hz–1 ster–1), we evaluated distances of 335 objects. Independent evidence of the correctness of the accepted scale are given. Then(D)-dependency is constructed and it is shown that atD<0.08 pc the mean electron density is higher than the one determined by the Seaton method. We showed that the filling factor diminishes with the increase of the PN diameter (1 atD0.08 pc and 0.2 atD0.4 pc). the ionized mass of 33 PNs is determined. With the diameter increase the ionized mass grows and atD0.4 pc reaches the valueM0.07M . We used the new distance scale when investigating the space distribution of PNs. The mean scale height =130±15 pc and the mean gradient of the change of surface densitym=0.37, which allowed us to estimate the total number of nebulae in the GalaxyN4×104. We divided the PNs according to their velocities (withV LSR>35 km s–1 andV LSR<35 km s–1) and permitted us to confirm that the PN belong to different sub-systems of the Galaxy. The estimated local formation rate of PNs [=(4.6±2.2)×10–12 pc–3 yr–1] is a little higher than the one of the white dwarfs. That can be explained by a large number of PNs having binary cores, which used in our sample. The statistical estimation of PN expansion velocity showed thatV ex increases from 5–7 km s–1 (atD0.03 pc) to 40–50 km s–1 (atD0.8 pc).  相似文献   

5.
This paper deals with molecular clouds discovered in the absorption spectra (z=2–3) of distant quasars. It is argued that these clouds belong to the gaseous subsystems of young galaxies. We estimate the gas concentration to ben<104 cm–3 in the cloud observed in the direction of the quasar PHL957. It is shown that this cloud is exposed to ultraviolet radiation. The UV-energy flux does not exceed the value typical for our Galaxy by an order of magnitude (F2×10–6 ergs cm–2 s–1 Å–1 at =2000 Å). The mechanisms maintaining the thermal balance in this cloud are discussed.  相似文献   

6.
We present simple two-layer models of Uranus with rocky core and polytropic envelope satisfying exactly the observed mass, radius and the gravitational moments. The models show that the value of the fourth order zonal harmonic isJ 4 –38×10–6, whileJ 6 10–6. More elaborate threelayer models fail to satisfy the observational constraints of the ice/rock ratio and/or of the rotation period. We conclude that three-layer models with uniform chemical composition in each layer may be too restrictive. More realistic models should account for variable chemical composition within each layer.  相似文献   

7.
The diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl II180°, 0°b II40° is analyzed in connection with the distributions of stars and dust grains as well as with optical properties of grains. Its intensity (starlight+scattered light) is about 6×10–7 erg cm–2 sec–1 sr–1 Å–1 in the direction ofb II0° andl II180°. The latitude dependence of the intensity is in approximate agreement with the plane parallel slab model of the galaxy with a reasonable set of parameters. The interstellar scattering gives an albedo close to unity and forward phase function of about 0.6, which are not inconsistent with the model of interstellar grains of Wickramasinghe. The upper limit of the extragalactic UV is 2×10–8 erg cm–2 sec–1 sr–1 Å–1 in the same region of wave-length.  相似文献   

8.
We use theoretical results derived in a previous paper (Lanzano, 1986) to numerically evaluate the temperature profile and radial deformation within a spherical, elastic Earth due to heat generated by the decay of radiogenic elements.We consider only the Uranium family and have assumed the diffusivity of the silicate mantle to be K = 8 × 10–3 cm2 s–1, the Poisson elastic ratio to be = 0.25 and the coefficient of thermal expansion to be = 2 × 10–5 (deg)–1. Our series solutions when applied to the interiors of the Moon, Mercury, and Mars yield results in agreement with Kopal's (1963) evaluations.  相似文献   

9.
A first detailed period study of the eclipsing RS CVn-binary system RW Com is presented. A new period (P=0d.2373455) based on 223 minima is given. The O–C diagrams of RW Com have been presented for the first time. Types of ten minima have been corrected judging the period trend. Period changes in different portions of the O–C diagram (Figure 2) have been estimated. The total change in period (P/P) ranges from 5.5×10–7 to 6.4×10–6. Thus, P ranges from 1.3×10–7 d to 1.5×10–6 d. Numerous minima are available in the time interval 1967 to 1986. This part of the O–C diagram (Figure 2) shows a sinusoidal variation, thus, it is suspected that RW Com could be a three-body system. The period of variation due to third body appears to be nearly 16 years.  相似文献   

10.
BUSS observations of the profiles of two well observed spectral lines in the ultraviolet spectrum of CMi (Procyon; F5 IV–V) are analysed with a Fourier transform method in order to determine values of various parameters of the velocity field of the upper photosphere. We find a microturbulent line-of-sight velocity componentL = 0.9 ± 0.4 km s–1, a macroturbulent velocity componentL M = 5.3 ± 0.2 km s–1, and a rotational velocity componentv R sini=10.0±1.2 km s–1. In these calculations a single-moded sinusoidal isotropic macroturbulent velocity function was assumed. The result appears to be sensitive to the assumed shape of the macroturbulence function: for an assumed Gaussian shape the observations can be described withv R sini=4 km s–1 andL M = 11.6 ± 2.7 km s–1. A comparison is made with other results and theoretical predictions.  相似文献   

11.
The analysis of the Th/U ratio in meteorites and the evolutionary ages of globular clusters favour values of the cosmic age of (19±5)×109 yr. This evidence together with a Hubble parameterH 0>70 km s–1 Mpc–1=(14×109 yr)–1 cannot be reconciled in a Friedmann model with =0. It requires a cosmological constant in the order of 10–56 cm–2, equivalent to a vacuum density v =10–29 g cm–3 The Friedmann-Lemaître models (>0) with a hot big-bang have been calculated. They are based on a present value of the baryonic matter density of 0=0.5×10–30 g cm–3 as derived from the primordial4He and2H abundances.For a Hubble parameter ofH 0=75 km s–1 Mpc–1, our analysis favours a set of models which can be represented by a model with Euclidean metric (density parameter 0=1.0, deceleration parameterq 0=–0.93, aget 0=19.7×109 yr) and by a closed model with perpetual expansion (0=1.072,q 0=–1.0, aget 0=21.4×109 yr). A present density parameter close to one can indeed be expected if the conjecture of an exponential inflation of the very early universe is correct.The possible behaviour of the vacuum density is demonstrated with the help of Streeruwitz' formula in the context of the closed model with an inflationary phase at very early times.  相似文献   

12.
It has been shown that Alfvén waves can drive non-inductive current in solar coronal loops via collisional or collisionless damping. Assuming that all the coronal-loop density of dissipated wave power (W= 10–3 erg cm–3 s–1), which is necessary to keep the plasma hot, is due to Alfvén wave electron heating, we have estimated the axial current density driven by Alfvén waves to be jz 103–105 statA cm–2. This current can indeed support the quasi-stationary equilibrium and stability of coronal loops and create the poloidal magnetic field up to B 1–5 G.  相似文献   

13.
Colliding comets in the Solar System may be an important source of gamma ray bursts. The spherical gamma ray comet cloud required by the results of the Venera Satellites (Mazets and Golenetskii, 1987) and the BATSE detector on the Compton Satellite (Meeganet al., 1992a, b) is neither the Oort Cloud nor the Kuiper Belt. To satisfy observations ofN(>P max) vsP max for the maximum gamma ray fluxes,P max > 10–5 erg cm–2 s–1 (about 30 bursts yr–1), the comet density,n, should increase asn a 1 from about 40 to 100 AU wherea is the comet heliocentric distance. The turnover above 100 AU requiresn a –1/2 to 200 AU to fit the Venera results andn a 1/4 to 400 AU to fit the BATSE data. Then the masses of comets in the 3 regions are from: 40–100 AU, about 9 earth masses,m E; 100–200 AU about 25m E; and 100–400 AU, about 900m E. The flux of 10–5 erg cm–2 s–1 corresponds to a luminosity at 100 AU of 3 × 1026 erg s–1. Two colliding spherical comets at a distance of 100 AU, each with nucleus of radiusR of 5 km, density of 0.5 g cm–3 and Keplerian velocity 3 km s–1 have a combined kinetic energy of 3 × 1028 erg, a factor of about 100 greater than required by the burst maximum fluxes that last for one second. Betatron acceleration in the compressed magnetic fields between the colliding comets could accelerate electrons to energies sufficient to produce the observed high energy gamma rays. Many of the additional observed features of gamma ray bursts can be explained by the solar comet collision source.  相似文献   

14.
We have modeled stellar coronal loops in static conditions for a wide range of loop length, plasma pressure at the base of the loop and stellar surface gravity, so as to describe physical conditions that can occur in coronae of stars ranging from low mass dwarfs to giants as well as on a significant fraction of the Main-Sequence stars.Three alternative formulations of heat conduction have been used in the energy balance equation, depending on the ratio 0/L Tbetween electron mean free path and temperature scale height: Spitzer's formulation for 0/L Tless than 2 × 10–3, the Luciani, Mora, and Virmont non-local formulation for 0/L Tbetween 2 × 10–3 and 6.67 × 10–3 and the limited free-streaming formulation for 0/L Tlarger than 6.67 × 10–3.We report the characteristics of all loop models studied, and present examples to illustrate how the temperature and density stratification can be drastically altered by the different conductivity regimes. Significant differences are evident in the differential emission measure distribution vs temperature, an important observable quantity. We also show how physical conditions of coronal plasma, and in particular thermal conduction, change with stellar surface gravity.We have found that, for fixed loop length and stellar gravity, a minimum of loop-top plasma temperature occurs, corresponding to the highest value of base plasma pressure for which the limited free-streaming conduction occurs. This value of temperature satisfies the appropriate scalingT 10–9 L g, in cgs units.  相似文献   

15.
Javaraiah  J. 《Solar physics》1999,189(2):289-304
We have analyzed data on sunspot groups compiled during 1874–1981 and investigated the following: (i) dependence of the `initial' meridional motion (v ini()) of sunspot groups on the life span () of the groups in the range 2–12 days, (ii) dependence of the meridional motion (v(t)) of sunspot groups of life spans 10–12 days on the age (t) of the spot groups, and (iii) variations in the mean meridional motion of spot groups of life span 2–12 days during the solar cycle. In each of the latitude intervals 0°–10°, 10°–20° and 20°–30°, the values of both v ini() and v(t) often differ significantly from zero. In the latitude interval 20°–30°, the forms of v ini() and v(t) are largely systematic and mutually similar in both the north and south hemispheres. The form of v(t) suggests existence of periodic variation in the solar meridional motion with period of 4 days and amplitude 10–20 m s–1. Using the anchoring depths of magnetic structures for spot groups of different and testimated earlier, (Javaraiah and Gokhale, 1997), we suggest that the forms of v ini() and v(t) may represent radial variation of meridional flow in the Sun's convection zone, rather than temporal variation of the flow. The meridional flows (v e(t)) determined from the data during the last few days (i.e., age t: 10–12 days) of spot groups of life spans of 10–12 days are found to have magnitudes (10–20 m s–1) and directions (poleward) similar to the those of the surface meridional plasma flows determined from the Dopplergrams and magnetograms. The mean meridional velocity of sunspot groups living 2–12 days seems to vary during the solar cycle. The velocity is not significantly different from zero during the rising phase of the cycle and there is a suggestion of equatorward motion (a few m s–1at lower latitudes and 10 m s–1at higher latitudes) during the declining phase (last few years) of the cycle. The variation during the odd numbered cycles seems to anticorrelate with the variation during the even numbered cycles, suggesting existence of 22-year periodicity in the solar meridional flow. The amplitude of the anticorrelation seems to be depending on latitude and the cycle phase. In the latitude interval 20°–30° the `surface plasma meridional motion', v e(t), is found to be poleward during maximum years (v e(t) 20 m s–1at 4th year) and equatorward during ending years of the cycle (v e(t) –17 m s–1at 10th year).  相似文献   

16.
Two models for superluminal radio sources predict sharp lower bounds for the apparent velocities of separation. The light echo model predicts a minimum velocityv min=2c, and the dipole field model predictsv min=4.446c. Yahil (1979) has suggested that, if either of these models is correct, thenv min provides a standard velocity which can be used to determine the cosmological parametersH andq 0. This is accomplished by estimating a lower envelope for the proper motion vs redshift relation. Yahil also argued that the procedure could easily be generalized to include a nonzero cosmical constant . We derive the formulas relating the proper motion to the redshiftz in a Friedmann universe with a nonzero . We show that the determination of a lower envelope for a given sample of measured points yields an estimate of the angle of inclination i for each source in the sample. We formulate the estimation of the lower envelope as a constrained maximum likelihood problem with the constraints specified by the expected value of the largest order statistic for the estimated i . We solve this problem numerically using an off-the-shelf nonlinearly constrained nonlinear optimization program from the NAg library. Assuming =0, we apply the estimation procedure to a sample of 27 sources with measured values , using both the light echo and the dipole field models. The fits giveH=103 km s–1 Mpc–1 for the light echo model andH=46 km s–1 Mpc–1 for the dipole field model. In both cases the fits giveq 0=0.4, but the uncertainty in this result is too large to rule out the possibility thatq 0>0.5. When is allowed to be a free parameter, we obtainH=105 km s–1 Mpc–1 for the light echo model andH=47 km s–1 Mpc–1 for the dipole field model. In both cases the fits giveq 0=–1 and /H 0 2 =6.7, but no significance can be attached to these results because of the paucity of measured data at hight redshifts. For all of the fits, we compute the corresponding estimates of the i and compare the cumulative distribution of these values with that expected from a sample of randomly oriented sources. In all cases we find a large excess of sources at low-inclination angles (high apparent velocities). The expected selection effect would produce such an excess, but the excess is large enough to suggest a strong contamination of the sample by relativistic beam sources which would only be seen at low inclination angles.Applied Research Corporation  相似文献   

17.
The production of magnetic monopoles-antimonopoles pairs is expected in the interactions of highly energetic particles in the outer gaps of a pulsar. We estimate upper limit for production of monopoles withm g 104 GeV c –2 ine + e – interactions of the order ee 10–32 cm–2.  相似文献   

18.
We report on eight X-ray bursts detected by ASTRON from the Rapid Burster (RB) on 13 and 28 April and 16 August, 1983. Six of them (trailing bursts), with durations of 1.5–2 min, rise times of 5–10 s and intervals of 1–1.5 hours, exhibit spectral softening during the burst decay and may be related to the type I bursts. Two of the bursts (triangle bursts) observed on 28 April at interval of 28 min with much longer rise times (30–50 s) and longer durations (3 min), do not show distinct spectral softening. Persistent flux from RB on 16 August was estimated asF p(2.0–2.4)×10–9 erg cm–2 s–1. Spectral evolution of two trailing bursts was investigated by fitting their spectra in consecutive time intervals with the blackbody (BB), isothermal scattering photosphere (SP) and thermal bremsstrahlung (TB) models. Around the burst maxima the SP model fits the data best whereas in the burst tails the TB model is generally better. The BB model is worse than at least one of the two others. Interpretation of the burst spectra in terms of the BB radiation leads to improbably small neutron star mass and radius (M<0.86M ,R NS<5 km) if the peak luminosity does not exceed the Eddington limit. Interpretation of the spectra around the burst maxima (3–15 s from the burst onset) in terms of an isothermal SP yields reasonable constraints onM,R NS, and distanceD. For instance, for the hydrogen photosphere we obtainedM=(1.0–2.1)M R NS=(7.1–16.4) km ifD=11 kpc. If one postulatesM=1.4M , thenD=(8.5–13) kpc for hydrogen photosphere; if, besides,D=11 kpc, thenR NS=(8.1–13.3) km. It follows also from the SP-interpretation that the photosphere radius may increase up to 20–30 km in maxima of the trailing bursts when the luminosity becomes close to the Eddington luminosity.  相似文献   

19.
A first period study of the eclipsing binary XY Ceti is presented. A new period (P=2d.7807135), based on all available times of minima, is given. Period changes in different portions of the O–C diagram, with a new period, have been estimated. The total change in period (P/P) ranges from 1.1×10–5 d to 1.2×10–4 d, thus, P ranges from 3.1×10–5 d to 3.3×10–4 d. The O–C diagram suggests that the trend of the period has changed around the year 1959. Two portions of increasing and decreasing trends also reveal that the period changes (P/P) of the order of 10–5 d are present, which are appreciably large.  相似文献   

20.
Elemental abundances of the VH group of cosmic radiation have been measured in the energy interval 250–550 MeV nucl–1 in a balloon exposure at Sioux Falls (South Dakota) of a plastic detector LeXAN stack. The so obtained abundances have been extrapolated to the sources in the frame of the homogeneous model correcting for energy loss. After taking into account solar modulation, the best fit to model values has led to a escape mean free path e = 5E –0.4 g cm–2, whereE is the energy in GeV nucl–1, forE>1 GeV nucl–1, and a constant e = 5 g cm–2 forE1 GeV nucl–1. When turning to the diffusion model, also including an energy loss term, a diffusion coefficientD=3×1028 cm2 s–1 has been estimated.  相似文献   

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