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1.
We study the fragmentation properties in the protoplanetary disk and properties of the resultant self-gravitating clumps using our newly constructed disk model. Our disk model includes the mass inflall term from a molecular cloud core and the photoevaporation winds effect. We adopt the conventional fragmentation criterion to judge whether a protoplanetary disk can fragment. In this work, we follow our previous work to investigate the properties of the resultant self-gravitating clumps. In our calculation, the initial masses of the resultant self-gravitating clumps lie in the range of tens of MJ to more than one hundred of MJ, where MJ is the Jupiter mass. These initial masses can seemingly account for the masses of extrasolar planets in magnitude. We also calculate the subsequent gas accretion of clumps in 1.27 × 104 yr after the formation of self-gravitating clumps. We find that the subsequent gas accretion of self-gravitating clumps is very efficient, and the clump masses grow to hundreds of MJ and the physical radii Rc of clumps increase to about 10 AU. Additionally, we also calculate the orbital migration of clumps. We find that most clumps have short migration timescale to be accreted onto the protostar, and only a small fraction of clumps have long migration timescale (>106 yr) to successfully become gas giant planets. These results are consistent with previous studies. 相似文献
2.
A. V. Tutukov 《Astronomy Reports》2005,49(1):13-23
The formation and evolution of supermassive (102?1010 M ⊙) black holes (SMBHs) in the dense cores of globular clusters and galaxies is investigated. The raw material for the construction of the SMBHs is stellar black holes produced during the evolution of massive (25?150M ⊙) stars. The first SMBHs, with masses of ～1000M ⊙, arise in the centers of the densest and most massive globular clusters. Current scenarios for the formation of SMBHs in the cores of globular clusters are analyzed. The dynamical deceleration of the most massive and slowly moving stellar-mass (< 100M ⊙) black holes, accompanied by the radiation of gravitational waves in late stages, is a probable scenario for the formation of SMBHs in the most massive and densest globular clusters. The dynamical friction of the most massive globular clusters close to the dense cores of their galaxies, with the formation of close binary black holes due to the radiation of gravitational waves, leads to the formation of SMBHs with masses ? 103 M ⊙ in these regions. The stars of these galaxies form galactic bulges, providing a possible explanation for the correlation between the masses of the bulge and of the central SMBHs. The deceleration of the most massive galaxies in the central regions of the most massive and dense clusters of galaxies could lead to the appearance of the most massive (to 1010 M ⊙) SMBHs in the cores of cD galaxies. A side product of this cascade scenario for the formation of massive galaxies with SMBHs in their cores is the appearance of stars with high spatial velocities (> 300 km/s). The velocities of neutron stars and stellar-mass black holes can reach ～105 km/s. 相似文献
3.
Our analysis of many years of infrared photometry of the unique object FG Sge indicates that the dust envelope formed around the supergiant in August 1992 is spherically symmetrical and contains compact, dense dust clouds. The emission from the spherically symmetrical dust envelope is consistent with the observed radiation from the star at 3.5–5 µm, and the presence of the dust clouds can explain the radiation observed at 1.25–2.2 µm. The mean integrated flux from the dust envelope in 1992–2001 was ～(1.0±0.2)×10?8 erg s?1cm?2. The variations of its optical depth in 1992–2001 were within 0.5–1.0. The maximum density of the dust envelope was recorded in the second half of 1993 and corresponded to mean optical depths as high as unity. Several times in the interval from 1992 to 2001, the dusty material of the envelope partially dissipated and was then replenished. For example, the optical depth of the dust cloud at λ=1.25 µm during the last brigthness minimum in the J band was τ1.25≈4.3, which is much higher than the optical depth of the dust envelope of FG Sge. During maxima of the J brightness, the mean spectral energy distribution at 0.36–5 µm can be represented as a combination of radiation from a G0 supergiant that is attenuated by a dust envelope with a mean optical depth of 0.65±0.15 and emission from the spherically symmetrical dust envelope itself, with the temperature of the graphite grains being 750±150 K. At minima of the J brightness, only radiation from the dust envelope is observed at 1.65–5 µm, with the radiation from the supergiant barely detectable at 1.25 µm. As a result, the integrated flux during J minima is almost half that during J maxima. The mean mass of the spherically symmetrical dust envelope of FG Sge in 1992–2001 was (3 ± 1) × 10?7M⊙. This envelope’s mass varied by nearly a factor of two during 1992–2001, in the range (2 – 4) × 10?7M⊙. In Autumn 1992, the mass-loss rate from the supergiant exceeded 2 × 10?7M⊙/yr. The average rate at which matter was injected into the envelope during 1993–2001 was 10?8M⊙/yr. The mean rate of dissipation of the dust envelope was about 1 × 10?8M⊙/yr. During 1992–2001, the supergiant lost about 8.7 × 10?7M⊙. The parameters of the dust envelope were relatively constant from 1999 until the middle of 2001. 相似文献
4.
Twenty-eight CS molecular clouds toward HII regions with Galactocentric distances from ～ 4 to 20 kpc have been studied based on observations obtained in the J=2→1 lines of CS and C34S on the 20-meter radio telescope of the Onsala Space Observatory (Sweden) in March 2001. All 28 clouds have been mapped with an angular resolution of ～40″. The peak intensity in the C34S line has been measured for 20 objects. An LTE analysis has been performed and the parameters of the molecular cloud cores derived. The core sizes are dA=0.3–4.8 pc, with a median value of ～1.6 pc. The mean hydrogen densities in the cloud cores are nH2=3.5×102–3.7 × 104 cm?3, with a median value of ～7.2×103 cm?3. The value of nH2 ends to decrease with increasing Galactocentric distance of the cloud. The masses of most clouds are 102?6×103M⊙, with the most probable value being MCS～103M⊙. The data follow the dependence MCS ∝d A (2.4–3.2) . As a rule, the cloud masses are lower than the virial masses for MCS<103M⊙. 相似文献
5.
The mean 1983–1996 UBV light curves of the dwarf nova SS Cyg are used to derive the binary parameters in the quiescent state. Solutions are obtained for a classical hot-spot model and a model with an energy source lying outside the accretion disk. Photometric and spectroscopic data are combined to infer the masses and radii of the binary components. The white dwarf in SS Cyg is one and a half times as massive as the red dwarf, q=M wd /M rd ～1.45, M rd ～0.46M ⊙ and M wd ～0.66M ⊙. The orbital inclination of the system is i?51°–54°. The contribution of the accretion disk to the total flux in the quiescent state is estimated to be ～47–49% and ～54% in the VU and B filters, respectively. The hot spot contributes less than ～3% to the total optical flux. In the “non-classical” hot-spot model, the disk and bulge contributions are 27 and 2–8%, respectively, depending on the orbital phase. The shape of the mean light curves of SS Cyg suggests asymmetric heating of the red-dwarf surface in the quiescent state by high-temperature radiation generated in the hot-spot region. 相似文献
6.
We consider the evolutionary status of observed close binary systems containing black holes and Wolf-Rayet (WR) stars. When the component masses and the orbital period of a system are known, the reason for the formation of a WR star in an initial massive system of two main-sequence stars can be established. Such WR stars can form due to the action of the stellar wind from a massive OB star (MOB≥50M⊙), conservative mass transfer between components with close initial masses, or the loss of the common envelope in a system with a large (up to ～25) initial component mass ratio. The strong impact of observational selection effects on the creation of samples of close binaries with black holes and WR stars is demonstrated. We estimate theoretical mass-loss rates for WR stars, which are essential for our understanding the observed ratio of the numbers of carbon and nitrogen WR stars in the Galaxy \(\dot M_{WR} (M_ \odot yr^{ - 1} ) = 5 \times 10^{ - 7} (M_{WR} /M_ \odot )^{1.3} \). We also estimate the minimum initial masses of the components in close binaries producing black holes and WR stars to be ～25M⊙. The spatial velocities of systems with black holes indicate that, during the formation of a black hole from a WR star, the mass loss reaches at least several solar masses. The rate of formation of rapidly rotating Kerr black holes in close binaries in the Galaxy is ～3×10?6 yr?1. Their formation may be accompanied by a burst of gamma radiation, possibly providing clues to the nature of gamma-ray bursts. The initial distribution of the component mass ratios for close binaries is dN～dq=dM2/M1 in the interval 0.04?q0≤1, suggesting a single mechanism for their formation. 相似文献
7.
A model for the formation of supermassive black holes at the center of a cluster of primordial black holes is developed. It is assumed that ～10?3 of the mass of the Universe consists of compact clusters of primordial black holes that arose as a result of phase transitions in the early Universe. These clusters also serve as centers for the condensation of dark matter. The formation of protogalaxies with masses of the order of 2 × 108 M ⊙ at redshift z = 15 containing clusters of black holes is investigated. The nuclei of these protogalaxies contain central black holes with masses ～105 M ⊙, and the protogalaxies themselves resemble dwarf spherical galaxies with their maximum density at their centers. Subsequent merging of these induced protogalaxies with ordinary halos of dark matter leads to the standard picture for the formation of the large-scale structure of the Universe. The merging of the primordial black holes leads to the formation of supermassive black holes in galactic nuclei and produces the observed correlation between the mass of the central black hole and the bulge velocity dispersion. 相似文献
8.
We consider the astrophysical evolution of the Galaxy over large time scales, from early stages (an age of ～108 yrs) to the end of traditional stellar evolution (～1011 yrs). Despite the fact that the basic parameters of our stellar system (such as its size, mass, and general structure) have varied little over this time, variations in the characteristics of stars (their total luminosity, color, mass function, and chemical composition) are rather substantial. The interaction of the Galaxy with other stellar systems becomes an important factor in its evolution 100–1000 Gyr after its origin; however, we take the Galaxy to be isolated. In the model considered, the basic stages of Galactic evolution are as follows. The Galaxy forms as the result of the contraction (collapse) of a protogalactic cloud. The beginning of the Milky Way’s life—the relaxation period, which lasts about 1–2 Gyr—is characterized by active star formation and final structurization. The luminosity and colors of the Galaxy are correlated to the star formation rate (SFR). The young Galaxy intensely radiates high-energy photons, which are mostly absorbed by dust and re-emitted at IR wavelengths. In the subsequent period of steady-state evolution, the gas content in the Galactic disk gradually decreases; accordingly, the SFR decreases, reaching 3–5M ⊙/yr at the present epoch and decreasing to 0.03M ⊙/yr by an age of 100 Gyr. Essentially all other basic parameters of the Galaxy vary little. Later, the decrease in the SFR accelerates, since the evolution of stars with masses exceeding 0.4M ⊙ (i.e., those able to lose matter and renew the supply of interstellar gas) comes to an end. The Galaxy enters a period of “dying”, and becomes fainter and redder. The variation of its chemical composition is manifested most appreciably in a dramatic enrichment of the interstellar gas in iron. The final “stellar epoch” in the life of the Galaxy is completed ～1013 yrs after its formation, when the evolution of the least massive stars comes to an end. By this time, the supplies of interstellar and intergalactic gas are exhausted, the remaining stars become dark, compact remnants, there is no further formation of new stars, and the Galactic disk no longer radiates. Eventually, infrequent outbursts originating from collisions of stellar remnants in the densest central regions of the Galaxy will remain the only source of emission. 相似文献
9.
The chemical and thermal evolution of the baryon component in the gravitational field of low-mass primordial dark-matter halos during their virialization is studied. We consider low-mass halos to be those for which the characteristic baryon cooling time can appreciably exceed the comoving Hubble time, so that the cooling process can continue to the current epoch (z～0). The virialization process is described in two scenarios: “quiet” virialization, in which the establishment of the virial state is assumed to be homogeneous over the entire volume considered, and “violent” virialization, in which the establishment of the virial state is assumed to be realized via the action of shock waves. In this second case, the efficiency of the formation of molecular hydrogen grows substantially, and can reach H2/H～0.01 in some cases, which exceeds current estimates by at least an order of magnitude. This eases the condition for the birth of the first gravitationally bound objects with comparatively low masses (M ? 2 × 105M⊙), possibly leading to an appreciable increase in the fraction of the mass contained in Population III objects, and also to a shift in the onset of the formation of the first stars toward higher redshifts. 相似文献
10.
We present the results of long-term (1978–1998) infrared and optical observations of the unique symbiotic system CH Cygni. The system’s IR brightness and color variations are generally consistent with a model in which the source is surrounded by a dust envelope with variable optical depth. There was evidence for a hot source in the CH Cyg system during the entire period from 1978 to 1998, with the exception of several hundred days in 1987–1989. Over the observation period, there was tendency for the system to gradually redden at 0.36–5 µm, accompanied by a brightness decrease at 0.36–2.2µm and a brightness increase at 3.5 and 5 µm. The “activation” of the cool sources in 1986–1989 nearly coincided with the disappearance of radiation from the hot source. The dust envelope of CH Cyg is not spherically symmetrical, and its optical depth along the line of sight is substantially lower than its emission coefficient, the mean values being τex(L)～0.06 and τem(L)～0.16. We confirm the presence of a 1800-to 2000-day period in both the optical and IR, both accounting for, and not accounting for, a linear trend. The spectral type of the cool star varied between M5III and M7III. The spectral type was M5III during the phase of maximum activity of the system’s hot source, while the spectral type was M7III when the star’s optical radiation was almost completely absent. The luminosity of the cool giant varied from (6300–9100)L ⊙; its radius varied by approximately 30%. The ratio of the luminosities of the dust envelope and the cool giant varied from 0.08 to 0.5; i.e., up to 50% of the cool star’s radiation could be absorbed in the envelope. The temperature of dust particles in the emitting envelope varied from 550 to 750 K; the radius of the envelope varied by more than a factor of 2. The expansion of the emitting dust envelope observed in 1979–1988 accelerated: its initial velocity (in 1979) was ～8 km/s, while the maximum velocity (in 1987–1989) was ～180 km/s. Beginning in 1988, the radiation radius of the dust envelope began to decrease, first at ～45 km/s and then (in 1996–1998) at ～3 km/s. From 1979 until 1996, the mass of the emitting dust envelope increased by approximately a factor of 27 (the masses in 1979 and 1988 were ～1.4×10?7 M ⊙ and ～3.8×10?6 M ⊙, respectively), after which (by 1999) it decreased by nearly a factor of 7. The mass-loss rate of the cool star increased in 1979–1989, reaching ～3.5×10?6 ～3.5×10?6 M ⊙/yr in 1988. Subsequently (up to the summer of 1999), the envelope itself began to lose mass at a rate exceeding that of the cool star. The largest input of matter to the envelope occurred after the phase of optical activity in 1978–1985. If the envelope’s gas-to-dust ratio is ～100, the mass of matter ejected in 1988 was ～4×10?4 M ⊙. 相似文献
11.
We analyze models for quasi-stationary, ultraluminous X-ray sources (ULXs) with luminosities 1038–1040 erg/s exceeding the Eddington limit for a ～1.4M⊙ neutron star. With the exception of relatively rare stationary ULXs that are associated with supernova remnants or background quasars, most ULXs are close binary systems containing a massive stellar black hole (BH) that accretes matter donated by a stellar companion. To explain the observed luminosities of ～1040 erg/s, the mass of the BH must be ～40M⊙ if the accreted matter is helium and ～60M⊙ if the accreted matter has the solar chemical composition. We consider donors in the form of main-sequence stars, red giants, red supergiants, degenerate helium dwarfs, heavy disks that are the remnants of disrupted degenerate dwarfs, helium nondegenerate stars, and Wolf-Rayet stars. The most common ULXs in galaxies with active star formation are BHs with Roche-lobe-filling main-sequence companions with masses ～7M⊙ or close Wolf-Rayet companions, which support the required mass-exchange rate via their strong stellar winds. The most probable candidate ULXs in old galaxies are BHs surrounded by massive disks and close binaries containing a BH and degenerate helium-dwarf, red-giant, or red-supergiant donor. 相似文献
12.
The sequence of events determining the initial stages of star formation is analyzed in framework of the self-enrichment scenario. The computations are based on a single-zone chemo-dynamical model. It is shown that the first episode of star formation was characterized by an initial mass function shifted toward massive stars (M ≥ 8M⊙). We argue that the transition to a star formation with a normal (Salpeter) initial mass function was due to more efficient radiative cooling of the proto-globular cluster gas after its enrichment to a metallicity of Z ～ 0.02 Z⊙ in agreement with those observed in globular clusters. 相似文献
13.
Observations of the molecular cloud G1.6-0.025 in the 2K-1K and J0-J?1E series and 5?1-40E line of CH3OH, the (2-1) and (3-2) lines of SiO, and the 7?7-6?6 line of HNCO are described. Maps of the previously observed extended cloud with Vlsr～50 km/s and high-velocity clump with Vlsr～160 km/s, as well as a newly detected clump with Vlsr～0 km/s, have been obtained. The extended cloud and high-velocity clump have a nonuniform structure. The linewidths associated with all the objects are between 20 and 35 km/s, as is typical of clouds of the Galactic center. In some directions, emission at velocities from 40 to 160 km/s and from ?10 to +75 km/s is observed at the clump boundaries, testifying to a connection between the extended cloud and the high-velocity clump and clump at Vlsr～0 km/s. Compact maser sources are probaby contributing appreciably to the emission of the extended cloud in the 5?1-40E CH3OH line. Non-LTE modeling of the methanol emission shows that the extended cloud and high-velocity clump have a relatively low hydrogen density (<104 cm?3). The specific column density of methanol in the extended cloud exceeds 6×108 cm?3s, and is 4×108?6×109 cm?3s in the high-velocity clump. The kinetic temperatures of the extended cloud and high-velocity clump are estimated to be <80 K and 150–200 K, respectively. Possible mechanisms that can explain the link between the extended cloud with Vlsr～50 km/s and the clumps with Vlsr～0 km/s and ～160 km/s are briefly discussed. 相似文献
14.
The radial velocity fields of molecular clouds, OB stars, and ionized hydrogen in the Cygnus arm (l ～ 72°–8°) are analyzed. A gradientΔV LSR/Δlin the mean line-of-sight velocities of molecular clouds and ionized hydrogen due to differential Galactic rotation is detected, and two groups of physically and genetically associated objects moving with different line-of-sight velocities are identified. One of the two molecular-cloud complexes (l～77.3°–80°) is located within 1 kpc of the Sun, closer to the inner edge of the arm, whereas the other complex (l～78.5°–85°) lies 1–1.5 kpc from the Sun and is farther from the inner edge of the arm. The residual azimuthal velocities of the objects in both groups are analyzed. The residual azimuthal velocities of the first molecular-cloud complex are directed opposite to the Galactic rotation (V Θ ～ ?7 km/s), while those of the second complex are near zero or in the direction of Galactic rotation, independent of the distance to the complex (V Θ ≥ 1 km/s). Like the molecular clouds, stars of the Cygnus arm form two kinematic groups with similar azimuthal velocities. On the whole, the mean azimuthal velocities V Θ for the ionized hydrogen averaged over large areas agree with the velocities of either the first or second molecular-cloud complex. In terms of density-wave theory, the observed differences between the magnitudes and directions of the azimuthal velocities of the kinematic groups considered could be due to their different locations within the arm. 相似文献
15.
G. L. Nord Jr. 《Physics and Chemistry of Minerals》1980,6(2):109-128
Lunar and terrestrial orthopyroxenes (Mg,Fe,Ca)2Si2O6 contain varying abundances of coherent, Ca-enriched Guinier-Preston (G.P.) zones. G.P. zones 5–6 unit cells thick have been found in one lunar sample whereas all other examples (lunar and terrestrial) are only one unit-cell-thick. Electron diffraction maxima from the larger lunar G.P. zones indicate that d 100=18.52 Å whereas, d 100=18.2 Å for the host. This increase in the a direction corresponds to an increase in calcium content in the G.P. zones over that of the host of ～25 mol% Ca2Si2O6. Diffraction patterns of the hk0 net from an area containing G.P. zones show extra spots (h=2n+1) not observed in the host orthopyroxene (Pbca), that violate the a-glide of the host. The G.P. zones, therefore, have space group Pbc21 if it is assumed that the c-glide of pyroxene is retained and the space group of the G.P. zone is a subgroup of Pbca. The loss of the a-glide in the G.P. zones results in 4 distinct silica chains and 4 distinct cation sites M1A, M1B, M2A, M2B; by symmetry, equivalent M2A or M2B sites are clustered together in only one-half of the unit cell. As one-fourth of the divalent cations in the G.P. zones are calcium, ordering of Ca on M2A or M2B would produce a zone 9 Å thick extended parallel to (100) with the composition of Ca(Mg,Fe)Si2O6, but constrained by the host to the structure of orthopyroxene. This zone and the Ca-poor half-unit-cell then constitute an 18 Å thick G.P. zone. Heating experiments of varying duration indicate that the zones become unstable with respect to the host orthopyroxene at ～950°C for Wo0.6 and ～1,050°C for Wo2.5. The zones are interpreted in terms of the pyroxene subsolidus as a metastable phase having either a solvus relationship with orthopyroxene or originating as a distinct phase. The size, distribution, composition and structure of G.P. zones may be an important indicator of the low-temperature thermal history of orthopyroxene. 相似文献
16.
Physical properties of the variations of the electric field of the earth preceding earthquakes. II. determination of epicenter and magnitude 总被引：1，自引：0，他引：1
As reported in the preceding paper, a transient change of the electric field of the earth (seismic electric signal), hereafter called SES, appears many hours before an earthquake (EQ). By measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction. Measurements in two directions (E-W and N-S) give the relative signal intensity Jrel at the station under consideration. By measuring Jrel at a number of stations and considering that it attenuates according to a 1/r-law, the epicenter can be determined with an accuracy usually around 100 km. Once the epicenter has been determined, the product Jrel · r can be evaluated so that the magnitude M can be estimated by resorting to an empirical log(Jrel · r) versus M plot. The uncertainty of M is around 0.5 units. Following Sobolev (1975) and for the statistics to be beyond any doubt, predictions were officially documented before the EQ-occurrence. For 23 earthquakes with a magnitude equal or greater than Ms = 5.0 two events were missed.The present method is compared to other electrical methods used in China, Japan and Soviet Union. A number of problems concerning the origin of the effect, its directivity and the attenuation with distance remain open for further studies. 相似文献
17.
M. K. Abubekerov E. A. Antokhina A. M. Cherepashchuk V. V. Shimanskii 《Astronomy Reports》2006,50(7):544-552
We interpret the observed radial-velocity curve of the optical star in the low-mass X-ray binary 2S 0921-630 using a Roche model, taking into account the X-ray heating of the optical star and screening of X-rays coming from the relativistic object by the accretion disk. Consequences of possible anisotropy of the X-ray radiation are considered. We obtain relations between the masses of the optical and compact (X-ray) components, m v and m x , for orbital inclinations i = 60°, 75°, and 90°. Including X-ray heating enabled us to reduce the compact object’s mass by ～0.5–1 M ⊙, compared to the case with no heating. Based on the K0III spectral type of the optical component (with a probable mass of m v ? 2.9 M ⊙), we concluded that m x ? 2.45?2.55 M ⊙ (for i = 75°?90°). If the K0III star has lost a substantial part of its mass as a result of mass exchange, as in the V404 Cyg and GRS 1905+105 systems, and its mass is m v ? 0.65?0.75 M ⊙, the compact object’s mass is close to the standard mass of a neutron star, m x ? 1.4 M ⊙ (for i = 75°?90°). Thus, it is probable that the X-ray source in the 2S 0921-630 binary is an accreting neutron star. 相似文献
18.
L. E. Pirogov V. M. Shul’ga I. I. Zinchenko P. M. Zemlyanukha A. N. Patoka M. Tomasson 《Astronomy Reports》2016,60(10):904-923
Five regions of massive-star formation have been observed in various molecular lines in the frequency range～85?89 GHz. The studied regions comprise dense cores, which host young stellar objects. The physical parameters of the cores are estimated, including the kinetic temperatures (～20?40 K), the sizes of the emitting regions (～0.1?0.6 pc), and the virial masses (～40?500 M⊙). The column densities and abundances of various molecules are calculated assuming Local Thermodynamical Equilibrium(LTE). The core in 99.982+4.17, which is associated with the weakest IRAS source, is characterized by reduced molecular abundances. The molecular line widths decrease with increasing distance from the core centers (b). For b ? 0.1 pc, the dependences ΔV (b) are close to power laws (∝b?p), where p varies from ~0.2 to ~0.5, depending on the object. In four cores, the asymmetries of the optically thick HCN(1–0) and HCO+(1–0) lines indicates systematicmotions along the line of sight: collapse in two cores and expansion in two others. Approximate estimates of the accretion rates in the collapsing cores indicate that the forming stars have masses exceeding the solar mass. 相似文献
19.
M. G. Townsend G. Longworth C. A. M. Ross R. Provencher 《Physics and Chemistry of Minerals》1987,15(1):64-70
Mössbauer spectra of glauconite and nontronite recorded at temperatures down to 1.3K and in applied fields up to 4.5 T show that Fe III spin configurations are respectively ferromagnetic and antiferromagnetic. It is shown that in a particular material depending on the distribution and concentration of Fe III in the silicate sheet either mode might occur. A new model of competing nearest-neighbour (J 1) and next-nearest-neighbour (J 2) magnetic exchange interactions in the triangular lattice is introduced to account for the results. From available magnetic susceptibilities we estimate ∣J 1∣～6∣J 2∣. The results lead to the conclusion that the Fe III cations are highly ordered in glauconite and occupy cis sites so as to maximize their mutual separations. 相似文献
20.
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