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1.
Time variation in impact probability is studied by assuming that the periodic flux of the Oort Cloud comets within 15 au arises from the motion of the Sun with respect to the Galactic mid-plane. The periodic flux clearly shows up in the impact rate of the captured Oort Cloud cometary population, with a phase shift caused by the orbital evolution. Depending on the assumed flux of comets and the size distribution of comets, the impact rate of the Oort Cloud comets of 1 km in diameter or greater is from 5 to 700 impacts Myr−1 on the Earth and from 0.5 to 70 impacts per 1000 yr on Jupiter. The relative fractions of impacts are 0.09, 0.11, 0.26 and 0.54 for long-period comets, Halley type comets, Jupiter family comets and near-Earth objects, respectively. For Jupiter, the corresponding fractions in the first three categories are 0.18, 0.31 and 0.51. If we consider physical fading of comet activity that is compatible with the observations, then the impact rates of active comets are two orders of magnitude smaller than the total impact rates by all kinds of comets and cometary asteroids of size 1 km or greater.  相似文献   

2.
近地小行星是一类可能对地球安全造成潜在威胁的太阳系小天体, 目前绝大部分的近地小行星是由地基望远镜发现的, 且数目仍在不断增加. 为了对我国未来开展近地小行星发现监测提供参考和借鉴, 利用国际小行星中心公开的数据库对所有近地小行星首次发现时刻的观测资料开展了多维度统计分析. 发现望远镜探测能力的限制会对近地小行星的发现造成选择效应, 导致不同轨道类型近地小行星发现的相对比例逐年变化且与直径有关. 另外, 结合数值模拟获得的轨道数据, 对近地小行星首次发现时的观测场景进行了还原, 获得了发现时刻近地小行星位置在不同天球坐标系的分布, 分析了其分布特征与季节、测站纬度和小行星直径的依赖关系. 最后, 通过分析数据定量考察了太阳、月球和银道面对近地小行星发现的影响, 发现地基望远镜一般难以发现来自太阳方向90$^\circ$范围内直径140m以下的近地小行星, 并且随着小行星直径的减小该限制范围也将变大; 月光污染对近地小行星发现的影响也非常显著, 望月前后几天的观测限制可导致约29%的目标无法被发现, 而且分析表明农历上半月发现的目标一般比下半月发现的更难以被跟踪观测; 银道面特别是银心方向会对近地小行星发现产生影响, 使得黄道面附近存在与季节相关的观测``盲区''.  相似文献   

3.
《Planetary and Space Science》2007,55(11):1541-1556
The exosphere of Mercury has been the object of many investigations and speculations regarding its composition, formation, depletion and dynamics. While vapourization of Mercurian surface materials by meteorite impacts has been often considered to be a less important contributor to the exosphere than other potential processes, larger objects coming from the Main Asteroid Belt could cause high local and transient enhancements in the density of the exosphere. Vapourization by such impacts is an almost stoichiometric process, and thus would contain valuable information about the surface composition. We investigate some exospheric effects of impact vapourization for meteorites with radii of 1, 10 cm, and 1 m, with particular reference to the missions that will explore Mercury during the next decade (MESSENGER and BepiColombo). Because of their higher probabilities, impacts of objects in the two smaller size ranges will surely occur during the lifetimes of the two missions. The enhancement of the exospheric density on the dayside of Mercury would be appreciable for the 10-cm and 1-m meteorites (some orders of magnitude, especially for Al, Mg, Si, and Ca). Such events could allow detection, for the first time, of refractory species like Al, Mg, and Si, which are expected to exist on the surface but have not yet been detected in the exosphere. Ca could be detectable in all cases, even if produced by impacting objects as small as 1 cm in radius. The lower exospheric background on the night side should allow easier identification of Na and K produced by impulsive events, even if their generally high background values make this eventuality less likely.  相似文献   

4.
We present the infrasonic observations of three large bolides that were observed at numerous International Monitoring System (IMS) infrasound arrays on a global scale. First, a simple procedure for the global association of infrasound detections from large infrasound events is outlined. Infrasound signals are associated with large events based on arrival time, backazimuth and uniqueness at a given IMS array. Next, we apply the algorithm to three bolides and investigate some of the factors affecting the detectability of infrasound from large events. Our findings suggest that site-noise effects significantly degrade the capability of the IMS infrasound network, suggesting that more effort is required to reduce ambient site noise. These results have implications for the use of infrasound measurements (in particular those from IMS stations) as a tool for evaluating the global flux of near-Earth objects.  相似文献   

5.
In the following paper we argue that each wind-driving star in relative motion with respect to the ambient interstellar medium experiences a force exerted on its central wind-generating body. The exact magnitude of this force depends on the actual geometry of the counterflow configuration of stellar and interstellar winds for a particular kinematic situation which is especially sensitive to whether the interstellar flow is subsonic or supersonic. It will, however, be demonstrated here that this force is of an accelerating nature, i.e., it operates like a rocket-motor, as long as the peculiar motion of the wind-driving star with respect to the ambient interstellar medium remains subsonic.Here we use a specific analytical model to describe theoretically the specific counterflow configuration for the case of the solar system in a subsonic peculiar motion with respect to the local interstellar medium assuming irrotational and incompressible flows. We can work out a quantitative number for the accelerating force governing the Sun's motion at present. The net reaction force exerted on the solar body is then mediated by the asymmetric boundary conditions to which the distant solar wind field has to adapt.Next we study the indirect action of such a force on orbiting Keplerian objects like planets, planetesimals and comets. Since this force only influences the central solar body, but not the planets themselves, the problem is different from the treatment of a constant perturbation force perturbing the Keplerian orbits. We present a perturbation analysis treating the action of a corresponding position-dependent perturbation force resulting in secular changes of the orbital elements of Keplerian objects. It is found that changes are accumulating more rapidly in time the closer to the sun the orbiting bodies are. Main axis and perihelion distances are systematically increasing. Especially pronounced are changes in the perihelion position angle of the objects. For solar wind mass losses larger than the Sun's present value by a factor of 1000 (T-Tauri phase of the Sun,) the migration periods calculated for the planet Mercury are of the same order of magnitude as that for corresponding general relativistic migration.  相似文献   

6.
Abstract— Scanning electron microscopy of 137 Australasian microtektites and fragments from 4 sediment cores in the Central Indian Ocean reveals more than 2000 impact‐generated features in the size range of 0.3 to 600 μm. Three distinct impact types are recognized: destructive, erosive, and accretionery. A large variation in impact energy is seen in terms of catastrophic destruction demonstrated by fragmented microtektites through erosive impacts comprising glass‐lined pit craters, stylus pit craters, pitless craters, and a small number of accretionery features as well. The size range of observed microtektites is from 180 to 2320 μm, and not only are the smaller microtektites seen to have the largest number of impacts, but most of these impacts are also of the erosive category, indicating that target temperature is an important factor for retaining impact‐generated features. Further, microcratering due to collisions in impact‐generated plumes seems to exist on a larger and more violent scale than previously known. Although the microcraters are produced in a terrestrially generated impact plume, they resemble lunar microcraters in many ways: 1) the size range of impacts and crater morphology variation with increasing size; 2) dominant crater number densities in μm and sub‐μm sizes. Therefore, tektite‐producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials, and for the lunar rocks to qualify as reliable cosmic dust flux detectors, their tumbling histories and lunar surface orientations have to be known precisely.  相似文献   

7.
Takaaki Takeda  Keiji Ohtsuki 《Icarus》2009,202(2):514-524
Expanding on our previous N-body simulation of impacts between initially non-rotating rubble-pile objects [Takeda, T., Ohtsuki, K., 2007. Icarus 189, 256-273], we examine effects of initial rotation of targets on mass dispersal and change of spin rates. Numerical results show that the collisional energy needed to disrupt a rubble-pile object is not sensitive to initial rotation of the target, in most of the parameter range studied in our simulations. We find that initial rotation of targets is slowed down through disruptive impacts for a wide range of parameters. The spin-down is caused by escape of high-velocity ejecta and asymmetric re-accumulation of fragments. When these effects are significant, rotation is slowed down even when the angular momentum added by an impactor is in the same direction as the initial rotation of the target. Spin-down is most efficient when the impact occurs in the equatorial plane of the target, because in this case most of the ejected fragments originate from the equatorial region of the target and a significant amount of angular momentum can be easily removed. In the case of impacts from directions inclined relative to the target's equatorial plane, spin-down still occurs with reduced degree, unless impacts occur onto the pole region from the vertical direction. Our results suggest that such spin-down through disruptive impacts may have played an important role in spin evolution of asteroids through collisions in the gravity-dominated regime.  相似文献   

8.
Parallax measurements allow distances to celestial objects to be determined. Together with measurements of their position on the celestial sphere, they give a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a distant source affected by weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. The gravitational deflection causes a retrograde apparent motion of the image, which is revealed as a negative parallax. It is shown that the distortions may significantly change the parallax values when they amount to several microseconds of arc. In particular, at this level many measured values of parallaxes should be negative.  相似文献   

9.
Classical Cepheids remain a cornerstone of the cosmic distance scale, and thus characterizing the dependence of their light amplitude on metallicity is important. Period-amplitude diagrams constructed for longer-period classical Cepheids in IC 1613, NGC 3109, SMC, NGC 6822, LMC, and the Milky Way imply that very metal-poor Cepheids typically exhibit smaller V-band amplitudes than their metal-rich counterparts. The results provide an alternate interpretation relative to arguments for a null and converse metallicity dependence. The empirical results can be employed to check predictions from theoretical models, to approximate mean abundances for target populations hosting numerous long-period Cepheids, and to facilitate the identification of potentially blended or peculiar objects.  相似文献   

10.
Galileo images of asteroid 951 Gaspra reveal numerous linear ridges, crater chains and valleys. I describe? all visible linear features and assess their significance. A few crater chains may be chance alignments of impact craters, but most linear features are probably surface expressions of deep fractures caused by impact, perhaps exploiting systems of joints. Some features not previously recognised are documented, and one previously described structure, a ‘chevron’ form near the north pole, is shown to be an artifact. The global distribution suggests a monolithic interior. Two separate sets of grooves may have been formed by different impacts. One group of four or five parallel, wide but extremely subdued depressions may indicate the presence of a thicker or more mobile regolith accumulated on a sloping, rotational leading surface.  相似文献   

11.
As is well known, black hole entropy is proportional to the area of the horizon suggesting a holographic principle wherein all degrees of freedom contributing to the entropy reside on the surface. In this note, we point out that large scale dark energy (such as a cosmological constant) constraining cosmic structures can imply a similar situation for the entropy of a hierarchy of such objects.  相似文献   

12.
Studies of impacts (impactor velocity about 5 km s−1) on icy targets were performed. The prime goal was to study the response of solid CO2 targets to impacts and to find the differences between the results of impacts on CO2 targets with those on H2O ice targets. The crater dimensions in CO2 ice were found to scale with impact energy, with little dependence on projectile density (which ranged from nylon to copper, i.e., 1150-8930 kg m−3). At equal temperatures, craters in CO2 ice were the same diameter as those in water ice, but were shallower and smaller in volume. In addition, the shape of the radial profiles of the craters was found to depend strongly on the type of ice and to change with impact energy. The impact speed of the data is comparable to that for impacts on many types of icy bodies in the outer Solar System (e.g., the satellites of the giant planets, the cometary nuclei and the Kuiper Belt objects), but the size and thus energy of the impactors is lower. Scaling with impact energy is demonstrated for the impacts on CO2 ice. The issue of impact disruption (rather than cratering) is discussed by analogy with that on water ice. Expressions for the critical energy density for the onset of disruption rather than cratering are established for water ice as a function of porosity and silicate content. Although the critical energy density for disruption of CO2 ice is not established, it is argued that the critical energy to disrupt a CO2 ice body will be greater than that for a (non-porous) water ice body of the similar mass.  相似文献   

13.
The occurrence and visibility of meteoroid impacts on the moon as seen from the earth were little more than speculation prior to November 1999. The best evidence of present-day impact activity came from the seismic experiments left on the Moon during the Apollo era. Past systematic attempts at earth-based observations to document lunar impacts revealed nothing conclusive. However, during the Leonid storms of 1999 and 2001, lunar impact events were for the first time confirmed by multiple independent observers. A total of 15 meteoritic impact flash events have been verified during these storms, with an additional 12 unconfirmed but likely events awaiting confirmation. Estimates of the mass of these meteoroids range from less than one gram for the faintest flashes to more than 10 kg for the brightest observed flash. The fraction of visible light to total energy produced by these events, a quantity known as luminous efficiency, averages about 0.001 for the established events. The confirmation of lunar meteoritic events on the Moon opens a new avenue in lunar and planetary research, one which could help bridge the gap between atmospheric sampling of the smallest components of meteoroid streams and interplanetary debris to the larger scale objects accessible to ground-based telescopes.  相似文献   

14.
Abstract— Imaging of asteroids Gaspra and Ida and laboratory studies of asteroidal meteorites show that impacts undoubtedly played an important role in the histories of asteroids and resulted in shock metamorphism and the formation of breccias and melt rocks. However, in recent years, impact has also been called upon by numerous authors as the heat source for some of the major geological processes that took place on asteroids, such as global thermal metamorphism of chondrite parent bodies and a variety of melting and igneous events. The latter were proposed to explain the origin of ureilites, aubrites, mesosiderites, the Eagle Station pallasites, acapulcoites, lodranites, and the IAB, IIICD, and HE irons. We considered fundamental observations from terrestrial impact craters, combined with results from laboratory shock experiments and theoretical considerations, to evaluate the efficiency of impact heating and melting of asteroids. Studies of terrestrial impact craters and relevant shock experiments suggest that impact heating of asteroids will produce two types of impact melts: (1) large-scale whole rock melts (total melts, not partial melts) at high shock pressure and (2) localized melts formed at the scale of the mineral constituents (mineral specific or grain boundary melting) at intermediate shock pressures. The localized melts form minuscule amounts of melt that quench and solidify in situ, thus preventing them from pooling into larger melt bodies. Partial melting as defined in petrology has not been observed in natural and experimental shock metamorphism and is thermodynamically impossible in a shock wave-induced transient compression of rocks. The total impact melts produced represent a minuscule portion of the displaced rock volume of the parent crater. Internal differentiation by fractional crystallization is absent in impact melt sheets of craters of sizes that can be tolerated by asteroids, and impact melt rocks are usually clast-laden. Thermal metamorphism of country rocks by impact is extremely minor. Experimental and theoretical considerations suggest that (1) single disruptive impacts cannot raise the average global temperature of strength- or gravity-dominated asteroids by more than a few degrees; (2) cumulative global heating of asteroids by multiple impacts is ineffective for asteroids less than a few hundred kilometers in diameter; (3) small crater size, low gravity, and low impact velocity suggest that impact melt volume in single asteroidal impacts is a very small (0.01–0.1%) fraction of the total displaced crater volume; (4) total impact melt volume formed during the typical lifetime of an asteroid is a small fraction (<0.001) of the volume of impact-generated debris; and (5) much of the impact melt generated on asteroidal targets is ejected from craters with velocities greater than escape velocity and, thus, not retained on the asteroid. The inescapable conclusion from these observations and calculations is that impacts cannot have been the heat source for the origin of the meteorite types listed above, and we must turn to processes other than impact, such as decay of short-lived radionuclides or electromagnetic induction during an early T-tauri phase of the Sun to explain heating and melting of the parent bodies of these meteorites.  相似文献   

15.
In this paper, we compare the outcome of high-velocity impact experiments on porous targets, composed of pumice, with the results of simulations by a 3D SPH hydrocode in which a porosity model has been implemented. The different populations of small bodies of our Solar System are believed to be composed, at least partially, of objects with a high degree of porosity. To describe the fragmentation of such porous objects, a different model is needed than that used for non-porous bodies. In the case of porous bodies, the impact process is not only driven by the presence of cracks which propagate when a stress threshold is reached, it is also influenced by the crushing of pores and compaction. Such processes can greatly affect the whole body's response to an impact. Therefore, another physical model is necessary to improve our understanding of the collisional process involving porous bodies. Such a model has been developed recently and introduced successfully in a 3D SPH hydrocode [Jutzi, M., Benz, W., Michel, P., 2008. Icarus 198, 242-255]. Basic tests have been performed which already showed that it is implemented in a consistent way and that theoretical solutions are well reproduced. However, its full validation requires that it is also capable of reproducing the results of real laboratory impact experiments. Here we present simulations of laboratory experiments on pumice targets for which several of the main material properties have been measured. We show that using the measured material properties and keeping the remaining free parameters fixed, our numerical model is able to reproduce the outcome of these experiments carried out under different impact conditions. This first complete validation of our model, which will be tested for other porous materials in the future, allows us to start addressing problems at larger scale related to small bodies of our Solar System, such as collisions in the Kuiper Belt or the formation of a family by the disruption of a porous parent body in the main asteroid belt.  相似文献   

16.
Numerical simulations have been used to study high velocity two-body impacts. In this paper a two-dimensional Lagrangian finite difference hydrocode and a three-dimensional smooth particle hydrocode (SPH) are described and initial results reported.

The 2D hydrocode has successfully reproduced both the fragment size distribution and the mean fragment velocities from laboratory impact experiments using basalt and cement mortar. Further, the hydrocode calculations have determined that the energy needed to fracture a body has a much stronger dependence on target size than predicted from most scaling theories. In addition, velocity distributions obtained (using homogeneous targets at impact velocities around 2 km s−1) indicate that mean ejecta speeds resulting from large-body collisions do not generally exceed escape velocities.

The SPH model provides a fully three-dimensional framework for studying impacts, so that phenomena such as oblique collisions or impacts into non-spherical targets may be studied. The gridless code allows for arbitrary levels of distortion, and is hence appropriate for modeling the large-scale deformations which accompany most impact events. Because fragments are modeled explicitly, greater numerical accuracy is achieved in the regions of large fragments than with the purely statistical approach of the 2D model. Of course, this accuracy comes at the expense of significantly greater computational requirements.

These codes can be, and have been, used to make specific predictions about particular objects in our solar system. But more significantly, they allow us to explore a broad range of collisional events. Certain parameters (size, time) can be studied only over a very restricted range within the laboratory; other parameters (initial spin, low gravity, exotic structure or composition) are difficult to study at all experimentally. The outcomes of numerical simulations lead to a more general and accurate understanding of impacts in their many forms.  相似文献   


17.
Abstract— Isotopic ages of meteorites that indicate chronometer resetting due to impact heating are summarized. Most of the ages were obtained by the 39Ar-40Ar technique, but several Rb-Sr, Pb-Pb, and Sm-Nd ages also suggest some degree of impact resetting. Considerations of experimental data on element diffusion in silicates suggest that various isotopic chronometers ought to differ in their ease of resetting during shock heating in the order K-Ar (easiest), Rb-Sr, Pb-Pb, and Sm-Nd, which is approximately the order observed in meteorites. Partial rather than total chronometer resetting by impacts appears to be the norm; consequently, interpretation of the event age is not always straightforward. Essentially all 39Ar-40Ar ages of eucrites and howardites indicate partial to total resetting in the relatively narrow time interval of 3.4–4.1 Ga ago (1 Ga = 109 years). Several disturbed Rb-Sr ages appear consistent with this age distribution. This grouping of ages and the brecciated nature of many eucrites and all howardites argues for a large-scale impact bombardment of the HED parent body during the same time period that the Moon received its cataclysmic bombardment. Other meteorite parent bodies such as those of mesosiderites, some chondrites, and IIE irons also may have experienced this bombardment. These data suggest that the early bombardment was not lunar specific but involved much of the inner Solar System, and may have been caused by breakup of a larger planetismal. Although a few chondrites show evidence of age resetting ～3.5–3.9 Ga ago, most impact ages of chondrites tend to fall below 1.3 Ga in age. A minimum of ～4 impact events, including events at 0.3, 0.5, 1.2, and possibly 0.9 Ga appear to be required to explain the younger ages of H, L, and LL chondrites, although additional events are possible. Most L chondrites show evidence of shock, and the majority of 39Ar-40Ar ages of L chondrites fall near 0.5 Ga. The L chondrite parent body apparently experienced a major impact at this time, which may have disrupted it. The observations (1) that lunar highland rocks experienced major impact resetting of various isotopic chronometers ～3.7–4.1 Ga ago; (2) that the HED parent body experienced widespread impact resetting of the K-Ar chronometer but only modest disturbance of other isotopic systems, during a similar time period; (3) that ordinary chondrite parent bodies show much more recent and less extensive impact resetting; and (4) that impacts, which initiated cosmic-ray exposure of most stone meteorites almost never reset isotopic chronometers, may all be a consequence of relative parent body size. Greater degrees of isotopic chronometer resetting occur in larger and warmer impact ejecta deposits that cool slowly. The relatively greater size of bodies like the Moon and Vesta (assumed to be the parent asteroid of HED meteorites) both permit such favorable ejecta deposits to occur more easily compared to smaller parent bodies (generally assumed for chondrites) and also protect parent objects from collisional disruption. Thus, impacts on larger bodies would tend to more easily reset chronometers, consistent with the observed relative ease of resetting of Moon (easiest), HED, chondrites and of K-Ar (easiest), Rb-Sr, other chronometers. In contrast, the more recent impact ages of chondrites are postulated to represent collisional disruption of smaller parent objects whose fragments are more readily removed from the meteorite source reservoirs. Impacts that initiate cosmic-ray exposure are mostly small in scale and produce little heating.  相似文献   

18.
Abstract— We describe the results of a variety of model calculations for predictions of observable results of the LCROSS mission to be launched in 2009. Several models covering different aspects of the event are described along with their results. Our aim is to bracket the range of expected results and produce a useful guide for mission planning. In this paper, we focus on several different questions, which are modeled by different methods. The questions include the size of impact crater, the mass, velocity, and visibility of impact ejecta, and the mass and temperature of the initial vapor plume. The mass and velocity profiles of the ejecta are of primary interest, as the ejecta will be the main target of the S‐S/C observations. In particular, we focus on such quantities as the amount of mass that reaches various heights. A height of 2 km above the target is of special interest, as we expect that the EDUS impact will take place on the floor of a moderate‐sized crater ?30 km in diameter, with a rim height of 1–2 km. The impact ejecta must rise above the crater rim at the target site in order to scatter sunlight and become visible to the detectors aboard the S‐S/C. We start with a brief discussion of crater scaling relationships as applied to the impact of the EDUS second stage and resulting estimated crater diameter and ejecta mass. Next we describe results from the RAGE hydrocode as applied to modeling the short time scale (t 0.1 s) thermal plume that is expected to occur immediately after the impact. We present results from several large‐scale smooth‐particle hydrodynamics (SPH) calculations, along with results from a ZEUS‐MP hydrocode model of the crater formation and ejecta mass‐velocity distribution. We finish with two semi‐analytic models, the first being a Monte Carlo model of the distribution of expected ejecta, based on scaling models using a plausible range of crater and ejecta parameters, and the second being a simple model of observational predictions for the shepherding spacecraft (S‐S/C) that will follow the impact for several minutes until its own impact into the lunar surface. For the initial thermal plume, we predict an initial expansion velocity of ?7 km s?1, and a maximum temperature of ?1200 K. Scaling relations for crater formation and the SPH calculation predict a crater with a diameter of ?15 m, a total ejecta mass of ?106kg, with ?104kg reaching an altitude of 2 km above the target. Both the SPH and ZEUS‐MP calculations predict a maximum ejecta velocity of ?1 km s?1. The semi‐analytic Monte Carlo calculations produce more conservative estimates (by a factor of ?5) for ejecta at 2 km, but with a large dispersion in possible results.  相似文献   

19.
Abstract— Chondrule-like objects and brown glasses were analyzed in the howardites, Bununu, Malvern, Monticello, Pavlovka, and Yamato 7308. The objects are very similar to chondrules in ordinary and carbonaceous chondrites. Like the brown glasses the chondrule-like objects could have been produced by impact melting that left some crystalline nuclei, followed by a slower cooling rate than for the glasses. Alternatively, these objects are chondrules implanted from chondrite impactors. They are, however, without rims or any adhering matrix. The brown glasses appear to represent melting of average regolithic surface material, except for Monticello and Y7308, both of which have some siliceous glasses. The siliceous glasses could not have been produced by vapor fractionation but by melting of differentiated lithologies such as fayalitic granites. Impact mechanics indicates that howardites with abundant brown glasses came from an asteroid larger than Vesta (>400 km radius), upon which impacts occurred at relative velocities of up to 5 km/s. Howardites with little or no brown glasses came from a smaller parent body. We conclude that at least two parent bodies are likely sources for the basaltic achondrites.  相似文献   

20.
Using a Newtonian approximation we developed a quantitative criterion for the collapse of a spherical distribution of matter under an isolated texture field. In particular, we found the evolution of an overdense region is strongly determined by two parameters: the energy scale of symmetry breaking ( η ) and the initial radius of the system. Applying our collapse criterion to typical galaxy scales we verified the formation of 1011‐M⊙ objects at z ≲9 and 1012‐M⊙ objects at z ≲5.  相似文献   

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