共查询到20条相似文献,搜索用时 15 毫秒
1.
Yan-JieXue 《中国天文和天体物理学报》2001,1(5):383-388
LETTERS1 INTRODUCTIONIn the hierarchical scenario of structure formation, massive dark ha1os fOrm by gravitationalaggregation of individual low-mass objects, whi1e the stel1ar disks of spiral galaxies like theMilky Way form by accretion of gas which cools and falls onto the galaxies from an extendedsurrounding reservoir. FOr a massive galaxy of M ~ 10"MO, the surrounding gas can be heatedto temperature of T ~ 106 K by gravitational1y-driven shocks, the dominant cooling is thus dueto … 相似文献
2.
Olga Popova 《Earth, Moon, and Planets》2004,95(1-4):303-319
The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from
ablation of small-sized meteors (roughly R≤0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the
atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance
properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the
falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere
allows describing two limiting cases: – large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical
models); – small meteoroid/or high altitudes – free molecule regime of interaction, which assumes no collisions between evaporated
meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion.
Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent
hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This
paper reviews existing ablation models and discusses model boundaries. 相似文献
3.
4.
H. McNamara J. Jones B. Kauffman R. Suggs W. Cooke S. Smith 《Earth, Moon, and Planets》2004,95(1-4):123-139
In an attempt to overcome some of the deficiencies of existing meteoroid models, NASA’s Space Environments and Effects (SEE)
Program sponsored a 3 year research effort at the University of Western Ontario. The resulting understanding of the sporadic
meteoroid environment – particularly the nature and distribution of the sporadic sources – were then incorporated into a new
Meteoroid Engineering Model (MEM) by members of the Space Environments Team at NASA’s Marshall Space Flight Center. This paper
discusses some of the revolutionary aspects of MEM which include (a) identification of the sporadic radiants with real sources
of meteoroids, such as comets, (b) a physics-based approach which yields accurate fluxes and directionality for interplanetary
spacecraft anywhere from 0.2 to 2.0 astronomical units (AU), and (c) velocity distributions obtained from theory and validated
against observation. Use of the model, which gives penetrating fluxes and average impact speeds on the surfaces of a cube-like
structure, is also described along with its current limitations and plans for future improvements. 相似文献
5.
The development of galactic fountain theories is reviewed with special emphasis on the different approaches and concepts that
have been used in the past. In particular the outstanding contribution of Franz Kahn to our physical understanding is appreciated. It is argued that galactic outflows represent an essential phase during galaxy evolution. The
dynamics of the outflow imprints its signature on the emission spectra of soft X-rays, which may well be observable with AXAF
and XMM. Finally, some remarks about winds in starburst galaxies and AGN are made.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
I. P. Williams 《Celestial Mechanics and Dynamical Astronomy》2001,81(1-2):103-113
Meteors are streaks of light seen in the upper atmosphere when particles from the inter-planetary dust complex collide with the Earth. Meteor showers originate from the impact of a coherent stream of such dust particles, generally assumed to have been recently ejected from a parent comet. The parent comets of these dust particles, or meteoroids, fortunately, for us tend not to collide with the Earth. Hence there has been orbital changes from one to the other so as to cause a relative movement of the nodes of the meteor orbits and that of the comet, implying changes in the energy and/or angular momentum. In this review, we will discuss these changes and their causes and through this place limits on the ejection process. Other forces also come into play in the longer term, for example perturbations from the planets, and the effects of radiation pressure and Poynting–Robertson drag. The effect of these will also be discussed with a view to understanding both the observed evolution in some meteor streams. Finally we will consider the final fate of meteor streams as contributors to the interplanetary dust complex. 相似文献
7.
I. G. Brykina 《Solar System Research》2018,52(5):426-434
The interaction between a large meteoroid and the atmosphere is modeled as its destruction into a cloud of fragments and vapors moving with a common shock wave. Under the action of aerodynamic forces the shape of this cloud is deformed—it is expanded in the direction transverse to the motion and compressed in the longitudinal direction. With allowance for the pressure distribution over the surface of a body varying its shape (it is assumed that the sphere is transformed into a flattened spheroid), the relation for the rate of increase in the midsection radius of a fragmented meteoroid has been obtained. This rate significantly depends on the degree of the meteoroid flattening which leads to a significantly smaller increase in the transverse size of the meteoroid along the trajectory as compared to similar models used in the literature where the influence of the body shape was not considered. The proposed model also takes into account the change in the density of the cloud of fragments due to an increase in gaps between them. An approximate analytical solution of equations of the physical theory of meteors with drag and heat transfer coefficients varying along the trajectory has been obtained for a fragmented meteoroid. The interaction of the Chelyabinsk meteoroid with the atmosphere is modeled and the solution obtained for the energy release curve is compared with the observational data. 相似文献
8.
Frans J. M. Rietmeijer 《Earth, Moon, and Planets》2008,102(1-4):461-471
Bulk compositions of aggregate meteoroids made of the originally accreted dust with its highly varied in mineral content and
chemistry and considerable grain size variations do not have a chondritic bulk composition. Deviations from CI element abundances
reflect indigenous variations within and among comet nuclei. These unmodified meteoroids that are heterogeneous in all their
properties are fundamentally different from meteoroids with a CI bulk composition that are fine-grained, equigranular materials
and chemically and mineralogically homogeneous. Collection and data reduction bias exists but the compositions of individual
fast meteors are entirely constrained by the measured main component meteor abundances. 相似文献
9.
N. G. Barri 《Solar System Research》2010,44(1):55-59
There is significant evidence that some fraction of meteoric bodies is destroyed in the atmosphere. The evolution of the fragment
cloud depends on a large number of factors, amound them: the meteoroid’s altitude and velocity at the moment of greakup, fragment
sizes and properties of a body material. The interaction of shock waves forming in front of the fragments may lead to both
an increase and decrease of the midsection area of the fragment cloud (Artem’eva & Shuvalov, 1996; Laurence et al., 2007).
In this work, we consider the interaction of the fragments in a supersonic flow. The configuration properties of two spherical
bodies of different radii are considered. Via numerical simulations, we calculate the pressure distribution in the flow around
the two bodies for different relative positions. We construct the functions of the coefficients of transverse and drag forces
from the angle between the central line of the two bodies and the flow direction for different distances between the two fragments.
We find the conditions for the collimation effect, i.e., fragment involving into the wake of the leading (usually, the largest)
fragment. We systematize the simulation results for drag and transverse forces and infer the basic aerodynamic properties
of the meteoroid fragments. 相似文献
10.
A.V. Arkhipov 《Astrophysics and Space Science》1997,252(1-2):67-71
It is shown that the Earth is a natural collector of extraterrestrial nonsterile artefacts that could impact our planet. Artefacts from 1.2 × 106 nearby stars could have reached the Earth over its history, and could be agents for spontaneous interstellar panspermia, even if alien civilizations pollute space only at the current terrestrial rate. 相似文献
11.
Wolfgang Kundt 《Astrophysics and Space Science》1996,235(2):319-327
Application of standard accretion-disk theory to the central parts of galactic disks implies that galactic nuclei should pass quasi-periodically through stages of (i) gravitational instability (Starburst), (ii) non-thermal disk activity (LINER), and (iii) central-plane nuclear burning (AGN). Nuclear disks can have large mass-to-light ratios, and store large masses at high (Keplerian) rotation velocities. Aburning disk can explain all the phenomena commonly attributed to a supermassive Black Hole. 相似文献
12.
I. P. Williams 《Earth, Moon, and Planets》1995,68(1-3):1-12
Summary There is a general agreement that meteoroid streams form through the ejection of dust grains, or meteoroids, up to a few centimeters in size from comets and possibly asteroids. After ejection these meteoroids are subject to forces arising from Solar radiation and the gravitational fields of the planets. Meteoroids may also break up into smaller ones through collisions and other effects. In many cases meteor showers have been observed for millennia, with material being fed into the stream throughout this period from the parent and material lost through the external effects mentioned. Much of the lost material forms the general sporadic background. This paper will review our state of knowledge of the processes involved above and will also aim to give some insight into the structure of the sporadic background 相似文献
13.
W. J. Baggaley 《Earth, Moon, and Planets》1995,71(1-2):85-97
H.F. radars are a powerful diagnostic for examining the orbits and spatial characteristics of the dust-meteoroid complex. Data on those properties are essential in realistic studies of the dynamics of small bodies in the solar system. An outline is here presented of the various programmes and their techniques employed in radar operations over the last 40 years. The particular value and contribution of radar facilities and comparisons with other methods is emphasised. Some of the features of the only current routine survey are discussed and, to illustrate to power of the system, examples of the types of data presentation are given. 相似文献
14.
F. Garzón P. L. Hammersley T. Mahoney X. Calbet M. J. Selby 《Astrophysics and Space Science》1994,217(1-2):177-180
The Two Micron Galactic Survey (TMGS) is the most sensitive large scale point source near infrared survey of the Galactic Plane yet attempted. The Galactic plane has been drift-scanned in several regions over the areas -5° <l < 30°, |b | 15° and 30° <l < 180°, |b | 5°. The survey is complete in the areas covered to magnitudem
k = +9.8. So far, a total area of 255 square degrees has been mapped and 500000 objects have been detected, the majority of these in the Galactic plane and with no visible counterparts. In this contribution we use data from the TMGS to probe the star distribution within the Galactic disk. 相似文献
15.
《Chinese Astronomy》1979,3(2):174-178
We discussed the possible role of active galactic nuclei, which would lead to the generation of strong winds from them. We show that the galaxies are swept clean of hot gases from dying stars by such winds, resulting in the observed upper limit in the gas-content of E and SO galaxies. 相似文献
16.
Mercedes Mollá Federico Ferrini †‡ Giacomo Gozzi 《Monthly notices of the Royal Astronomical Society》2000,316(2):345-356
We model the evolution of the Galactic bulge and of the bulges of a selected sample of external spiral galaxies, via the multiphase multizone evolution model. We address a few questions concerning the role of the bulges within galactic evolution schemes and the properties of bulge stellar populations. We provide solutions to the problems of chemical abundances and spectral indices, the two main observational constraints to bulge structure. 相似文献
17.
F. D. Kahn 《Astrophysics and Space Science》1994,216(1-2):325-332
The hot intercloud medium (ICM) streams away from the Galactic disk into the fountain. It is generally agreed that a flow consisting of thermal gas only reaches a maximum height of a few (3 to 5) kpc. The dynamics is more complicated if cosmic rays are also present. In that case it is possible for some of the gas to be ejected from the Galaxy altogether. 相似文献
18.
M. D. Campbell-Brown S. Close 《Monthly notices of the Royal Astronomical Society》2007,382(3):1309-1316
The Advanced Research Project Agency (ARPA) Long Range Tracking and Instrumentation Radar has recorded thousands of head echoes from small meteoroids, which include detailed trajectory information as well as ionization measurements. In total, 25 complete ionization curves have been matched using a detailed model of meteoroid ablation, though the solutions are not necessarily unique. While measurements of the spread along the trajectory of the echoes indicate that most meteors in this size range do not have large separations among fragments, the ionization curves are consistent with fragmenting bodies in the most cases. Very precise radar measurements of meteors can be a valuable source of data on the chemical and physical properties of small meteoroids. 相似文献
19.
Paul A. Wiegert 《Earth, Moon, and Planets》2008,102(1-4):15-26
The Canadian Meteor Orbit Radar (CMOR) has collected information on a number of weak meteor showers that have not been well
characterized in the literature. A subsample of these showers (1) do not show a strong orbital resemblance to any known comets
or asteroids, (2) have highly inclined orbits, (3) are at low perihelion distances ( AU) and (4) are at small semimajor axes (<2 AU). Though one might conclude that the absence of a parent object could be the
result of its disruption, it is unclear how this relatively inaccessible (dynamically speaking) region of phase space might
have been populated by parents in the first place. It will be shown that the Kozai secular resonance and/or Poynting–Robertson
drag can modify meteor stream orbits rapidly (on time scales comparable to a precession cycle) and may be responsible for
placing some of these streams into their current locations. These same effects are also argued to act on these streams so
as to contribute to the high-ecliptic latitude north and south toroidal sporadic meteor sources. There remain some differences
between the simple model results presented here and observations, but there may be no need to invoke a substantial population
of high-inclination parents for the observed high-inclination meteoroid streams with small perihelion distances. 相似文献
20.
The recent systematic monitoring of the skies has led to the discovery of an increasingly large number of objects on Earth
approaching orbits. Not surprisingly, an increasing number of this population have also been associated with meteoroid streams
in the literature. We will review the history of this topic. We have also conducted our own search for asteroids moving on
orbits that are similar to the orbits of known fireball streams. As NEOs are moving in prograde orbits with low geocentric
velocities, any potential streams will have large radiant areas and in consequence, may have been identified as several "sub-streams".
This greatly hampers both their detection and their recognition as single meteoroid streams. With the large number of Near
Earth Asteroids detected, the probability of two orbits being similar at the present time by coincidence is high. We have
therefore also investigated the evolution of the orbits and only include as real asteroid-stream pairs those where the evolution
is also similar over 5000 years. We have identified nine pairs, including the well known pair of the Geminid meteoroid stream
and asteroid 3200 Phaethon. Currently there are a number of papers being published on the pairing of asteroid 2003 EH1 and
the Quadrantid meteoroid stream. Because of the newness of the research and the fact that this is a high inclination pair,
we have excluded this pair from our discussions. 相似文献